3694 lines
141 KiB
Plaintext
3694 lines
141 KiB
Plaintext
@node I/O on Streams, Low-Level I/O, I/O Overview, Top
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@chapter Input/Output on Streams
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This chapter describes the functions for creating streams and performing
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input and output operations on them. As discussed in @ref{I/O
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Overview}, a stream is a fairly abstract, high-level concept
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representing a communications channel to a file, device, or process.
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@menu
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* Streams:: About the data type representing a stream.
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* Standard Streams:: Streams to the standard input and output
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devices are created for you.
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* Opening Streams:: How to create a stream to talk to a file.
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* Closing Streams:: Close a stream when you are finished with it.
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* Simple Output:: Unformatted output by characters and lines.
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* Character Input:: Unformatted input by characters and words.
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* Line Input:: Reading a line or a record from a stream.
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* Unreading:: Peeking ahead/pushing back input just read.
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* Block Input/Output:: Input and output operations on blocks of data.
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* Formatted Output:: @code{printf} and related functions.
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* Customizing Printf:: You can define new conversion specifiers for
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@code{printf} and friends.
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* Formatted Input:: @code{scanf} and related functions.
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* EOF and Errors:: How you can tell if an I/O error happens.
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* Binary Streams:: Some systems distinguish between text files
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and binary files.
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* File Positioning:: About random-access streams.
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* Portable Positioning:: Random access on peculiar ISO C systems.
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* Stream Buffering:: How to control buffering of streams.
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* Other Kinds of Streams:: Streams that do not necessarily correspond
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to an open file.
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@end menu
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@node Streams
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@section Streams
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For historical reasons, the type of the C data structure that represents
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a stream is called @code{FILE} rather than ``stream''. Since most of
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the library functions deal with objects of type @code{FILE *}, sometimes
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the term @dfn{file pointer} is also used to mean ``stream''. This leads
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to unfortunate confusion over terminology in many books on C. This
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manual, however, is careful to use the terms ``file'' and ``stream''
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only in the technical sense.
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@cindex file pointer
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@pindex stdio.h
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The @code{FILE} type is declared in the header file @file{stdio.h}.
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@comment stdio.h
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@comment ISO
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@deftp {Data Type} FILE
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This is the data type used to represent stream objects. A @code{FILE}
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object holds all of the internal state information about the connection
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to the associated file, including such things as the file position
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indicator and buffering information. Each stream also has error and
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end-of-file status indicators that can be tested with the @code{ferror}
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and @code{feof} functions; see @ref{EOF and Errors}.
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@end deftp
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@code{FILE} objects are allocated and managed internally by the
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input/output library functions. Don't try to create your own objects of
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type @code{FILE}; let the library do it. Your programs should
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deal only with pointers to these objects (that is, @code{FILE *} values)
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rather than the objects themselves.
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@c !!! should say that FILE's have "No user-serviceable parts inside."
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@node Standard Streams
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@section Standard Streams
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@cindex standard streams
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@cindex streams, standard
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When the @code{main} function of your program is invoked, it already has
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three predefined streams open and available for use. These represent
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the ``standard'' input and output channels that have been established
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for the process.
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These streams are declared in the header file @file{stdio.h}.
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@pindex stdio.h
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@comment stdio.h
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@comment ISO
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@deftypevar {FILE *} stdin
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The @dfn{standard input} stream, which is the normal source of input for the
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program.
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@end deftypevar
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@cindex standard input stream
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@comment stdio.h
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@comment ISO
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@deftypevar {FILE *} stdout
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The @dfn{standard output} stream, which is used for normal output from
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the program.
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@end deftypevar
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@cindex standard output stream
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@comment stdio.h
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@comment ISO
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@deftypevar {FILE *} stderr
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The @dfn{standard error} stream, which is used for error messages and
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diagnostics issued by the program.
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@end deftypevar
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@cindex standard error stream
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In the GNU system, you can specify what files or processes correspond to
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these streams using the pipe and redirection facilities provided by the
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shell. (The primitives shells use to implement these facilities are
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described in @ref{File System Interface}.) Most other operating systems
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provide similar mechanisms, but the details of how to use them can vary.
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In the GNU C library, @code{stdin}, @code{stdout}, and @code{stderr} are
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normal variables which you can set just like any others. For example, to redirect
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the standard output to a file, you could do:
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@smallexample
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fclose (stdout);
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stdout = fopen ("standard-output-file", "w");
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@end smallexample
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Note however, that in other systems @code{stdin}, @code{stdout}, and
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@code{stderr} are macros that you cannot assign to in the normal way.
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But you can use @code{freopen} to get the effect of closing one and
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reopening it. @xref{Opening Streams}.
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@node Opening Streams
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@section Opening Streams
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@cindex opening a stream
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Opening a file with the @code{fopen} function creates a new stream and
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establishes a connection between the stream and a file. This may
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involve creating a new file.
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@pindex stdio.h
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Everything described in this section is declared in the header file
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@file{stdio.h}.
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@comment stdio.h
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@comment ISO
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@deftypefun {FILE *} fopen (const char *@var{filename}, const char *@var{opentype})
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The @code{fopen} function opens a stream for I/O to the file
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@var{filename}, and returns a pointer to the stream.
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The @var{opentype} argument is a string that controls how the file is
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opened and specifies attributes of the resulting stream. It must begin
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with one of the following sequences of characters:
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@table @samp
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@item r
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Open an existing file for reading only.
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@item w
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Open the file for writing only. If the file already exists, it is
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truncated to zero length. Otherwise a new file is created.
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@item a
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Open a file for append access; that is, writing at the end of file only.
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If the file already exists, its initial contents are unchanged and
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output to the stream is appended to the end of the file.
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Otherwise, a new, empty file is created.
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@item r+
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Open an existing file for both reading and writing. The initial contents
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of the file are unchanged and the initial file position is at the
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beginning of the file.
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@item w+
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Open a file for both reading and writing. If the file already exists, it
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is truncated to zero length. Otherwise, a new file is created.
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@item a+
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Open or create file for both reading and appending. If the file exists,
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its initial contents are unchanged. Otherwise, a new file is created.
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The initial file position for reading is at the beginning of the file,
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but output is always appended to the end of the file.
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@end table
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As you can see, @samp{+} requests a stream that can do both input and
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output. The ISO standard says that when using such a stream, you must
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call @code{fflush} (@pxref{Stream Buffering}) or a file positioning
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function such as @code{fseek} (@pxref{File Positioning}) when switching
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from reading to writing or vice versa. Otherwise, internal buffers
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might not be emptied properly. The GNU C library does not have this
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limitation; you can do arbitrary reading and writing operations on a
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stream in whatever order.
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Additional characters may appear after these to specify flags for the
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call. Always put the mode (@samp{r}, @samp{w+}, etc.) first; that is
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the only part you are guaranteed will be understood by all systems.
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The GNU C library defines one additional character for use in
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@var{opentype}: the character @samp{x} insists on creating a new
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file---if a file @var{filename} already exists, @code{fopen} fails
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rather than opening it. If you use @samp{x} you can are guaranteed that
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you will not clobber an existing file. This is equivalent to the
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@code{O_EXCL} option to the @code{open} function (@pxref{Opening and
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Closing Files}).
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The character @samp{b} in @var{opentype} has a standard meaning; it
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requests a binary stream rather than a text stream. But this makes no
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difference in POSIX systems (including the GNU system). If both
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@samp{+} and @samp{b} are specified, they can appear in either order.
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@xref{Binary Streams}.
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Any other characters in @var{opentype} are simply ignored. They may be
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meaningful in other systems.
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If the open fails, @code{fopen} returns a null pointer.
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@end deftypefun
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You can have multiple streams (or file descriptors) pointing to the same
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file open at the same time. If you do only input, this works
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straightforwardly, but you must be careful if any output streams are
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included. @xref{Stream/Descriptor Precautions}. This is equally true
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whether the streams are in one program (not usual) or in several
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programs (which can easily happen). It may be advantageous to use the
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file locking facilities to avoid simultaneous access. @xref{File
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Locks}.
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@comment stdio.h
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@comment ISO
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@deftypevr Macro int FOPEN_MAX
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The value of this macro is an integer constant expression that
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represents the minimum number of streams that the implementation
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guarantees can be open simultaneously. You might be able to open more
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than this many streams, but that is not guaranteed. The value of this
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constant is at least eight, which includes the three standard streams
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@code{stdin}, @code{stdout}, and @code{stderr}. In POSIX.1 systems this
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value is determined by the @code{OPEN_MAX} parameter; @pxref{General
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Limits}. In BSD and GNU, it is controlled by the @code{RLIMIT_NOFILE}
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resource limit; @pxref{Limits on Resources}.
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@end deftypevr
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@comment stdio.h
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@comment ISO
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@deftypefun {FILE *} freopen (const char *@var{filename}, const char *@var{opentype}, FILE *@var{stream})
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This function is like a combination of @code{fclose} and @code{fopen}.
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It first closes the stream referred to by @var{stream}, ignoring any
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errors that are detected in the process. (Because errors are ignored,
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you should not use @code{freopen} on an output stream if you have
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actually done any output using the stream.) Then the file named by
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@var{filename} is opened with mode @var{opentype} as for @code{fopen},
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and associated with the same stream object @var{stream}.
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If the operation fails, a null pointer is returned; otherwise,
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@code{freopen} returns @var{stream}.
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@code{freopen} has traditionally been used to connect a standard stream
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such as @code{stdin} with a file of your own choice. This is useful in
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programs in which use of a standard stream for certain purposes is
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hard-coded. In the GNU C library, you can simply close the standard
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streams and open new ones with @code{fopen}. But other systems lack
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this ability, so using @code{freopen} is more portable.
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@end deftypefun
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@node Closing Streams
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@section Closing Streams
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@cindex closing a stream
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When a stream is closed with @code{fclose}, the connection between the
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stream and the file is cancelled. After you have closed a stream, you
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cannot perform any additional operations on it.
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@comment stdio.h
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@comment ISO
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@deftypefun int fclose (FILE *@var{stream})
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This function causes @var{stream} to be closed and the connection to
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the corresponding file to be broken. Any buffered output is written
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and any buffered input is discarded. The @code{fclose} function returns
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a value of @code{0} if the file was closed successfully, and @code{EOF}
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if an error was detected.
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It is important to check for errors when you call @code{fclose} to close
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an output stream, because real, everyday errors can be detected at this
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time. For example, when @code{fclose} writes the remaining buffered
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output, it might get an error because the disk is full. Even if you
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know the buffer is empty, errors can still occur when closing a file if
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you are using NFS.
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The function @code{fclose} is declared in @file{stdio.h}.
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@end deftypefun
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To close all streams currently available the GNU C Library provides
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another function.
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@comment stdio.h
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@comment GNU
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@deftypefun int fcloseall (void)
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This function causes all open streams of the process to be closed and
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the connection to corresponding files to be broken. All buffered data
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is written and any buffered inputis discarded. The @code{fcloseall}
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function returns a value of @code{0} if all the files were closed
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successfully, and @code{EOF} if an error was detected.
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This function should be used in only in special situation, e.g., when an
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error occurred and the program must be aborted. Normally each single
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stream should be closed separately so that problems with one stream can
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be identifier. It is also problematic since the standard streams
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(@pxref{Standard Streams}) will also be closed.
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The function @code{fcloseall} is declared in @file{stdio.h}.
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@end deftypefun
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If the @code{main} function to your program returns, or if you call the
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@code{exit} function (@pxref{Normal Termination}), all open streams are
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automatically closed properly. If your program terminates in any other
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manner, such as by calling the @code{abort} function (@pxref{Aborting a
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Program}) or from a fatal signal (@pxref{Signal Handling}), open streams
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might not be closed properly. Buffered output might not be flushed and
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files may be incomplete. For more information on buffering of streams,
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see @ref{Stream Buffering}.
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@node Simple Output
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@section Simple Output by Characters or Lines
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@cindex writing to a stream, by characters
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This section describes functions for performing character- and
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line-oriented output.
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These functions are declared in the header file @file{stdio.h}.
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@pindex stdio.h
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@comment stdio.h
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@comment ISO
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@deftypefun int fputc (int @var{c}, FILE *@var{stream})
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The @code{fputc} function converts the character @var{c} to type
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@code{unsigned char}, and writes it to the stream @var{stream}.
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@code{EOF} is returned if a write error occurs; otherwise the
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character @var{c} is returned.
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@end deftypefun
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@comment stdio.h
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@comment ISO
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@deftypefun int putc (int @var{c}, FILE *@var{stream})
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This is just like @code{fputc}, except that most systems implement it as
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a macro, making it faster. One consequence is that it may evaluate the
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@var{stream} argument more than once, which is an exception to the
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general rule for macros. @code{putc} is usually the best function to
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use for writing a single character.
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@end deftypefun
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@comment stdio.h
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@comment ISO
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@deftypefun int putchar (int @var{c})
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The @code{putchar} function is equivalent to @code{putc} with
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@code{stdout} as the value of the @var{stream} argument.
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@end deftypefun
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@comment stdio.h
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@comment ISO
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@deftypefun int fputs (const char *@var{s}, FILE *@var{stream})
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The function @code{fputs} writes the string @var{s} to the stream
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@var{stream}. The terminating null character is not written.
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This function does @emph{not} add a newline character, either.
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It outputs only the characters in the string.
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This function returns @code{EOF} if a write error occurs, and otherwise
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a non-negative value.
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For example:
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@smallexample
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fputs ("Are ", stdout);
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fputs ("you ", stdout);
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fputs ("hungry?\n", stdout);
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@end smallexample
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@noindent
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outputs the text @samp{Are you hungry?} followed by a newline.
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@end deftypefun
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@comment stdio.h
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@comment ISO
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@deftypefun int puts (const char *@var{s})
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The @code{puts} function writes the string @var{s} to the stream
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@code{stdout} followed by a newline. The terminating null character of
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the string is not written. (Note that @code{fputs} does @emph{not}
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write a newline as this function does.)
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@code{puts} is the most convenient function for printing simple
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messages. For example:
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@smallexample
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puts ("This is a message.");
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@end smallexample
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@end deftypefun
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@comment stdio.h
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@comment SVID
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@deftypefun int putw (int @var{w}, FILE *@var{stream})
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This function writes the word @var{w} (that is, an @code{int}) to
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@var{stream}. It is provided for compatibility with SVID, but we
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recommend you use @code{fwrite} instead (@pxref{Block Input/Output}).
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@end deftypefun
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@node Character Input
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@section Character Input
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@cindex reading from a stream, by characters
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This section describes functions for performing character-oriented input.
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These functions are declared in the header file @file{stdio.h}.
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@pindex stdio.h
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These functions return an @code{int} value that is either a character of
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input, or the special value @code{EOF} (usually -1). It is important to
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store the result of these functions in a variable of type @code{int}
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instead of @code{char}, even when you plan to use it only as a
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character. Storing @code{EOF} in a @code{char} variable truncates its
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|
value to the size of a character, so that it is no longer
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distinguishable from the valid character @samp{(char) -1}. So always
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use an @code{int} for the result of @code{getc} and friends, and check
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for @code{EOF} after the call; once you've verified that the result is
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not @code{EOF}, you can be sure that it will fit in a @samp{char}
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variable without loss of information.
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@comment stdio.h
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@comment ISO
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@deftypefun int fgetc (FILE *@var{stream})
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This function reads the next character as an @code{unsigned char} from
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the stream @var{stream} and returns its value, converted to an
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@code{int}. If an end-of-file condition or read error occurs,
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@code{EOF} is returned instead.
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@end deftypefun
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@comment stdio.h
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|
@comment ISO
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|
@deftypefun int getc (FILE *@var{stream})
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|
This is just like @code{fgetc}, except that it is permissible (and
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|
typical) for it to be implemented as a macro that evaluates the
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@var{stream} argument more than once. @code{getc} is often highly
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|
optimized, so it is usually the best function to use to read a single
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character.
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@end deftypefun
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@comment stdio.h
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@comment ISO
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|
@deftypefun int getchar (void)
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The @code{getchar} function is equivalent to @code{getc} with @code{stdin}
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as the value of the @var{stream} argument.
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@end deftypefun
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|
Here is an example of a function that does input using @code{fgetc}. It
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|
would work just as well using @code{getc} instead, or using
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@code{getchar ()} instead of @w{@code{fgetc (stdin)}}.
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@smallexample
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int
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y_or_n_p (const char *question)
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@{
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fputs (question, stdout);
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while (1)
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@{
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int c, answer;
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/* @r{Write a space to separate answer from question.} */
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fputc (' ', stdout);
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/* @r{Read the first character of the line.}
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@r{This should be the answer character, but might not be.} */
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c = tolower (fgetc (stdin));
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answer = c;
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/* @r{Discard rest of input line.} */
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while (c != '\n' && c != EOF)
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c = fgetc (stdin);
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/* @r{Obey the answer if it was valid.} */
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if (answer == 'y')
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return 1;
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if (answer == 'n')
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return 0;
|
|
/* @r{Answer was invalid: ask for valid answer.} */
|
|
fputs ("Please answer y or n:", stdout);
|
|
@}
|
|
@}
|
|
@end smallexample
|
|
|
|
@comment stdio.h
|
|
@comment SVID
|
|
@deftypefun int getw (FILE *@var{stream})
|
|
This function reads a word (that is, an @code{int}) from @var{stream}.
|
|
It's provided for compatibility with SVID. We recommend you use
|
|
@code{fread} instead (@pxref{Block Input/Output}). Unlike @code{getc},
|
|
any @code{int} value could be a valid result. @code{getw} returns
|
|
@code{EOF} when it encounters end-of-file or an error, but there is no
|
|
way to distinguish this from an input word with value -1.
|
|
@end deftypefun
|
|
|
|
@node Line Input
|
|
@section Line-Oriented Input
|
|
|
|
Since many programs interpret input on the basis of lines, it's
|
|
convenient to have functions to read a line of text from a stream.
|
|
|
|
Standard C has functions to do this, but they aren't very safe: null
|
|
characters and even (for @code{gets}) long lines can confuse them. So
|
|
the GNU library provides the nonstandard @code{getline} function that
|
|
makes it easy to read lines reliably.
|
|
|
|
Another GNU extension, @code{getdelim}, generalizes @code{getline}. It
|
|
reads a delimited record, defined as everything through the next
|
|
occurrence of a specified delimiter character.
|
|
|
|
All these functions are declared in @file{stdio.h}.
|
|
|
|
@comment stdio.h
|
|
@comment GNU
|
|
@deftypefun ssize_t getline (char **@var{lineptr}, size_t *@var{n}, FILE *@var{stream})
|
|
This function reads an entire line from @var{stream}, storing the text
|
|
(including the newline and a terminating null character) in a buffer
|
|
and storing the buffer address in @code{*@var{lineptr}}.
|
|
|
|
Before calling @code{getline}, you should place in @code{*@var{lineptr}}
|
|
the address of a buffer @code{*@var{n}} bytes long, allocated with
|
|
@code{malloc}. If this buffer is long enough to hold the line,
|
|
@code{getline} stores the line in this buffer. Otherwise,
|
|
@code{getline} makes the buffer bigger using @code{realloc}, storing the
|
|
new buffer address back in @code{*@var{lineptr}} and the increased size
|
|
back in @code{*@var{n}}.
|
|
@xref{Unconstrained Allocation}.
|
|
|
|
If you set @code{*@var{lineptr}} to a null pointer, and @code{*@var{n}}
|
|
to zero, before the call, then @code{getline} allocates the initial
|
|
buffer for you by calling @code{malloc}.
|
|
|
|
In either case, when @code{getline} returns, @code{*@var{lineptr}} is
|
|
a @code{char *} which points to the text of the line.
|
|
|
|
When @code{getline} is successful, it returns the number of characters
|
|
read (including the newline, but not including the terminating null).
|
|
This value enables you to distinguish null characters that are part of
|
|
the line from the null character inserted as a terminator.
|
|
|
|
This function is a GNU extension, but it is the recommended way to read
|
|
lines from a stream. The alternative standard functions are unreliable.
|
|
|
|
If an error occurs or end of file is reached, @code{getline} returns
|
|
@code{-1}.
|
|
@end deftypefun
|
|
|
|
@comment stdio.h
|
|
@comment GNU
|
|
@deftypefun ssize_t getdelim (char **@var{lineptr}, size_t *@var{n}, int @var{delimiter}, FILE *@var{stream})
|
|
This function is like @code{getline} except that the character which
|
|
tells it to stop reading is not necessarily newline. The argument
|
|
@var{delimiter} specifies the delimiter character; @code{getdelim} keeps
|
|
reading until it sees that character (or end of file).
|
|
|
|
The text is stored in @var{lineptr}, including the delimiter character
|
|
and a terminating null. Like @code{getline}, @code{getdelim} makes
|
|
@var{lineptr} bigger if it isn't big enough.
|
|
|
|
@code{getline} is in fact implemented in terms of @code{getdelim}, just
|
|
like this:
|
|
|
|
@smallexample
|
|
ssize_t
|
|
getline (char **lineptr, size_t *n, FILE *stream)
|
|
@{
|
|
return getdelim (lineptr, n, '\n', stream);
|
|
@}
|
|
@end smallexample
|
|
@end deftypefun
|
|
|
|
@comment stdio.h
|
|
@comment ISO
|
|
@deftypefun {char *} fgets (char *@var{s}, int @var{count}, FILE *@var{stream})
|
|
The @code{fgets} function reads characters from the stream @var{stream}
|
|
up to and including a newline character and stores them in the string
|
|
@var{s}, adding a null character to mark the end of the string. You
|
|
must supply @var{count} characters worth of space in @var{s}, but the
|
|
number of characters read is at most @var{count} @minus{} 1. The extra
|
|
character space is used to hold the null character at the end of the
|
|
string.
|
|
|
|
If the system is already at end of file when you call @code{fgets}, then
|
|
the contents of the array @var{s} are unchanged and a null pointer is
|
|
returned. A null pointer is also returned if a read error occurs.
|
|
Otherwise, the return value is the pointer @var{s}.
|
|
|
|
@strong{Warning:} If the input data has a null character, you can't tell.
|
|
So don't use @code{fgets} unless you know the data cannot contain a null.
|
|
Don't use it to read files edited by the user because, if the user inserts
|
|
a null character, you should either handle it properly or print a clear
|
|
error message. We recommend using @code{getline} instead of @code{fgets}.
|
|
@end deftypefun
|
|
|
|
@comment stdio.h
|
|
@comment ISO
|
|
@deftypefn {Deprecated function} {char *} gets (char *@var{s})
|
|
The function @code{gets} reads characters from the stream @code{stdin}
|
|
up to the next newline character, and stores them in the string @var{s}.
|
|
The newline character is discarded (note that this differs from the
|
|
behavior of @code{fgets}, which copies the newline character into the
|
|
string). If @code{gets} encounters a read error or end-of-file, it
|
|
returns a null pointer; otherwise it returns @var{s}.
|
|
|
|
@strong{Warning:} The @code{gets} function is @strong{very dangerous}
|
|
because it provides no protection against overflowing the string
|
|
@var{s}. The GNU library includes it for compatibility only. You
|
|
should @strong{always} use @code{fgets} or @code{getline} instead. To
|
|
remind you of this, the linker (if using GNU @code{ld}) will issue a
|
|
warning whenever you use @code{gets}.
|
|
@end deftypefn
|
|
|
|
@node Unreading
|
|
@section Unreading
|
|
@cindex peeking at input
|
|
@cindex unreading characters
|
|
@cindex pushing input back
|
|
|
|
In parser programs it is often useful to examine the next character in
|
|
the input stream without removing it from the stream. This is called
|
|
``peeking ahead'' at the input because your program gets a glimpse of
|
|
the input it will read next.
|
|
|
|
Using stream I/O, you can peek ahead at input by first reading it and
|
|
then @dfn{unreading} it (also called @dfn{pushing it back} on the stream).
|
|
Unreading a character makes it available to be input again from the stream,
|
|
by the next call to @code{fgetc} or other input function on that stream.
|
|
|
|
@menu
|
|
* Unreading Idea:: An explanation of unreading with pictures.
|
|
* How Unread:: How to call @code{ungetc} to do unreading.
|
|
@end menu
|
|
|
|
@node Unreading Idea
|
|
@subsection What Unreading Means
|
|
|
|
Here is a pictorial explanation of unreading. Suppose you have a
|
|
stream reading a file that contains just six characters, the letters
|
|
@samp{foobar}. Suppose you have read three characters so far. The
|
|
situation looks like this:
|
|
|
|
@smallexample
|
|
f o o b a r
|
|
^
|
|
@end smallexample
|
|
|
|
@noindent
|
|
so the next input character will be @samp{b}.
|
|
|
|
@c @group Invalid outside @example
|
|
If instead of reading @samp{b} you unread the letter @samp{o}, you get a
|
|
situation like this:
|
|
|
|
@smallexample
|
|
f o o b a r
|
|
|
|
|
o--
|
|
^
|
|
@end smallexample
|
|
|
|
@noindent
|
|
so that the next input characters will be @samp{o} and @samp{b}.
|
|
@c @end group
|
|
|
|
@c @group
|
|
If you unread @samp{9} instead of @samp{o}, you get this situation:
|
|
|
|
@smallexample
|
|
f o o b a r
|
|
|
|
|
9--
|
|
^
|
|
@end smallexample
|
|
|
|
@noindent
|
|
so that the next input characters will be @samp{9} and @samp{b}.
|
|
@c @end group
|
|
|
|
@node How Unread
|
|
@subsection Using @code{ungetc} To Do Unreading
|
|
|
|
The function to unread a character is called @code{ungetc}, because it
|
|
reverses the action of @code{getc}.
|
|
|
|
@comment stdio.h
|
|
@comment ISO
|
|
@deftypefun int ungetc (int @var{c}, FILE *@var{stream})
|
|
The @code{ungetc} function pushes back the character @var{c} onto the
|
|
input stream @var{stream}. So the next input from @var{stream} will
|
|
read @var{c} before anything else.
|
|
|
|
If @var{c} is @code{EOF}, @code{ungetc} does nothing and just returns
|
|
@code{EOF}. This lets you call @code{ungetc} with the return value of
|
|
@code{getc} without needing to check for an error from @code{getc}.
|
|
|
|
The character that you push back doesn't have to be the same as the last
|
|
character that was actually read from the stream. In fact, it isn't
|
|
necessary to actually read any characters from the stream before
|
|
unreading them with @code{ungetc}! But that is a strange way to write
|
|
a program; usually @code{ungetc} is used only to unread a character
|
|
that was just read from the same stream.
|
|
|
|
The GNU C library only supports one character of pushback---in other
|
|
words, it does not work to call @code{ungetc} twice without doing input
|
|
in between. Other systems might let you push back multiple characters;
|
|
then reading from the stream retrieves the characters in the reverse
|
|
order that they were pushed.
|
|
|
|
Pushing back characters doesn't alter the file; only the internal
|
|
buffering for the stream is affected. If a file positioning function
|
|
(such as @code{fseek} or @code{rewind}; @pxref{File Positioning}) is
|
|
called, any pending pushed-back characters are discarded.
|
|
|
|
Unreading a character on a stream that is at end of file clears the
|
|
end-of-file indicator for the stream, because it makes the character of
|
|
input available. After you read that character, trying to read again
|
|
will encounter end of file.
|
|
@end deftypefun
|
|
|
|
Here is an example showing the use of @code{getc} and @code{ungetc} to
|
|
skip over whitespace characters. When this function reaches a
|
|
non-whitespace character, it unreads that character to be seen again on
|
|
the next read operation on the stream.
|
|
|
|
@smallexample
|
|
#include <stdio.h>
|
|
#include <ctype.h>
|
|
|
|
void
|
|
skip_whitespace (FILE *stream)
|
|
@{
|
|
int c;
|
|
do
|
|
/* @r{No need to check for @code{EOF} because it is not}
|
|
@r{@code{isspace}, and @code{ungetc} ignores @code{EOF}.} */
|
|
c = getc (stream);
|
|
while (isspace (c));
|
|
ungetc (c, stream);
|
|
@}
|
|
@end smallexample
|
|
|
|
@node Block Input/Output
|
|
@section Block Input/Output
|
|
|
|
This section describes how to do input and output operations on blocks
|
|
of data. You can use these functions to read and write binary data, as
|
|
well as to read and write text in fixed-size blocks instead of by
|
|
characters or lines.
|
|
@cindex binary I/O to a stream
|
|
@cindex block I/O to a stream
|
|
@cindex reading from a stream, by blocks
|
|
@cindex writing to a stream, by blocks
|
|
|
|
Binary files are typically used to read and write blocks of data in the
|
|
same format as is used to represent the data in a running program. In
|
|
other words, arbitrary blocks of memory---not just character or string
|
|
objects---can be written to a binary file, and meaningfully read in
|
|
again by the same program.
|
|
|
|
Storing data in binary form is often considerably more efficient than
|
|
using the formatted I/O functions. Also, for floating-point numbers,
|
|
the binary form avoids possible loss of precision in the conversion
|
|
process. On the other hand, binary files can't be examined or modified
|
|
easily using many standard file utilities (such as text editors), and
|
|
are not portable between different implementations of the language, or
|
|
different kinds of computers.
|
|
|
|
These functions are declared in @file{stdio.h}.
|
|
@pindex stdio.h
|
|
|
|
@comment stdio.h
|
|
@comment ISO
|
|
@deftypefun size_t fread (void *@var{data}, size_t @var{size}, size_t @var{count}, FILE *@var{stream})
|
|
This function reads up to @var{count} objects of size @var{size} into
|
|
the array @var{data}, from the stream @var{stream}. It returns the
|
|
number of objects actually read, which might be less than @var{count} if
|
|
a read error occurs or the end of the file is reached. This function
|
|
returns a value of zero (and doesn't read anything) if either @var{size}
|
|
or @var{count} is zero.
|
|
|
|
If @code{fread} encounters end of file in the middle of an object, it
|
|
returns the number of complete objects read, and discards the partial
|
|
object. Therefore, the stream remains at the actual end of the file.
|
|
@end deftypefun
|
|
|
|
@comment stdio.h
|
|
@comment ISO
|
|
@deftypefun size_t fwrite (const void *@var{data}, size_t @var{size}, size_t @var{count}, FILE *@var{stream})
|
|
This function writes up to @var{count} objects of size @var{size} from
|
|
the array @var{data}, to the stream @var{stream}. The return value is
|
|
normally @var{count}, if the call succeeds. Any other value indicates
|
|
some sort of error, such as running out of space.
|
|
@end deftypefun
|
|
|
|
@node Formatted Output
|
|
@section Formatted Output
|
|
|
|
@cindex format string, for @code{printf}
|
|
@cindex template, for @code{printf}
|
|
@cindex formatted output to a stream
|
|
@cindex writing to a stream, formatted
|
|
The functions described in this section (@code{printf} and related
|
|
functions) provide a convenient way to perform formatted output. You
|
|
call @code{printf} with a @dfn{format string} or @dfn{template string}
|
|
that specifies how to format the values of the remaining arguments.
|
|
|
|
Unless your program is a filter that specifically performs line- or
|
|
character-oriented processing, using @code{printf} or one of the other
|
|
related functions described in this section is usually the easiest and
|
|
most concise way to perform output. These functions are especially
|
|
useful for printing error messages, tables of data, and the like.
|
|
|
|
@menu
|
|
* Formatted Output Basics:: Some examples to get you started.
|
|
* Output Conversion Syntax:: General syntax of conversion
|
|
specifications.
|
|
* Table of Output Conversions:: Summary of output conversions and
|
|
what they do.
|
|
* Integer Conversions:: Details about formatting of integers.
|
|
* Floating-Point Conversions:: Details about formatting of
|
|
floating-point numbers.
|
|
* Other Output Conversions:: Details about formatting of strings,
|
|
characters, pointers, and the like.
|
|
* Formatted Output Functions:: Descriptions of the actual functions.
|
|
* Dynamic Output:: Functions that allocate memory for the output.
|
|
* Variable Arguments Output:: @code{vprintf} and friends.
|
|
* Parsing a Template String:: What kinds of args does a given template
|
|
call for?
|
|
* Example of Parsing:: Sample program using @code{parse_printf_format}.
|
|
@end menu
|
|
|
|
@node Formatted Output Basics
|
|
@subsection Formatted Output Basics
|
|
|
|
The @code{printf} function can be used to print any number of arguments.
|
|
The template string argument you supply in a call provides
|
|
information not only about the number of additional arguments, but also
|
|
about their types and what style should be used for printing them.
|
|
|
|
Ordinary characters in the template string are simply written to the
|
|
output stream as-is, while @dfn{conversion specifications} introduced by
|
|
a @samp{%} character in the template cause subsequent arguments to be
|
|
formatted and written to the output stream. For example,
|
|
@cindex conversion specifications (@code{printf})
|
|
|
|
@smallexample
|
|
int pct = 37;
|
|
char filename[] = "foo.txt";
|
|
printf ("Processing of `%s' is %d%% finished.\nPlease be patient.\n",
|
|
filename, pct);
|
|
@end smallexample
|
|
|
|
@noindent
|
|
produces output like
|
|
|
|
@smallexample
|
|
Processing of `foo.txt' is 37% finished.
|
|
Please be patient.
|
|
@end smallexample
|
|
|
|
This example shows the use of the @samp{%d} conversion to specify that
|
|
an @code{int} argument should be printed in decimal notation, the
|
|
@samp{%s} conversion to specify printing of a string argument, and
|
|
the @samp{%%} conversion to print a literal @samp{%} character.
|
|
|
|
There are also conversions for printing an integer argument as an
|
|
unsigned value in octal, decimal, or hexadecimal radix (@samp{%o},
|
|
@samp{%u}, or @samp{%x}, respectively); or as a character value
|
|
(@samp{%c}).
|
|
|
|
Floating-point numbers can be printed in normal, fixed-point notation
|
|
using the @samp{%f} conversion or in exponential notation using the
|
|
@samp{%e} conversion. The @samp{%g} conversion uses either @samp{%e}
|
|
or @samp{%f} format, depending on what is more appropriate for the
|
|
magnitude of the particular number.
|
|
|
|
You can control formatting more precisely by writing @dfn{modifiers}
|
|
between the @samp{%} and the character that indicates which conversion
|
|
to apply. These slightly alter the ordinary behavior of the conversion.
|
|
For example, most conversion specifications permit you to specify a
|
|
minimum field width and a flag indicating whether you want the result
|
|
left- or right-justified within the field.
|
|
|
|
The specific flags and modifiers that are permitted and their
|
|
interpretation vary depending on the particular conversion. They're all
|
|
described in more detail in the following sections. Don't worry if this
|
|
all seems excessively complicated at first; you can almost always get
|
|
reasonable free-format output without using any of the modifiers at all.
|
|
The modifiers are mostly used to make the output look ``prettier'' in
|
|
tables.
|
|
|
|
@node Output Conversion Syntax
|
|
@subsection Output Conversion Syntax
|
|
|
|
This section provides details about the precise syntax of conversion
|
|
specifications that can appear in a @code{printf} template
|
|
string.
|
|
|
|
Characters in the template string that are not part of a
|
|
conversion specification are printed as-is to the output stream.
|
|
Multibyte character sequences (@pxref{Extended Characters}) are permitted in
|
|
a template string.
|
|
|
|
The conversion specifications in a @code{printf} template string have
|
|
the general form:
|
|
|
|
@example
|
|
% @var{flags} @var{width} @r{[} . @var{precision} @r{]} @var{type} @var{conversion}
|
|
@end example
|
|
|
|
For example, in the conversion specifier @samp{%-10.8ld}, the @samp{-}
|
|
is a flag, @samp{10} specifies the field width, the precision is
|
|
@samp{8}, the letter @samp{l} is a type modifier, and @samp{d} specifies
|
|
the conversion style. (This particular type specifier says to
|
|
print a @code{long int} argument in decimal notation, with a minimum of
|
|
8 digits left-justified in a field at least 10 characters wide.)
|
|
|
|
In more detail, output conversion specifications consist of an
|
|
initial @samp{%} character followed in sequence by:
|
|
|
|
@itemize @bullet
|
|
@item
|
|
Zero or more @dfn{flag characters} that modify the normal behavior of
|
|
the conversion specification.
|
|
@cindex flag character (@code{printf})
|
|
|
|
@item
|
|
An optional decimal integer specifying the @dfn{minimum field width}.
|
|
If the normal conversion produces fewer characters than this, the field
|
|
is padded with spaces to the specified width. This is a @emph{minimum}
|
|
value; if the normal conversion produces more characters than this, the
|
|
field is @emph{not} truncated. Normally, the output is right-justified
|
|
within the field.
|
|
@cindex minimum field width (@code{printf})
|
|
|
|
You can also specify a field width of @samp{*}. This means that the
|
|
next argument in the argument list (before the actual value to be
|
|
printed) is used as the field width. The value must be an @code{int}.
|
|
If the value is negative, this means to set the @samp{-} flag (see
|
|
below) and to use the absolute value as the field width.
|
|
|
|
@item
|
|
An optional @dfn{precision} to specify the number of digits to be
|
|
written for the numeric conversions. If the precision is specified, it
|
|
consists of a period (@samp{.}) followed optionally by a decimal integer
|
|
(which defaults to zero if omitted).
|
|
@cindex precision (@code{printf})
|
|
|
|
You can also specify a precision of @samp{*}. This means that the next
|
|
argument in the argument list (before the actual value to be printed) is
|
|
used as the precision. The value must be an @code{int}, and is ignored
|
|
if it is negative. If you specify @samp{*} for both the field width and
|
|
precision, the field width argument precedes the precision argument.
|
|
Other C library versions may not recognize this syntax.
|
|
|
|
@item
|
|
An optional @dfn{type modifier character}, which is used to specify the
|
|
data type of the corresponding argument if it differs from the default
|
|
type. (For example, the integer conversions assume a type of @code{int},
|
|
but you can specify @samp{h}, @samp{l}, or @samp{L} for other integer
|
|
types.)
|
|
@cindex type modifier character (@code{printf})
|
|
|
|
@item
|
|
A character that specifies the conversion to be applied.
|
|
@end itemize
|
|
|
|
The exact options that are permitted and how they are interpreted vary
|
|
between the different conversion specifiers. See the descriptions of the
|
|
individual conversions for information about the particular options that
|
|
they use.
|
|
|
|
With the @samp{-Wformat} option, the GNU C compiler checks calls to
|
|
@code{printf} and related functions. It examines the format string and
|
|
verifies that the correct number and types of arguments are supplied.
|
|
There is also a GNU C syntax to tell the compiler that a function you
|
|
write uses a @code{printf}-style format string.
|
|
@xref{Function Attributes, , Declaring Attributes of Functions,
|
|
gcc.info, Using GNU CC}, for more information.
|
|
|
|
@node Table of Output Conversions
|
|
@subsection Table of Output Conversions
|
|
@cindex output conversions, for @code{printf}
|
|
|
|
Here is a table summarizing what all the different conversions do:
|
|
|
|
@table @asis
|
|
@item @samp{%d}, @samp{%i}
|
|
Print an integer as a signed decimal number. @xref{Integer
|
|
Conversions}, for details. @samp{%d} and @samp{%i} are synonymous for
|
|
output, but are different when used with @code{scanf} for input
|
|
(@pxref{Table of Input Conversions}).
|
|
|
|
@item @samp{%o}
|
|
Print an integer as an unsigned octal number. @xref{Integer
|
|
Conversions}, for details.
|
|
|
|
@item @samp{%u}
|
|
Print an integer as an unsigned decimal number. @xref{Integer
|
|
Conversions}, for details.
|
|
|
|
@item @samp{%x}, @samp{%X}
|
|
Print an integer as an unsigned hexadecimal number. @samp{%x} uses
|
|
lower-case letters and @samp{%X} uses upper-case. @xref{Integer
|
|
Conversions}, for details.
|
|
|
|
@item @samp{%f}
|
|
Print a floating-point number in normal (fixed-point) notation.
|
|
@xref{Floating-Point Conversions}, for details.
|
|
|
|
@item @samp{%e}, @samp{%E}
|
|
Print a floating-point number in exponential notation. @samp{%e} uses
|
|
lower-case letters and @samp{%E} uses upper-case. @xref{Floating-Point
|
|
Conversions}, for details.
|
|
|
|
@item @samp{%g}, @samp{%G}
|
|
Print a floating-point number in either normal or exponential notation,
|
|
whichever is more appropriate for its magnitude. @samp{%g} uses
|
|
lower-case letters and @samp{%G} uses upper-case. @xref{Floating-Point
|
|
Conversions}, for details.
|
|
|
|
@item @samp{%c}
|
|
Print a single character. @xref{Other Output Conversions}.
|
|
|
|
@item @samp{%s}
|
|
Print a string. @xref{Other Output Conversions}.
|
|
|
|
@item @samp{%p}
|
|
Print the value of a pointer. @xref{Other Output Conversions}.
|
|
|
|
@item @samp{%n}
|
|
Get the number of characters printed so far. @xref{Other Output Conversions}.
|
|
Note that this conversion specification never produces any output.
|
|
|
|
@item @samp{%m}
|
|
Print the string corresponding to the value of @code{errno}.
|
|
(This is a GNU extension.)
|
|
@xref{Other Output Conversions}.
|
|
|
|
@item @samp{%%}
|
|
Print a literal @samp{%} character. @xref{Other Output Conversions}.
|
|
@end table
|
|
|
|
If the syntax of a conversion specification is invalid, unpredictable
|
|
things will happen, so don't do this. If there aren't enough function
|
|
arguments provided to supply values for all the conversion
|
|
specifications in the template string, or if the arguments are not of
|
|
the correct types, the results are unpredictable. If you supply more
|
|
arguments than conversion specifications, the extra argument values are
|
|
simply ignored; this is sometimes useful.
|
|
|
|
@node Integer Conversions
|
|
@subsection Integer Conversions
|
|
|
|
This section describes the options for the @samp{%d}, @samp{%i},
|
|
@samp{%o}, @samp{%u}, @samp{%x}, and @samp{%X} conversion
|
|
specifications. These conversions print integers in various formats.
|
|
|
|
The @samp{%d} and @samp{%i} conversion specifications both print an
|
|
@code{int} argument as a signed decimal number; while @samp{%o},
|
|
@samp{%u}, and @samp{%x} print the argument as an unsigned octal,
|
|
decimal, or hexadecimal number (respectively). The @samp{%X} conversion
|
|
specification is just like @samp{%x} except that it uses the characters
|
|
@samp{ABCDEF} as digits instead of @samp{abcdef}.
|
|
|
|
The following flags are meaningful:
|
|
|
|
@table @asis
|
|
@item @samp{-}
|
|
Left-justify the result in the field (instead of the normal
|
|
right-justification).
|
|
|
|
@item @samp{+}
|
|
For the signed @samp{%d} and @samp{%i} conversions, print a
|
|
plus sign if the value is positive.
|
|
|
|
@item @samp{ }
|
|
For the signed @samp{%d} and @samp{%i} conversions, if the result
|
|
doesn't start with a plus or minus sign, prefix it with a space
|
|
character instead. Since the @samp{+} flag ensures that the result
|
|
includes a sign, this flag is ignored if you supply both of them.
|
|
|
|
@item @samp{#}
|
|
For the @samp{%o} conversion, this forces the leading digit to be
|
|
@samp{0}, as if by increasing the precision. For @samp{%x} or
|
|
@samp{%X}, this prefixes a leading @samp{0x} or @samp{0X} (respectively)
|
|
to the result. This doesn't do anything useful for the @samp{%d},
|
|
@samp{%i}, or @samp{%u} conversions. Using this flag produces output
|
|
which can be parsed by the @code{strtoul} function (@pxref{Parsing of
|
|
Integers}) and @code{scanf} with the @samp{%i} conversion
|
|
(@pxref{Numeric Input Conversions}).
|
|
|
|
@item @samp{'}
|
|
Separate the digits into groups as specified by the locale specified for
|
|
the @code{LC_NUMERIC} category; @pxref{General Numeric}. This flag is a
|
|
GNU extension.
|
|
|
|
@item @samp{0}
|
|
Pad the field with zeros instead of spaces. The zeros are placed after
|
|
any indication of sign or base. This flag is ignored if the @samp{-}
|
|
flag is also specified, or if a precision is specified.
|
|
@end table
|
|
|
|
If a precision is supplied, it specifies the minimum number of digits to
|
|
appear; leading zeros are produced if necessary. If you don't specify a
|
|
precision, the number is printed with as many digits as it needs. If
|
|
you convert a value of zero with an explicit precision of zero, then no
|
|
characters at all are produced.
|
|
|
|
Without a type modifier, the corresponding argument is treated as an
|
|
@code{int} (for the signed conversions @samp{%i} and @samp{%d}) or
|
|
@code{unsigned int} (for the unsigned conversions @samp{%o}, @samp{%u},
|
|
@samp{%x}, and @samp{%X}). Recall that since @code{printf} and friends
|
|
are variadic, any @code{char} and @code{short} arguments are
|
|
automatically converted to @code{int} by the default argument
|
|
promotions. For arguments of other integer types, you can use these
|
|
modifiers:
|
|
|
|
@table @samp
|
|
@item h
|
|
Specifies that the argument is a @code{short int} or @code{unsigned
|
|
short int}, as appropriate. A @code{short} argument is converted to an
|
|
@code{int} or @code{unsigned int} by the default argument promotions
|
|
anyway, but the @samp{h} modifier says to convert it back to a
|
|
@code{short} again.
|
|
|
|
@item l
|
|
Specifies that the argument is a @code{long int} or @code{unsigned long
|
|
int}, as appropriate. Two @samp{l} characters is like the @samp{L}
|
|
modifier, below.
|
|
|
|
@item L
|
|
@itemx ll
|
|
@itemx q
|
|
Specifies that the argument is a @code{long long int}. (This type is
|
|
an extension supported by the GNU C compiler. On systems that don't
|
|
support extra-long integers, this is the same as @code{long int}.)
|
|
|
|
The @samp{q} modifier is another name for the same thing, which comes
|
|
from 4.4 BSD; a @w{@code{long long int}} is sometimes called a ``quad''
|
|
@code{int}.
|
|
|
|
@item Z
|
|
Specifies that the argument is a @code{size_t}. This is a GNU extension.
|
|
@end table
|
|
|
|
Here is an example. Using the template string:
|
|
|
|
@smallexample
|
|
"|%5d|%-5d|%+5d|%+-5d|% 5d|%05d|%5.0d|%5.2d|%d|\n"
|
|
@end smallexample
|
|
|
|
@noindent
|
|
to print numbers using the different options for the @samp{%d}
|
|
conversion gives results like:
|
|
|
|
@smallexample
|
|
| 0|0 | +0|+0 | 0|00000| | 00|0|
|
|
| 1|1 | +1|+1 | 1|00001| 1| 01|1|
|
|
| -1|-1 | -1|-1 | -1|-0001| -1| -01|-1|
|
|
|100000|100000|+100000| 100000|100000|100000|100000|100000|
|
|
@end smallexample
|
|
|
|
In particular, notice what happens in the last case where the number
|
|
is too large to fit in the minimum field width specified.
|
|
|
|
Here are some more examples showing how unsigned integers print under
|
|
various format options, using the template string:
|
|
|
|
@smallexample
|
|
"|%5u|%5o|%5x|%5X|%#5o|%#5x|%#5X|%#10.8x|\n"
|
|
@end smallexample
|
|
|
|
@smallexample
|
|
| 0| 0| 0| 0| 0| 0x0| 0X0|0x00000000|
|
|
| 1| 1| 1| 1| 01| 0x1| 0X1|0x00000001|
|
|
|100000|303240|186a0|186A0|0303240|0x186a0|0X186A0|0x000186a0|
|
|
@end smallexample
|
|
|
|
|
|
@node Floating-Point Conversions
|
|
@subsection Floating-Point Conversions
|
|
|
|
This section discusses the conversion specifications for floating-point
|
|
numbers: the @samp{%f}, @samp{%e}, @samp{%E}, @samp{%g}, and @samp{%G}
|
|
conversions.
|
|
|
|
The @samp{%f} conversion prints its argument in fixed-point notation,
|
|
producing output of the form
|
|
@w{[@code{-}]@var{ddd}@code{.}@var{ddd}},
|
|
where the number of digits following the decimal point is controlled
|
|
by the precision you specify.
|
|
|
|
The @samp{%e} conversion prints its argument in exponential notation,
|
|
producing output of the form
|
|
@w{[@code{-}]@var{d}@code{.}@var{ddd}@code{e}[@code{+}|@code{-}]@var{dd}}.
|
|
Again, the number of digits following the decimal point is controlled by
|
|
the precision. The exponent always contains at least two digits. The
|
|
@samp{%E} conversion is similar but the exponent is marked with the letter
|
|
@samp{E} instead of @samp{e}.
|
|
|
|
The @samp{%g} and @samp{%G} conversions print the argument in the style
|
|
of @samp{%e} or @samp{%E} (respectively) if the exponent would be less
|
|
than -4 or greater than or equal to the precision; otherwise they use the
|
|
@samp{%f} style. Trailing zeros are removed from the fractional portion
|
|
of the result and a decimal-point character appears only if it is
|
|
followed by a digit.
|
|
|
|
The following flags can be used to modify the behavior:
|
|
|
|
@comment We use @asis instead of @samp so we can have ` ' as an item.
|
|
@table @asis
|
|
@item @samp{-}
|
|
Left-justify the result in the field. Normally the result is
|
|
right-justified.
|
|
|
|
@item @samp{+}
|
|
Always include a plus or minus sign in the result.
|
|
|
|
@item @samp{ }
|
|
If the result doesn't start with a plus or minus sign, prefix it with a
|
|
space instead. Since the @samp{+} flag ensures that the result includes
|
|
a sign, this flag is ignored if you supply both of them.
|
|
|
|
@item @samp{#}
|
|
Specifies that the result should always include a decimal point, even
|
|
if no digits follow it. For the @samp{%g} and @samp{%G} conversions,
|
|
this also forces trailing zeros after the decimal point to be left
|
|
in place where they would otherwise be removed.
|
|
|
|
@item @samp{'}
|
|
Separate the digits of the integer part of the result into groups as
|
|
specified by the locale specified for the @code{LC_NUMERIC} category;
|
|
@pxref{General Numeric}. This flag is a GNU extension.
|
|
|
|
@item @samp{0}
|
|
Pad the field with zeros instead of spaces; the zeros are placed
|
|
after any sign. This flag is ignored if the @samp{-} flag is also
|
|
specified.
|
|
@end table
|
|
|
|
The precision specifies how many digits follow the decimal-point
|
|
character for the @samp{%f}, @samp{%e}, and @samp{%E} conversions. For
|
|
these conversions, the default precision is @code{6}. If the precision
|
|
is explicitly @code{0}, this suppresses the decimal point character
|
|
entirely. For the @samp{%g} and @samp{%G} conversions, the precision
|
|
specifies how many significant digits to print. Significant digits are
|
|
the first digit before the decimal point, and all the digits after it.
|
|
If the precision @code{0} or not specified for @samp{%g} or @samp{%G},
|
|
it is treated like a value of @code{1}. If the value being printed
|
|
cannot be expressed accurately in the specified number of digits, the
|
|
value is rounded to the nearest number that fits.
|
|
|
|
Without a type modifier, the floating-point conversions use an argument
|
|
of type @code{double}. (By the default argument promotions, any
|
|
@code{float} arguments are automatically converted to @code{double}.)
|
|
The following type modifier is supported:
|
|
|
|
@table @samp
|
|
@item L
|
|
An uppercase @samp{L} specifies that the argument is a @code{long
|
|
double}.
|
|
@end table
|
|
|
|
Here are some examples showing how numbers print using the various
|
|
floating-point conversions. All of the numbers were printed using
|
|
this template string:
|
|
|
|
@smallexample
|
|
"|%12.4f|%12.4e|%12.4g|\n"
|
|
@end smallexample
|
|
|
|
Here is the output:
|
|
|
|
@smallexample
|
|
| 0.0000| 0.0000e+00| 0|
|
|
| 1.0000| 1.0000e+00| 1|
|
|
| -1.0000| -1.0000e+00| -1|
|
|
| 100.0000| 1.0000e+02| 100|
|
|
| 1000.0000| 1.0000e+03| 1000|
|
|
| 10000.0000| 1.0000e+04| 1e+04|
|
|
| 12345.0000| 1.2345e+04| 1.234e+04|
|
|
| 100000.0000| 1.0000e+05| 1e+05|
|
|
| 123456.0000| 1.2346e+05| 1.234e+05|
|
|
@end smallexample
|
|
|
|
Notice how the @samp{%g} conversion drops trailing zeros.
|
|
|
|
@node Other Output Conversions
|
|
@subsection Other Output Conversions
|
|
|
|
This section describes miscellaneous conversions for @code{printf}.
|
|
|
|
The @samp{%c} conversion prints a single character. The @code{int}
|
|
argument is first converted to an @code{unsigned char}. The @samp{-}
|
|
flag can be used to specify left-justification in the field, but no
|
|
other flags are defined, and no precision or type modifier can be given.
|
|
For example:
|
|
|
|
@smallexample
|
|
printf ("%c%c%c%c%c", 'h', 'e', 'l', 'l', 'o');
|
|
@end smallexample
|
|
|
|
@noindent
|
|
prints @samp{hello}.
|
|
|
|
The @samp{%s} conversion prints a string. The corresponding argument
|
|
must be of type @code{char *} (or @code{const char *}). A precision can
|
|
be specified to indicate the maximum number of characters to write;
|
|
otherwise characters in the string up to but not including the
|
|
terminating null character are written to the output stream. The
|
|
@samp{-} flag can be used to specify left-justification in the field,
|
|
but no other flags or type modifiers are defined for this conversion.
|
|
For example:
|
|
|
|
@smallexample
|
|
printf ("%3s%-6s", "no", "where");
|
|
@end smallexample
|
|
|
|
@noindent
|
|
prints @samp{ nowhere }.
|
|
|
|
If you accidentally pass a null pointer as the argument for a @samp{%s}
|
|
conversion, the GNU library prints it as @samp{(null)}. We think this
|
|
is more useful than crashing. But it's not good practice to pass a null
|
|
argument intentionally.
|
|
|
|
The @samp{%m} conversion prints the string corresponding to the error
|
|
code in @code{errno}. @xref{Error Messages}. Thus:
|
|
|
|
@smallexample
|
|
fprintf (stderr, "can't open `%s': %m\n", filename);
|
|
@end smallexample
|
|
|
|
@noindent
|
|
is equivalent to:
|
|
|
|
@smallexample
|
|
fprintf (stderr, "can't open `%s': %s\n", filename, strerror (errno));
|
|
@end smallexample
|
|
|
|
@noindent
|
|
The @samp{%m} conversion is a GNU C library extension.
|
|
|
|
The @samp{%p} conversion prints a pointer value. The corresponding
|
|
argument must be of type @code{void *}. In practice, you can use any
|
|
type of pointer.
|
|
|
|
In the GNU system, non-null pointers are printed as unsigned integers,
|
|
as if a @samp{%#x} conversion were used. Null pointers print as
|
|
@samp{(nil)}. (Pointers might print differently in other systems.)
|
|
|
|
For example:
|
|
|
|
@smallexample
|
|
printf ("%p", "testing");
|
|
@end smallexample
|
|
|
|
@noindent
|
|
prints @samp{0x} followed by a hexadecimal number---the address of the
|
|
string constant @code{"testing"}. It does not print the word
|
|
@samp{testing}.
|
|
|
|
You can supply the @samp{-} flag with the @samp{%p} conversion to
|
|
specify left-justification, but no other flags, precision, or type
|
|
modifiers are defined.
|
|
|
|
The @samp{%n} conversion is unlike any of the other output conversions.
|
|
It uses an argument which must be a pointer to an @code{int}, but
|
|
instead of printing anything it stores the number of characters printed
|
|
so far by this call at that location. The @samp{h} and @samp{l} type
|
|
modifiers are permitted to specify that the argument is of type
|
|
@code{short int *} or @code{long int *} instead of @code{int *}, but no
|
|
flags, field width, or precision are permitted.
|
|
|
|
For example,
|
|
|
|
@smallexample
|
|
int nchar;
|
|
printf ("%d %s%n\n", 3, "bears", &nchar);
|
|
@end smallexample
|
|
|
|
@noindent
|
|
prints:
|
|
|
|
@smallexample
|
|
3 bears
|
|
@end smallexample
|
|
|
|
@noindent
|
|
and sets @code{nchar} to @code{7}, because @samp{3 bears} is seven
|
|
characters.
|
|
|
|
|
|
The @samp{%%} conversion prints a literal @samp{%} character. This
|
|
conversion doesn't use an argument, and no flags, field width,
|
|
precision, or type modifiers are permitted.
|
|
|
|
|
|
@node Formatted Output Functions
|
|
@subsection Formatted Output Functions
|
|
|
|
This section describes how to call @code{printf} and related functions.
|
|
Prototypes for these functions are in the header file @file{stdio.h}.
|
|
Because these functions take a variable number of arguments, you
|
|
@emph{must} declare prototypes for them before using them. Of course,
|
|
the easiest way to make sure you have all the right prototypes is to
|
|
just include @file{stdio.h}.
|
|
@pindex stdio.h
|
|
|
|
@comment stdio.h
|
|
@comment ISO
|
|
@deftypefun int printf (const char *@var{template}, @dots{})
|
|
The @code{printf} function prints the optional arguments under the
|
|
control of the template string @var{template} to the stream
|
|
@code{stdout}. It returns the number of characters printed, or a
|
|
negative value if there was an output error.
|
|
@end deftypefun
|
|
|
|
@comment stdio.h
|
|
@comment ISO
|
|
@deftypefun int fprintf (FILE *@var{stream}, const char *@var{template}, @dots{})
|
|
This function is just like @code{printf}, except that the output is
|
|
written to the stream @var{stream} instead of @code{stdout}.
|
|
@end deftypefun
|
|
|
|
@comment stdio.h
|
|
@comment ISO
|
|
@deftypefun int sprintf (char *@var{s}, const char *@var{template}, @dots{})
|
|
This is like @code{printf}, except that the output is stored in the character
|
|
array @var{s} instead of written to a stream. A null character is written
|
|
to mark the end of the string.
|
|
|
|
The @code{sprintf} function returns the number of characters stored in
|
|
the array @var{s}, not including the terminating null character.
|
|
|
|
The behavior of this function is undefined if copying takes place
|
|
between objects that overlap---for example, if @var{s} is also given
|
|
as an argument to be printed under control of the @samp{%s} conversion.
|
|
@xref{Copying and Concatenation}.
|
|
|
|
@strong{Warning:} The @code{sprintf} function can be @strong{dangerous}
|
|
because it can potentially output more characters than can fit in the
|
|
allocation size of the string @var{s}. Remember that the field width
|
|
given in a conversion specification is only a @emph{minimum} value.
|
|
|
|
To avoid this problem, you can use @code{snprintf} or @code{asprintf},
|
|
described below.
|
|
@end deftypefun
|
|
|
|
@comment stdio.h
|
|
@comment GNU
|
|
@deftypefun int snprintf (char *@var{s}, size_t @var{size}, const char *@var{template}, @dots{})
|
|
The @code{snprintf} function is similar to @code{sprintf}, except that
|
|
the @var{size} argument specifies the maximum number of characters to
|
|
produce. The trailing null character is counted towards this limit, so
|
|
you should allocate at least @var{size} characters for the string @var{s}.
|
|
|
|
The return value is the number of characters stored, not including the
|
|
terminating null. If this value equals @code{@var{size} - 1}, then
|
|
there was not enough space in @var{s} for all the output. You should
|
|
try again with a bigger output string. Here is an example of doing
|
|
this:
|
|
|
|
@smallexample
|
|
@group
|
|
/* @r{Construct a message describing the value of a variable}
|
|
@r{whose name is @var{name} and whose value is @var{value}.} */
|
|
char *
|
|
make_message (char *name, char *value)
|
|
@{
|
|
/* @r{Guess we need no more than 100 chars of space.} */
|
|
int size = 100;
|
|
char *buffer = (char *) xmalloc (size);
|
|
@end group
|
|
@group
|
|
while (1)
|
|
@{
|
|
/* @r{Try to print in the allocated space.} */
|
|
int nchars = snprintf (buffer, size,
|
|
"value of %s is %s",
|
|
name, value);
|
|
/* @r{If that worked, return the string.} */
|
|
if (nchars < size)
|
|
return buffer;
|
|
/* @r{Else try again with twice as much space.} */
|
|
size *= 2;
|
|
buffer = (char *) xrealloc (size, buffer);
|
|
@}
|
|
@}
|
|
@end group
|
|
@end smallexample
|
|
|
|
In practice, it is often easier just to use @code{asprintf}, below.
|
|
@end deftypefun
|
|
|
|
@node Dynamic Output
|
|
@subsection Dynamically Allocating Formatted Output
|
|
|
|
The functions in this section do formatted output and place the results
|
|
in dynamically allocated memory.
|
|
|
|
@comment stdio.h
|
|
@comment GNU
|
|
@deftypefun int asprintf (char **@var{ptr}, const char *@var{template}, @dots{})
|
|
This function is similar to @code{sprintf}, except that it dynamically
|
|
allocates a string (as with @code{malloc}; @pxref{Unconstrained
|
|
Allocation}) to hold the output, instead of putting the output in a
|
|
buffer you allocate in advance. The @var{ptr} argument should be the
|
|
address of a @code{char *} object, and @code{asprintf} stores a pointer
|
|
to the newly allocated string at that location.
|
|
|
|
Here is how to use @code{asprintf} to get the same result as the
|
|
@code{snprintf} example, but more easily:
|
|
|
|
@smallexample
|
|
/* @r{Construct a message describing the value of a variable}
|
|
@r{whose name is @var{name} and whose value is @var{value}.} */
|
|
char *
|
|
make_message (char *name, char *value)
|
|
@{
|
|
char *result;
|
|
asprintf (&result, "value of %s is %s", name, value);
|
|
return result;
|
|
@}
|
|
@end smallexample
|
|
@end deftypefun
|
|
|
|
@comment stdio.h
|
|
@comment GNU
|
|
@deftypefun int obstack_printf (struct obstack *@var{obstack}, const char *@var{template}, @dots{})
|
|
This function is similar to @code{asprintf}, except that it uses the
|
|
obstack @var{obstack} to allocate the space. @xref{Obstacks}.
|
|
|
|
The characters are written onto the end of the current object.
|
|
To get at them, you must finish the object with @code{obstack_finish}
|
|
(@pxref{Growing Objects}).@refill
|
|
@end deftypefun
|
|
|
|
@node Variable Arguments Output
|
|
@subsection Variable Arguments Output Functions
|
|
|
|
The functions @code{vprintf} and friends are provided so that you can
|
|
define your own variadic @code{printf}-like functions that make use of
|
|
the same internals as the built-in formatted output functions.
|
|
|
|
The most natural way to define such functions would be to use a language
|
|
construct to say, ``Call @code{printf} and pass this template plus all
|
|
of my arguments after the first five.'' But there is no way to do this
|
|
in C, and it would be hard to provide a way, since at the C language
|
|
level there is no way to tell how many arguments your function received.
|
|
|
|
Since that method is impossible, we provide alternative functions, the
|
|
@code{vprintf} series, which lets you pass a @code{va_list} to describe
|
|
``all of my arguments after the first five.''
|
|
|
|
When it is sufficient to define a macro rather than a real function,
|
|
the GNU C compiler provides a way to do this much more easily with macros.
|
|
For example:
|
|
|
|
@smallexample
|
|
#define myprintf(a, b, c, d, e, rest...) printf (mytemplate , ## rest...)
|
|
@end smallexample
|
|
|
|
@noindent
|
|
@xref{Macro Varargs, , Macros with Variable Numbers of Arguments,
|
|
gcc.info, Using GNU CC}, for details. But this is limited to macros,
|
|
and does not apply to real functions at all.
|
|
|
|
Before calling @code{vprintf} or the other functions listed in this
|
|
section, you @emph{must} call @code{va_start} (@pxref{Variadic
|
|
Functions}) to initialize a pointer to the variable arguments. Then you
|
|
can call @code{va_arg} to fetch the arguments that you want to handle
|
|
yourself. This advances the pointer past those arguments.
|
|
|
|
Once your @code{va_list} pointer is pointing at the argument of your
|
|
choice, you are ready to call @code{vprintf}. That argument and all
|
|
subsequent arguments that were passed to your function are used by
|
|
@code{vprintf} along with the template that you specified separately.
|
|
|
|
In some other systems, the @code{va_list} pointer may become invalid
|
|
after the call to @code{vprintf}, so you must not use @code{va_arg}
|
|
after you call @code{vprintf}. Instead, you should call @code{va_end}
|
|
to retire the pointer from service. However, you can safely call
|
|
@code{va_start} on another pointer variable and begin fetching the
|
|
arguments again through that pointer. Calling @code{vprintf} does not
|
|
destroy the argument list of your function, merely the particular
|
|
pointer that you passed to it.
|
|
|
|
GNU C does not have such restrictions. You can safely continue to fetch
|
|
arguments from a @code{va_list} pointer after passing it to
|
|
@code{vprintf}, and @code{va_end} is a no-op. (Note, however, that
|
|
subsequent @code{va_arg} calls will fetch the same arguments which
|
|
@code{vprintf} previously used.)
|
|
|
|
Prototypes for these functions are declared in @file{stdio.h}.
|
|
@pindex stdio.h
|
|
|
|
@comment stdio.h
|
|
@comment ISO
|
|
@deftypefun int vprintf (const char *@var{template}, va_list @var{ap})
|
|
This function is similar to @code{printf} except that, instead of taking
|
|
a variable number of arguments directly, it takes an argument list
|
|
pointer @var{ap}.
|
|
@end deftypefun
|
|
|
|
@comment stdio.h
|
|
@comment ISO
|
|
@deftypefun int vfprintf (FILE *@var{stream}, const char *@var{template}, va_list @var{ap})
|
|
This is the equivalent of @code{fprintf} with the variable argument list
|
|
specified directly as for @code{vprintf}.
|
|
@end deftypefun
|
|
|
|
@comment stdio.h
|
|
@comment ISO
|
|
@deftypefun int vsprintf (char *@var{s}, const char *@var{template}, va_list @var{ap})
|
|
This is the equivalent of @code{sprintf} with the variable argument list
|
|
specified directly as for @code{vprintf}.
|
|
@end deftypefun
|
|
|
|
@comment stdio.h
|
|
@comment GNU
|
|
@deftypefun int vsnprintf (char *@var{s}, size_t @var{size}, const char *@var{template}, va_list @var{ap})
|
|
This is the equivalent of @code{snprintf} with the variable argument list
|
|
specified directly as for @code{vprintf}.
|
|
@end deftypefun
|
|
|
|
@comment stdio.h
|
|
@comment GNU
|
|
@deftypefun int vasprintf (char **@var{ptr}, const char *@var{template}, va_list @var{ap})
|
|
The @code{vasprintf} function is the equivalent of @code{asprintf} with the
|
|
variable argument list specified directly as for @code{vprintf}.
|
|
@end deftypefun
|
|
|
|
@comment stdio.h
|
|
@comment GNU
|
|
@deftypefun int obstack_vprintf (struct obstack *@var{obstack}, const char *@var{template}, va_list @var{ap})
|
|
The @code{obstack_vprintf} function is the equivalent of
|
|
@code{obstack_printf} with the variable argument list specified directly
|
|
as for @code{vprintf}.@refill
|
|
@end deftypefun
|
|
|
|
Here's an example showing how you might use @code{vfprintf}. This is a
|
|
function that prints error messages to the stream @code{stderr}, along
|
|
with a prefix indicating the name of the program
|
|
(@pxref{Error Messages}, for a description of
|
|
@code{program_invocation_short_name}).
|
|
|
|
@smallexample
|
|
@group
|
|
#include <stdio.h>
|
|
#include <stdarg.h>
|
|
|
|
void
|
|
eprintf (const char *template, ...)
|
|
@{
|
|
va_list ap;
|
|
extern char *program_invocation_short_name;
|
|
|
|
fprintf (stderr, "%s: ", program_invocation_short_name);
|
|
va_start (ap, count);
|
|
vfprintf (stderr, template, ap);
|
|
va_end (ap);
|
|
@}
|
|
@end group
|
|
@end smallexample
|
|
|
|
@noindent
|
|
You could call @code{eprintf} like this:
|
|
|
|
@smallexample
|
|
eprintf ("file `%s' does not exist\n", filename);
|
|
@end smallexample
|
|
|
|
In GNU C, there is a special construct you can use to let the compiler
|
|
know that a function uses a @code{printf}-style format string. Then it
|
|
can check the number and types of arguments in each call to the
|
|
function, and warn you when they do not match the format string.
|
|
For example, take this declaration of @code{eprintf}:
|
|
|
|
@smallexample
|
|
void eprintf (const char *template, ...)
|
|
__attribute__ ((format (printf, 1, 2)));
|
|
@end smallexample
|
|
|
|
@noindent
|
|
This tells the compiler that @code{eprintf} uses a format string like
|
|
@code{printf} (as opposed to @code{scanf}; @pxref{Formatted Input});
|
|
the format string appears as the first argument;
|
|
and the arguments to satisfy the format begin with the second.
|
|
@xref{Function Attributes, , Declaring Attributes of Functions,
|
|
gcc.info, Using GNU CC}, for more information.
|
|
|
|
@node Parsing a Template String
|
|
@subsection Parsing a Template String
|
|
@cindex parsing a template string
|
|
|
|
You can use the function @code{parse_printf_format} to obtain
|
|
information about the number and types of arguments that are expected by
|
|
a given template string. This function permits interpreters that
|
|
provide interfaces to @code{printf} to avoid passing along invalid
|
|
arguments from the user's program, which could cause a crash.
|
|
|
|
All the symbols described in this section are declared in the header
|
|
file @file{printf.h}.
|
|
|
|
@comment printf.h
|
|
@comment GNU
|
|
@deftypefun size_t parse_printf_format (const char *@var{template}, size_t @var{n}, int *@var{argtypes})
|
|
This function returns information about the number and types of
|
|
arguments expected by the @code{printf} template string @var{template}.
|
|
The information is stored in the array @var{argtypes}; each element of
|
|
this array describes one argument. This information is encoded using
|
|
the various @samp{PA_} macros, listed below.
|
|
|
|
The @var{n} argument specifies the number of elements in the array
|
|
@var{argtypes}. This is the most elements that
|
|
@code{parse_printf_format} will try to write.
|
|
|
|
@code{parse_printf_format} returns the total number of arguments required
|
|
by @var{template}. If this number is greater than @var{n}, then the
|
|
information returned describes only the first @var{n} arguments. If you
|
|
want information about more than that many arguments, allocate a bigger
|
|
array and call @code{parse_printf_format} again.
|
|
@end deftypefun
|
|
|
|
The argument types are encoded as a combination of a basic type and
|
|
modifier flag bits.
|
|
|
|
@comment printf.h
|
|
@comment GNU
|
|
@deftypevr Macro int PA_FLAG_MASK
|
|
This macro is a bitmask for the type modifier flag bits. You can write
|
|
the expression @code{(argtypes[i] & PA_FLAG_MASK)} to extract just the
|
|
flag bits for an argument, or @code{(argtypes[i] & ~PA_FLAG_MASK)} to
|
|
extract just the basic type code.
|
|
@end deftypevr
|
|
|
|
Here are symbolic constants that represent the basic types; they stand
|
|
for integer values.
|
|
|
|
@table @code
|
|
@comment printf.h
|
|
@comment GNU
|
|
@item PA_INT
|
|
@vindex PA_INT
|
|
This specifies that the base type is @code{int}.
|
|
|
|
@comment printf.h
|
|
@comment GNU
|
|
@item PA_CHAR
|
|
@vindex PA_CHAR
|
|
This specifies that the base type is @code{int}, cast to @code{char}.
|
|
|
|
@comment printf.h
|
|
@comment GNU
|
|
@item PA_STRING
|
|
@vindex PA_STRING
|
|
This specifies that the base type is @code{char *}, a null-terminated string.
|
|
|
|
@comment printf.h
|
|
@comment GNU
|
|
@item PA_POINTER
|
|
@vindex PA_POINTER
|
|
This specifies that the base type is @code{void *}, an arbitrary pointer.
|
|
|
|
@comment printf.h
|
|
@comment GNU
|
|
@item PA_FLOAT
|
|
@vindex PA_FLOAT
|
|
This specifies that the base type is @code{float}.
|
|
|
|
@comment printf.h
|
|
@comment GNU
|
|
@item PA_DOUBLE
|
|
@vindex PA_DOUBLE
|
|
This specifies that the base type is @code{double}.
|
|
|
|
@comment printf.h
|
|
@comment GNU
|
|
@item PA_LAST
|
|
@vindex PA_LAST
|
|
You can define additional base types for your own programs as offsets
|
|
from @code{PA_LAST}. For example, if you have data types @samp{foo}
|
|
and @samp{bar} with their own specialized @code{printf} conversions,
|
|
you could define encodings for these types as:
|
|
|
|
@smallexample
|
|
#define PA_FOO PA_LAST
|
|
#define PA_BAR (PA_LAST + 1)
|
|
@end smallexample
|
|
@end table
|
|
|
|
Here are the flag bits that modify a basic type. They are combined with
|
|
the code for the basic type using inclusive-or.
|
|
|
|
@table @code
|
|
@comment printf.h
|
|
@comment GNU
|
|
@item PA_FLAG_PTR
|
|
@vindex PA_FLAG_PTR
|
|
If this bit is set, it indicates that the encoded type is a pointer to
|
|
the base type, rather than an immediate value.
|
|
For example, @samp{PA_INT|PA_FLAG_PTR} represents the type @samp{int *}.
|
|
|
|
@comment printf.h
|
|
@comment GNU
|
|
@item PA_FLAG_SHORT
|
|
@vindex PA_FLAG_SHORT
|
|
If this bit is set, it indicates that the base type is modified with
|
|
@code{short}. (This corresponds to the @samp{h} type modifier.)
|
|
|
|
@comment printf.h
|
|
@comment GNU
|
|
@item PA_FLAG_LONG
|
|
@vindex PA_FLAG_LONG
|
|
If this bit is set, it indicates that the base type is modified with
|
|
@code{long}. (This corresponds to the @samp{l} type modifier.)
|
|
|
|
@comment printf.h
|
|
@comment GNU
|
|
@item PA_FLAG_LONG_LONG
|
|
@vindex PA_FLAG_LONG_LONG
|
|
If this bit is set, it indicates that the base type is modified with
|
|
@code{long long}. (This corresponds to the @samp{L} type modifier.)
|
|
|
|
@comment printf.h
|
|
@comment GNU
|
|
@item PA_FLAG_LONG_DOUBLE
|
|
@vindex PA_FLAG_LONG_DOUBLE
|
|
This is a synonym for @code{PA_FLAG_LONG_LONG}, used by convention with
|
|
a base type of @code{PA_DOUBLE} to indicate a type of @code{long double}.
|
|
@end table
|
|
|
|
@ifinfo
|
|
For an example of using these facilities, see @ref{Example of Parsing}.
|
|
@end ifinfo
|
|
|
|
@node Example of Parsing
|
|
@subsection Example of Parsing a Template String
|
|
|
|
Here is an example of decoding argument types for a format string. We
|
|
assume this is part of an interpreter which contains arguments of type
|
|
@code{NUMBER}, @code{CHAR}, @code{STRING} and @code{STRUCTURE} (and
|
|
perhaps others which are not valid here).
|
|
|
|
@smallexample
|
|
/* @r{Test whether the @var{nargs} specified objects}
|
|
@r{in the vector @var{args} are valid}
|
|
@r{for the format string @var{format}:}
|
|
@r{if so, return 1.}
|
|
@r{If not, return 0 after printing an error message.} */
|
|
|
|
int
|
|
validate_args (char *format, int nargs, OBJECT *args)
|
|
@{
|
|
int *argtypes;
|
|
int nwanted;
|
|
|
|
/* @r{Get the information about the arguments.}
|
|
@r{Each conversion specification must be at least two characters}
|
|
@r{long, so there cannot be more specifications than half the}
|
|
@r{length of the string.} */
|
|
|
|
argtypes = (int *) alloca (strlen (format) / 2 * sizeof (int));
|
|
nwanted = parse_printf_format (string, nelts, argtypes);
|
|
|
|
/* @r{Check the number of arguments.} */
|
|
if (nwanted > nargs)
|
|
@{
|
|
error ("too few arguments (at least %d required)", nwanted);
|
|
return 0;
|
|
@}
|
|
|
|
/* @r{Check the C type wanted for each argument}
|
|
@r{and see if the object given is suitable.} */
|
|
for (i = 0; i < nwanted; i++)
|
|
@{
|
|
int wanted;
|
|
|
|
if (argtypes[i] & PA_FLAG_PTR)
|
|
wanted = STRUCTURE;
|
|
else
|
|
switch (argtypes[i] & ~PA_FLAG_MASK)
|
|
@{
|
|
case PA_INT:
|
|
case PA_FLOAT:
|
|
case PA_DOUBLE:
|
|
wanted = NUMBER;
|
|
break;
|
|
case PA_CHAR:
|
|
wanted = CHAR;
|
|
break;
|
|
case PA_STRING:
|
|
wanted = STRING;
|
|
break;
|
|
case PA_POINTER:
|
|
wanted = STRUCTURE;
|
|
break;
|
|
@}
|
|
if (TYPE (args[i]) != wanted)
|
|
@{
|
|
error ("type mismatch for arg number %d", i);
|
|
return 0;
|
|
@}
|
|
@}
|
|
return 1;
|
|
@}
|
|
@end smallexample
|
|
|
|
@node Customizing Printf
|
|
@section Customizing @code{printf}
|
|
@cindex customizing @code{printf}
|
|
@cindex defining new @code{printf} conversions
|
|
@cindex extending @code{printf}
|
|
|
|
The GNU C library lets you define your own custom conversion specifiers
|
|
for @code{printf} template strings, to teach @code{printf} clever ways
|
|
to print the important data structures of your program.
|
|
|
|
The way you do this is by registering the conversion with the function
|
|
@code{register_printf_function}; see @ref{Registering New Conversions}.
|
|
One of the arguments you pass to this function is a pointer to a handler
|
|
function that produces the actual output; see @ref{Defining the Output
|
|
Handler}, for information on how to write this function.
|
|
|
|
You can also install a function that just returns information about the
|
|
number and type of arguments expected by the conversion specifier.
|
|
@xref{Parsing a Template String}, for information about this.
|
|
|
|
The facilities of this section are declared in the header file
|
|
@file{printf.h}.
|
|
|
|
@menu
|
|
* Registering New Conversions:: Using @code{register_printf_function}
|
|
to register a new output conversion.
|
|
* Conversion Specifier Options:: The handler must be able to get
|
|
the options specified in the
|
|
template when it is called.
|
|
* Defining the Output Handler:: Defining the handler and arginfo
|
|
functions that are passed as arguments
|
|
to @code{register_printf_function}.
|
|
* Printf Extension Example:: How to define a @code{printf}
|
|
handler function.
|
|
@end menu
|
|
|
|
@strong{Portability Note:} The ability to extend the syntax of
|
|
@code{printf} template strings is a GNU extension. ISO standard C has
|
|
nothing similar.
|
|
|
|
@node Registering New Conversions
|
|
@subsection Registering New Conversions
|
|
|
|
The function to register a new output conversion is
|
|
@code{register_printf_function}, declared in @file{printf.h}.
|
|
@pindex printf.h
|
|
|
|
@comment printf.h
|
|
@comment GNU
|
|
@deftypefun int register_printf_function (int @var{spec}, printf_function @var{handler-function}, printf_arginfo_function @var{arginfo-function})
|
|
This function defines the conversion specifier character @var{spec}.
|
|
Thus, if @var{spec} is @code{'z'}, it defines the conversion @samp{%z}.
|
|
You can redefine the built-in conversions like @samp{%s}, but flag
|
|
characters like @samp{#} and type modifiers like @samp{l} can never be
|
|
used as conversions; calling @code{register_printf_function} for those
|
|
characters has no effect.
|
|
|
|
The @var{handler-function} is the function called by @code{printf} and
|
|
friends when this conversion appears in a template string.
|
|
@xref{Defining the Output Handler}, for information about how to define
|
|
a function to pass as this argument. If you specify a null pointer, any
|
|
existing handler function for @var{spec} is removed.
|
|
|
|
The @var{arginfo-function} is the function called by
|
|
@code{parse_printf_format} when this conversion appears in a
|
|
template string. @xref{Parsing a Template String}, for information
|
|
about this.
|
|
|
|
@c The following is not true anymore. The `parse_printf_format' function
|
|
@c is now also called from `vfprintf' via `parse_one_spec'.
|
|
@c --drepper@gnu, 1996/11/14
|
|
@c
|
|
@c Normally, you install both functions for a conversion at the same time,
|
|
@c but if you are never going to call @code{parse_printf_format}, you do
|
|
@c not need to define an arginfo function.
|
|
|
|
@strong{Attention:} In the GNU C library version before 2.0 the
|
|
@var{arginfo-function} function did not need to be installed unless
|
|
the user uses the @code{parse_printf_format} function. This changed.
|
|
Now a call to any of the @code{printf} functions will call this
|
|
function when this format specifier appears in the format string.
|
|
|
|
The return value is @code{0} on success, and @code{-1} on failure
|
|
(which occurs if @var{spec} is out of range).
|
|
|
|
You can redefine the standard output conversions, but this is probably
|
|
not a good idea because of the potential for confusion. Library routines
|
|
written by other people could break if you do this.
|
|
@end deftypefun
|
|
|
|
@node Conversion Specifier Options
|
|
@subsection Conversion Specifier Options
|
|
|
|
If you define a meaning for @samp{%A}, what if the template contains
|
|
@samp{%+23A} or @samp{%-#A}? To implement a sensible meaning for these,
|
|
the handler when called needs to be able to get the options specified in
|
|
the template.
|
|
|
|
Both the @var{handler-function} and @var{arginfo-function} arguments
|
|
to @code{register_printf_function} accept an argument that points to a
|
|
@code{struct printf_info}, which contains information about the options
|
|
appearing in an instance of the conversion specifier. This data type
|
|
is declared in the header file @file{printf.h}.
|
|
@pindex printf.h
|
|
|
|
@comment printf.h
|
|
@comment GNU
|
|
@deftp {Type} {struct printf_info}
|
|
This structure is used to pass information about the options appearing
|
|
in an instance of a conversion specifier in a @code{printf} template
|
|
string to the handler and arginfo functions for that specifier. It
|
|
contains the following members:
|
|
|
|
@table @code
|
|
@item int prec
|
|
This is the precision specified. The value is @code{-1} if no precision
|
|
was specified. If the precision was given as @samp{*}, the
|
|
@code{printf_info} structure passed to the handler function contains the
|
|
actual value retrieved from the argument list. But the structure passed
|
|
to the arginfo function contains a value of @code{INT_MIN}, since the
|
|
actual value is not known.
|
|
|
|
@item int width
|
|
This is the minimum field width specified. The value is @code{0} if no
|
|
width was specified. If the field width was given as @samp{*}, the
|
|
@code{printf_info} structure passed to the handler function contains the
|
|
actual value retrieved from the argument list. But the structure passed
|
|
to the arginfo function contains a value of @code{INT_MIN}, since the
|
|
actual value is not known.
|
|
|
|
@item wchar_t spec
|
|
This is the conversion specifier character specified. It's stored in
|
|
the structure so that you can register the same handler function for
|
|
multiple characters, but still have a way to tell them apart when the
|
|
handler function is called.
|
|
|
|
@item unsigned int is_long_double
|
|
This is a boolean that is true if the @samp{L}, @samp{ll}, or @samp{q}
|
|
type modifier was specified. For integer conversions, this indicates
|
|
@code{long long int}, as opposed to @code{long double} for floating
|
|
point conversions.
|
|
|
|
@item unsigned int is_short
|
|
This is a boolean that is true if the @samp{h} type modifier was specified.
|
|
|
|
@item unsigned int is_long
|
|
This is a boolean that is true if the @samp{l} type modifier was specified.
|
|
|
|
@item unsigned int alt
|
|
This is a boolean that is true if the @samp{#} flag was specified.
|
|
|
|
@item unsigned int space
|
|
This is a boolean that is true if the @samp{ } flag was specified.
|
|
|
|
@item unsigned int left
|
|
This is a boolean that is true if the @samp{-} flag was specified.
|
|
|
|
@item unsigned int showsign
|
|
This is a boolean that is true if the @samp{+} flag was specified.
|
|
|
|
@item unsigned int group
|
|
This is a boolean that is true if the @samp{'} flag was specified.
|
|
|
|
@item unsigned int extra
|
|
This flag has a special meaning depending on the context. It could
|
|
be used freely by the user-defined handlers but when called from
|
|
the @code{printf} function this variable always contains the value
|
|
@code{0}.
|
|
|
|
@item wchar_t pad
|
|
This is the character to use for padding the output to the minimum field
|
|
width. The value is @code{'0'} if the @samp{0} flag was specified, and
|
|
@code{' '} otherwise.
|
|
@end table
|
|
@end deftp
|
|
|
|
|
|
@node Defining the Output Handler
|
|
@subsection Defining the Output Handler
|
|
|
|
Now let's look at how to define the handler and arginfo functions
|
|
which are passed as arguments to @code{register_printf_function}.
|
|
|
|
@strong{Compatibility Note:} The interface change in the GNU libc
|
|
version 2.0. Previously the third argument was of type
|
|
@code{va_list *}.
|
|
|
|
You should define your handler functions with a prototype like:
|
|
|
|
@smallexample
|
|
int @var{function} (FILE *stream, const struct printf_info *info,
|
|
const void *const *args)
|
|
@end smallexample
|
|
|
|
The @var{stream} argument passed to the handler function is the stream to
|
|
which it should write output.
|
|
|
|
The @var{info} argument is a pointer to a structure that contains
|
|
information about the various options that were included with the
|
|
conversion in the template string. You should not modify this structure
|
|
inside your handler function. @xref{Conversion Specifier Options}, for
|
|
a description of this data structure.
|
|
|
|
@c The following changes some time back. --drepper@gnu, 1996/11/14
|
|
@c
|
|
@c The @code{ap_pointer} argument is used to pass the tail of the variable
|
|
@c argument list containing the values to be printed to your handler.
|
|
@c Unlike most other functions that can be passed an explicit variable
|
|
@c argument list, this is a @emph{pointer} to a @code{va_list}, rather than
|
|
@c the @code{va_list} itself. Thus, you should fetch arguments by
|
|
@c means of @code{va_arg (*ap_pointer, @var{type})}.
|
|
@c
|
|
@c (Passing a pointer here allows the function that calls your handler
|
|
@c function to update its own @code{va_list} variable to account for the
|
|
@c arguments that your handler processes. @xref{Variadic Functions}.)
|
|
|
|
The @var{args} is a vector of pointers to the arguments data.
|
|
The number of arguments were determined by calling the argument
|
|
information function provided by the user.
|
|
|
|
Your handler function should return a value just like @code{printf}
|
|
does: it should return the number of characters it has written, or a
|
|
negative value to indicate an error.
|
|
|
|
@comment printf.h
|
|
@comment GNU
|
|
@deftp {Data Type} printf_function
|
|
This is the data type that a handler function should have.
|
|
@end deftp
|
|
|
|
If you are going to use @w{@code{parse_printf_format}} in your
|
|
application, you must also define a function to pass as the
|
|
@var{arginfo-function} argument for each new conversion you install with
|
|
@code{register_printf_function}.
|
|
|
|
You have to define these functions with a prototype like:
|
|
|
|
@smallexample
|
|
int @var{function} (const struct printf_info *info,
|
|
size_t n, int *argtypes)
|
|
@end smallexample
|
|
|
|
The return value from the function should be the number of arguments the
|
|
conversion expects. The function should also fill in no more than
|
|
@var{n} elements of the @var{argtypes} array with information about the
|
|
types of each of these arguments. This information is encoded using the
|
|
various @samp{PA_} macros. (You will notice that this is the same
|
|
calling convention @code{parse_printf_format} itself uses.)
|
|
|
|
@comment printf.h
|
|
@comment GNU
|
|
@deftp {Data Type} printf_arginfo_function
|
|
This type is used to describe functions that return information about
|
|
the number and type of arguments used by a conversion specifier.
|
|
@end deftp
|
|
|
|
@node Printf Extension Example
|
|
@subsection @code{printf} Extension Example
|
|
|
|
Here is an example showing how to define a @code{printf} handler function.
|
|
This program defines a data structure called a @code{Widget} and
|
|
defines the @samp{%W} conversion to print information about @w{@code{Widget *}}
|
|
arguments, including the pointer value and the name stored in the data
|
|
structure. The @samp{%W} conversion supports the minimum field width and
|
|
left-justification options, but ignores everything else.
|
|
|
|
@smallexample
|
|
@include rprintf.c.texi
|
|
@end smallexample
|
|
|
|
The output produced by this program looks like:
|
|
|
|
@smallexample
|
|
|<Widget 0xffeffb7c: mywidget>|
|
|
| <Widget 0xffeffb7c: mywidget>|
|
|
|<Widget 0xffeffb7c: mywidget> |
|
|
@end smallexample
|
|
|
|
@node Formatted Input
|
|
@section Formatted Input
|
|
|
|
@cindex formatted input from a stream
|
|
@cindex reading from a stream, formatted
|
|
@cindex format string, for @code{scanf}
|
|
@cindex template, for @code{scanf}
|
|
The functions described in this section (@code{scanf} and related
|
|
functions) provide facilities for formatted input analogous to the
|
|
formatted output facilities. These functions provide a mechanism for
|
|
reading arbitrary values under the control of a @dfn{format string} or
|
|
@dfn{template string}.
|
|
|
|
@menu
|
|
* Formatted Input Basics:: Some basics to get you started.
|
|
* Input Conversion Syntax:: Syntax of conversion specifications.
|
|
* Table of Input Conversions:: Summary of input conversions and what they do.
|
|
* Numeric Input Conversions:: Details of conversions for reading numbers.
|
|
* String Input Conversions:: Details of conversions for reading strings.
|
|
* Dynamic String Input:: String conversions that @code{malloc} the buffer.
|
|
* Other Input Conversions:: Details of miscellaneous other conversions.
|
|
* Formatted Input Functions:: Descriptions of the actual functions.
|
|
* Variable Arguments Input:: @code{vscanf} and friends.
|
|
@end menu
|
|
|
|
@node Formatted Input Basics
|
|
@subsection Formatted Input Basics
|
|
|
|
Calls to @code{scanf} are superficially similar to calls to
|
|
@code{printf} in that arbitrary arguments are read under the control of
|
|
a template string. While the syntax of the conversion specifications in
|
|
the template is very similar to that for @code{printf}, the
|
|
interpretation of the template is oriented more towards free-format
|
|
input and simple pattern matching, rather than fixed-field formatting.
|
|
For example, most @code{scanf} conversions skip over any amount of
|
|
``white space'' (including spaces, tabs, and newlines) in the input
|
|
file, and there is no concept of precision for the numeric input
|
|
conversions as there is for the corresponding output conversions.
|
|
Ordinarily, non-whitespace characters in the template are expected to
|
|
match characters in the input stream exactly, but a matching failure is
|
|
distinct from an input error on the stream.
|
|
@cindex conversion specifications (@code{scanf})
|
|
|
|
Another area of difference between @code{scanf} and @code{printf} is
|
|
that you must remember to supply pointers rather than immediate values
|
|
as the optional arguments to @code{scanf}; the values that are read are
|
|
stored in the objects that the pointers point to. Even experienced
|
|
programmers tend to forget this occasionally, so if your program is
|
|
getting strange errors that seem to be related to @code{scanf}, you
|
|
might want to double-check this.
|
|
|
|
When a @dfn{matching failure} occurs, @code{scanf} returns immediately,
|
|
leaving the first non-matching character as the next character to be
|
|
read from the stream. The normal return value from @code{scanf} is the
|
|
number of values that were assigned, so you can use this to determine if
|
|
a matching error happened before all the expected values were read.
|
|
@cindex matching failure, in @code{scanf}
|
|
|
|
The @code{scanf} function is typically used for things like reading in
|
|
the contents of tables. For example, here is a function that uses
|
|
@code{scanf} to initialize an array of @code{double}:
|
|
|
|
@smallexample
|
|
void
|
|
readarray (double *array, int n)
|
|
@{
|
|
int i;
|
|
for (i=0; i<n; i++)
|
|
if (scanf (" %lf", &(array[i])) != 1)
|
|
invalid_input_error ();
|
|
@}
|
|
@end smallexample
|
|
|
|
The formatted input functions are not used as frequently as the
|
|
formatted output functions. Partly, this is because it takes some care
|
|
to use them properly. Another reason is that it is difficult to recover
|
|
from a matching error.
|
|
|
|
If you are trying to read input that doesn't match a single, fixed
|
|
pattern, you may be better off using a tool such as Flex to generate a
|
|
lexical scanner, or Bison to generate a parser, rather than using
|
|
@code{scanf}. For more information about these tools, see @ref{, , ,
|
|
flex.info, Flex: The Lexical Scanner Generator}, and @ref{, , ,
|
|
bison.info, The Bison Reference Manual}.
|
|
|
|
@node Input Conversion Syntax
|
|
@subsection Input Conversion Syntax
|
|
|
|
A @code{scanf} template string is a string that contains ordinary
|
|
multibyte characters interspersed with conversion specifications that
|
|
start with @samp{%}.
|
|
|
|
Any whitespace character (as defined by the @code{isspace} function;
|
|
@pxref{Classification of Characters}) in the template causes any number
|
|
of whitespace characters in the input stream to be read and discarded.
|
|
The whitespace characters that are matched need not be exactly the same
|
|
whitespace characters that appear in the template string. For example,
|
|
write @samp{ , } in the template to recognize a comma with optional
|
|
whitespace before and after.
|
|
|
|
Other characters in the template string that are not part of conversion
|
|
specifications must match characters in the input stream exactly; if
|
|
this is not the case, a matching failure occurs.
|
|
|
|
The conversion specifications in a @code{scanf} template string
|
|
have the general form:
|
|
|
|
@smallexample
|
|
% @var{flags} @var{width} @var{type} @var{conversion}
|
|
@end smallexample
|
|
|
|
In more detail, an input conversion specification consists of an initial
|
|
@samp{%} character followed in sequence by:
|
|
|
|
@itemize @bullet
|
|
@item
|
|
An optional @dfn{flag character} @samp{*}, which says to ignore the text
|
|
read for this specification. When @code{scanf} finds a conversion
|
|
specification that uses this flag, it reads input as directed by the
|
|
rest of the conversion specification, but it discards this input, does
|
|
not use a pointer argument, and does not increment the count of
|
|
successful assignments.
|
|
@cindex flag character (@code{scanf})
|
|
|
|
@item
|
|
An optional flag character @samp{a} (valid with string conversions only)
|
|
which requests allocation of a buffer long enough to store the string in.
|
|
(This is a GNU extension.)
|
|
@xref{Dynamic String Input}.
|
|
|
|
@item
|
|
An optional decimal integer that specifies the @dfn{maximum field
|
|
width}. Reading of characters from the input stream stops either when
|
|
this maximum is reached or when a non-matching character is found,
|
|
whichever happens first. Most conversions discard initial whitespace
|
|
characters (those that don't are explicitly documented), and these
|
|
discarded characters don't count towards the maximum field width.
|
|
String input conversions store a null character to mark the end of the
|
|
input; the maximum field width does not include this terminator.
|
|
@cindex maximum field width (@code{scanf})
|
|
|
|
@item
|
|
An optional @dfn{type modifier character}. For example, you can
|
|
specify a type modifier of @samp{l} with integer conversions such as
|
|
@samp{%d} to specify that the argument is a pointer to a @code{long int}
|
|
rather than a pointer to an @code{int}.
|
|
@cindex type modifier character (@code{scanf})
|
|
|
|
@item
|
|
A character that specifies the conversion to be applied.
|
|
@end itemize
|
|
|
|
The exact options that are permitted and how they are interpreted vary
|
|
between the different conversion specifiers. See the descriptions of the
|
|
individual conversions for information about the particular options that
|
|
they allow.
|
|
|
|
With the @samp{-Wformat} option, the GNU C compiler checks calls to
|
|
@code{scanf} and related functions. It examines the format string and
|
|
verifies that the correct number and types of arguments are supplied.
|
|
There is also a GNU C syntax to tell the compiler that a function you
|
|
write uses a @code{scanf}-style format string.
|
|
@xref{Function Attributes, , Declaring Attributes of Functions,
|
|
gcc.info, Using GNU CC}, for more information.
|
|
|
|
@node Table of Input Conversions
|
|
@subsection Table of Input Conversions
|
|
@cindex input conversions, for @code{scanf}
|
|
|
|
Here is a table that summarizes the various conversion specifications:
|
|
|
|
@table @asis
|
|
@item @samp{%d}
|
|
Matches an optionally signed integer written in decimal. @xref{Numeric
|
|
Input Conversions}.
|
|
|
|
@item @samp{%i}
|
|
Matches an optionally signed integer in any of the formats that the C
|
|
language defines for specifying an integer constant. @xref{Numeric
|
|
Input Conversions}.
|
|
|
|
@item @samp{%o}
|
|
Matches an unsigned integer written in octal radix.
|
|
@xref{Numeric Input Conversions}.
|
|
|
|
@item @samp{%u}
|
|
Matches an unsigned integer written in decimal radix.
|
|
@xref{Numeric Input Conversions}.
|
|
|
|
@item @samp{%x}, @samp{%X}
|
|
Matches an unsigned integer written in hexadecimal radix.
|
|
@xref{Numeric Input Conversions}.
|
|
|
|
@item @samp{%e}, @samp{%f}, @samp{%g}, @samp{%E}, @samp{%G}
|
|
Matches an optionally signed floating-point number. @xref{Numeric Input
|
|
Conversions}.
|
|
|
|
@item @samp{%s}
|
|
Matches a string containing only non-whitespace characters.
|
|
@xref{String Input Conversions}.
|
|
|
|
@item @samp{%[}
|
|
Matches a string of characters that belong to a specified set.
|
|
@xref{String Input Conversions}.
|
|
|
|
@item @samp{%c}
|
|
Matches a string of one or more characters; the number of characters
|
|
read is controlled by the maximum field width given for the conversion.
|
|
@xref{String Input Conversions}.
|
|
|
|
@item @samp{%p}
|
|
Matches a pointer value in the same implementation-defined format used
|
|
by the @samp{%p} output conversion for @code{printf}. @xref{Other Input
|
|
Conversions}.
|
|
|
|
@item @samp{%n}
|
|
This conversion doesn't read any characters; it records the number of
|
|
characters read so far by this call. @xref{Other Input Conversions}.
|
|
|
|
@item @samp{%%}
|
|
This matches a literal @samp{%} character in the input stream. No
|
|
corresponding argument is used. @xref{Other Input Conversions}.
|
|
@end table
|
|
|
|
If the syntax of a conversion specification is invalid, the behavior is
|
|
undefined. If there aren't enough function arguments provided to supply
|
|
addresses for all the conversion specifications in the template strings
|
|
that perform assignments, or if the arguments are not of the correct
|
|
types, the behavior is also undefined. On the other hand, extra
|
|
arguments are simply ignored.
|
|
|
|
@node Numeric Input Conversions
|
|
@subsection Numeric Input Conversions
|
|
|
|
This section describes the @code{scanf} conversions for reading numeric
|
|
values.
|
|
|
|
The @samp{%d} conversion matches an optionally signed integer in decimal
|
|
radix. The syntax that is recognized is the same as that for the
|
|
@code{strtol} function (@pxref{Parsing of Integers}) with the value
|
|
@code{10} for the @var{base} argument.
|
|
|
|
The @samp{%i} conversion matches an optionally signed integer in any of
|
|
the formats that the C language defines for specifying an integer
|
|
constant. The syntax that is recognized is the same as that for the
|
|
@code{strtol} function (@pxref{Parsing of Integers}) with the value
|
|
@code{0} for the @var{base} argument. (You can print integers in this
|
|
syntax with @code{printf} by using the @samp{#} flag character with the
|
|
@samp{%x}, @samp{%o}, or @samp{%d} conversion. @xref{Integer Conversions}.)
|
|
|
|
For example, any of the strings @samp{10}, @samp{0xa}, or @samp{012}
|
|
could be read in as integers under the @samp{%i} conversion. Each of
|
|
these specifies a number with decimal value @code{10}.
|
|
|
|
The @samp{%o}, @samp{%u}, and @samp{%x} conversions match unsigned
|
|
integers in octal, decimal, and hexadecimal radices, respectively. The
|
|
syntax that is recognized is the same as that for the @code{strtoul}
|
|
function (@pxref{Parsing of Integers}) with the appropriate value
|
|
(@code{8}, @code{10}, or @code{16}) for the @var{base} argument.
|
|
|
|
The @samp{%X} conversion is identical to the @samp{%x} conversion. They
|
|
both permit either uppercase or lowercase letters to be used as digits.
|
|
|
|
The default type of the corresponding argument for the @code{%d} and
|
|
@code{%i} conversions is @code{int *}, and @code{unsigned int *} for the
|
|
other integer conversions. You can use the following type modifiers to
|
|
specify other sizes of integer:
|
|
|
|
@table @samp
|
|
@item h
|
|
Specifies that the argument is a @code{short int *} or @code{unsigned
|
|
short int *}.
|
|
|
|
@item l
|
|
Specifies that the argument is a @code{long int *} or @code{unsigned
|
|
long int *}. Two @samp{l} characters is like the @samp{L} modifier, below.
|
|
|
|
@need 100
|
|
@item ll
|
|
@itemx L
|
|
@itemx q
|
|
Specifies that the argument is a @code{long long int *} or @code{unsigned long long int *}. (The @code{long long} type is an extension supported by the
|
|
GNU C compiler. For systems that don't provide extra-long integers, this
|
|
is the same as @code{long int}.)
|
|
|
|
The @samp{q} modifier is another name for the same thing, which comes
|
|
from 4.4 BSD; a @w{@code{long long int}} is sometimes called a ``quad''
|
|
@code{int}.
|
|
@end table
|
|
|
|
All of the @samp{%e}, @samp{%f}, @samp{%g}, @samp{%E}, and @samp{%G}
|
|
input conversions are interchangeable. They all match an optionally
|
|
signed floating point number, in the same syntax as for the
|
|
@code{strtod} function (@pxref{Parsing of Floats}).
|
|
|
|
For the floating-point input conversions, the default argument type is
|
|
@code{float *}. (This is different from the corresponding output
|
|
conversions, where the default type is @code{double}; remember that
|
|
@code{float} arguments to @code{printf} are converted to @code{double}
|
|
by the default argument promotions, but @code{float *} arguments are
|
|
not promoted to @code{double *}.) You can specify other sizes of float
|
|
using these type modifiers:
|
|
|
|
@table @samp
|
|
@item l
|
|
Specifies that the argument is of type @code{double *}.
|
|
|
|
@item L
|
|
Specifies that the argument is of type @code{long double *}.
|
|
@end table
|
|
|
|
For all the above number parsing formats there is an additional optional
|
|
flag @samp{'}. When this flag is given the @code{scanf} function
|
|
expects the number represented in the input string to be formatted
|
|
according to the grouping rules of the currently selected locale
|
|
(@pxref{General Numeric}).
|
|
|
|
If the @code{"C"} or @code{"POSIX"} locale is selected there is no
|
|
difference. But for a locale which specifies values for the appropriate
|
|
fields in the locale the input must have the correct form in the input.
|
|
Otherwise the longest prefix with a correct form is processed.
|
|
|
|
@node String Input Conversions
|
|
@subsection String Input Conversions
|
|
|
|
This section describes the @code{scanf} input conversions for reading
|
|
string and character values: @samp{%s}, @samp{%[}, and @samp{%c}.
|
|
|
|
You have two options for how to receive the input from these
|
|
conversions:
|
|
|
|
@itemize @bullet
|
|
@item
|
|
Provide a buffer to store it in. This is the default. You
|
|
should provide an argument of type @code{char *}.
|
|
|
|
@strong{Warning:} To make a robust program, you must make sure that the
|
|
input (plus its terminating null) cannot possibly exceed the size of the
|
|
buffer you provide. In general, the only way to do this is to specify a
|
|
maximum field width one less than the buffer size. @strong{If you
|
|
provide the buffer, always specify a maximum field width to prevent
|
|
overflow.}
|
|
|
|
@item
|
|
Ask @code{scanf} to allocate a big enough buffer, by specifying the
|
|
@samp{a} flag character. This is a GNU extension. You should provide
|
|
an argument of type @code{char **} for the buffer address to be stored
|
|
in. @xref{Dynamic String Input}.
|
|
@end itemize
|
|
|
|
The @samp{%c} conversion is the simplest: it matches a fixed number of
|
|
characters, always. The maximum field with says how many characters to
|
|
read; if you don't specify the maximum, the default is 1. This
|
|
conversion doesn't append a null character to the end of the text it
|
|
reads. It also does not skip over initial whitespace characters. It
|
|
reads precisely the next @var{n} characters, and fails if it cannot get
|
|
that many. Since there is always a maximum field width with @samp{%c}
|
|
(whether specified, or 1 by default), you can always prevent overflow by
|
|
making the buffer long enough.
|
|
|
|
The @samp{%s} conversion matches a string of non-whitespace characters.
|
|
It skips and discards initial whitespace, but stops when it encounters
|
|
more whitespace after having read something. It stores a null character
|
|
at the end of the text that it reads.
|
|
|
|
For example, reading the input:
|
|
|
|
@smallexample
|
|
hello, world
|
|
@end smallexample
|
|
|
|
@noindent
|
|
with the conversion @samp{%10c} produces @code{" hello, wo"}, but
|
|
reading the same input with the conversion @samp{%10s} produces
|
|
@code{"hello,"}.
|
|
|
|
@strong{Warning:} If you do not specify a field width for @samp{%s},
|
|
then the number of characters read is limited only by where the next
|
|
whitespace character appears. This almost certainly means that invalid
|
|
input can make your program crash---which is a bug.
|
|
|
|
To read in characters that belong to an arbitrary set of your choice,
|
|
use the @samp{%[} conversion. You specify the set between the @samp{[}
|
|
character and a following @samp{]} character, using the same syntax used
|
|
in regular expressions. As special cases:
|
|
|
|
@itemize @bullet
|
|
@item
|
|
A literal @samp{]} character can be specified as the first character
|
|
of the set.
|
|
|
|
@item
|
|
An embedded @samp{-} character (that is, one that is not the first or
|
|
last character of the set) is used to specify a range of characters.
|
|
|
|
@item
|
|
If a caret character @samp{^} immediately follows the initial @samp{[},
|
|
then the set of allowed input characters is the everything @emph{except}
|
|
the characters listed.
|
|
@end itemize
|
|
|
|
The @samp{%[} conversion does not skip over initial whitespace
|
|
characters.
|
|
|
|
Here are some examples of @samp{%[} conversions and what they mean:
|
|
|
|
@table @samp
|
|
@item %25[1234567890]
|
|
Matches a string of up to 25 digits.
|
|
|
|
@item %25[][]
|
|
Matches a string of up to 25 square brackets.
|
|
|
|
@item %25[^ \f\n\r\t\v]
|
|
Matches a string up to 25 characters long that doesn't contain any of
|
|
the standard whitespace characters. This is slightly different from
|
|
@samp{%s}, because if the input begins with a whitespace character,
|
|
@samp{%[} reports a matching failure while @samp{%s} simply discards the
|
|
initial whitespace.
|
|
|
|
@item %25[a-z]
|
|
Matches up to 25 lowercase characters.
|
|
@end table
|
|
|
|
One more reminder: the @samp{%s} and @samp{%[} conversions are
|
|
@strong{dangerous} if you don't specify a maximum width or use the
|
|
@samp{a} flag, because input too long would overflow whatever buffer you
|
|
have provided for it. No matter how long your buffer is, a user could
|
|
supply input that is longer. A well-written program reports invalid
|
|
input with a comprehensible error message, not with a crash.
|
|
|
|
@node Dynamic String Input
|
|
@subsection Dynamically Allocating String Conversions
|
|
|
|
A GNU extension to formatted input lets you safely read a string with no
|
|
maximum size. Using this feature, you don't supply a buffer; instead,
|
|
@code{scanf} allocates a buffer big enough to hold the data and gives
|
|
you its address. To use this feature, write @samp{a} as a flag
|
|
character, as in @samp{%as} or @samp{%a[0-9a-z]}.
|
|
|
|
The pointer argument you supply for where to store the input should have
|
|
type @code{char **}. The @code{scanf} function allocates a buffer and
|
|
stores its address in the word that the argument points to. You should
|
|
free the buffer with @code{free} when you no longer need it.
|
|
|
|
Here is an example of using the @samp{a} flag with the @samp{%[@dots{}]}
|
|
conversion specification to read a ``variable assignment'' of the form
|
|
@samp{@var{variable} = @var{value}}.
|
|
|
|
@smallexample
|
|
@{
|
|
char *variable, *value;
|
|
|
|
if (2 > scanf ("%a[a-zA-Z0-9] = %a[^\n]\n",
|
|
&variable, &value))
|
|
@{
|
|
invalid_input_error ();
|
|
return 0;
|
|
@}
|
|
|
|
@dots{}
|
|
@}
|
|
@end smallexample
|
|
|
|
@node Other Input Conversions
|
|
@subsection Other Input Conversions
|
|
|
|
This section describes the miscellaneous input conversions.
|
|
|
|
The @samp{%p} conversion is used to read a pointer value. It recognizes
|
|
the same syntax as is used by the @samp{%p} output conversion for
|
|
@code{printf} (@pxref{Other Output Conversions}); that is, a hexadecimal
|
|
number just as the @samp{%x} conversion accepts. The corresponding
|
|
argument should be of type @code{void **}; that is, the address of a
|
|
place to store a pointer.
|
|
|
|
The resulting pointer value is not guaranteed to be valid if it was not
|
|
originally written during the same program execution that reads it in.
|
|
|
|
The @samp{%n} conversion produces the number of characters read so far
|
|
by this call. The corresponding argument should be of type @code{int *}.
|
|
This conversion works in the same way as the @samp{%n} conversion for
|
|
@code{printf}; see @ref{Other Output Conversions}, for an example.
|
|
|
|
The @samp{%n} conversion is the only mechanism for determining the
|
|
success of literal matches or conversions with suppressed assignments.
|
|
If the @samp{%n} follows the locus of a matching failure, then no value
|
|
is stored for it since @code{scanf} returns before processing the
|
|
@samp{%n}. If you store @code{-1} in that argument slot before calling
|
|
@code{scanf}, the presence of @code{-1} after @code{scanf} indicates an
|
|
error occurred before the @samp{%n} was reached.
|
|
|
|
Finally, the @samp{%%} conversion matches a literal @samp{%} character
|
|
in the input stream, without using an argument. This conversion does
|
|
not permit any flags, field width, or type modifier to be specified.
|
|
|
|
@node Formatted Input Functions
|
|
@subsection Formatted Input Functions
|
|
|
|
Here are the descriptions of the functions for performing formatted
|
|
input.
|
|
Prototypes for these functions are in the header file @file{stdio.h}.
|
|
@pindex stdio.h
|
|
|
|
@comment stdio.h
|
|
@comment ISO
|
|
@deftypefun int scanf (const char *@var{template}, @dots{})
|
|
The @code{scanf} function reads formatted input from the stream
|
|
@code{stdin} under the control of the template string @var{template}.
|
|
The optional arguments are pointers to the places which receive the
|
|
resulting values.
|
|
|
|
The return value is normally the number of successful assignments. If
|
|
an end-of-file condition is detected before any matches are performed
|
|
(including matches against whitespace and literal characters in the
|
|
template), then @code{EOF} is returned.
|
|
@end deftypefun
|
|
|
|
@comment stdio.h
|
|
@comment ISO
|
|
@deftypefun int fscanf (FILE *@var{stream}, const char *@var{template}, @dots{})
|
|
This function is just like @code{scanf}, except that the input is read
|
|
from the stream @var{stream} instead of @code{stdin}.
|
|
@end deftypefun
|
|
|
|
@comment stdio.h
|
|
@comment ISO
|
|
@deftypefun int sscanf (const char *@var{s}, const char *@var{template}, @dots{})
|
|
This is like @code{scanf}, except that the characters are taken from the
|
|
null-terminated string @var{s} instead of from a stream. Reaching the
|
|
end of the string is treated as an end-of-file condition.
|
|
|
|
The behavior of this function is undefined if copying takes place
|
|
between objects that overlap---for example, if @var{s} is also given
|
|
as an argument to receive a string read under control of the @samp{%s}
|
|
conversion.
|
|
@end deftypefun
|
|
|
|
@node Variable Arguments Input
|
|
@subsection Variable Arguments Input Functions
|
|
|
|
The functions @code{vscanf} and friends are provided so that you can
|
|
define your own variadic @code{scanf}-like functions that make use of
|
|
the same internals as the built-in formatted output functions.
|
|
These functions are analogous to the @code{vprintf} series of output
|
|
functions. @xref{Variable Arguments Output}, for important
|
|
information on how to use them.
|
|
|
|
@strong{Portability Note:} The functions listed in this section are GNU
|
|
extensions.
|
|
|
|
@comment stdio.h
|
|
@comment GNU
|
|
@deftypefun int vscanf (const char *@var{template}, va_list @var{ap})
|
|
This function is similar to @code{scanf} except that, instead of taking
|
|
a variable number of arguments directly, it takes an argument list
|
|
pointer @var{ap} of type @code{va_list} (@pxref{Variadic Functions}).
|
|
@end deftypefun
|
|
|
|
@comment stdio.h
|
|
@comment GNU
|
|
@deftypefun int vfscanf (FILE *@var{stream}, const char *@var{template}, va_list @var{ap})
|
|
This is the equivalent of @code{fscanf} with the variable argument list
|
|
specified directly as for @code{vscanf}.
|
|
@end deftypefun
|
|
|
|
@comment stdio.h
|
|
@comment GNU
|
|
@deftypefun int vsscanf (const char *@var{s}, const char *@var{template}, va_list @var{ap})
|
|
This is the equivalent of @code{sscanf} with the variable argument list
|
|
specified directly as for @code{vscanf}.
|
|
@end deftypefun
|
|
|
|
In GNU C, there is a special construct you can use to let the compiler
|
|
know that a function uses a @code{scanf}-style format string. Then it
|
|
can check the number and types of arguments in each call to the
|
|
function, and warn you when they do not match the format string.
|
|
@xref{Function Attributes, , Declaring Attributes of Functions,
|
|
gcc.info, Using GNU CC}, for details.
|
|
|
|
@node EOF and Errors
|
|
@section End-Of-File and Errors
|
|
|
|
@cindex end of file, on a stream
|
|
Many of the functions described in this chapter return the value of the
|
|
macro @code{EOF} to indicate unsuccessful completion of the operation.
|
|
Since @code{EOF} is used to report both end of file and random errors,
|
|
it's often better to use the @code{feof} function to check explicitly
|
|
for end of file and @code{ferror} to check for errors. These functions
|
|
check indicators that are part of the internal state of the stream
|
|
object, indicators set if the appropriate condition was detected by a
|
|
previous I/O operation on that stream.
|
|
|
|
These symbols are declared in the header file @file{stdio.h}.
|
|
@pindex stdio.h
|
|
|
|
@comment stdio.h
|
|
@comment ISO
|
|
@deftypevr Macro int EOF
|
|
This macro is an integer value that is returned by a number of functions
|
|
to indicate an end-of-file condition, or some other error situation.
|
|
With the GNU library, @code{EOF} is @code{-1}. In other libraries, its
|
|
value may be some other negative number.
|
|
@end deftypevr
|
|
|
|
@comment stdio.h
|
|
@comment ISO
|
|
@deftypefun void clearerr (FILE *@var{stream})
|
|
This function clears the end-of-file and error indicators for the
|
|
stream @var{stream}.
|
|
|
|
The file positioning functions (@pxref{File Positioning}) also clear the
|
|
end-of-file indicator for the stream.
|
|
@end deftypefun
|
|
|
|
@comment stdio.h
|
|
@comment ISO
|
|
@deftypefun int feof (FILE *@var{stream})
|
|
The @code{feof} function returns nonzero if and only if the end-of-file
|
|
indicator for the stream @var{stream} is set.
|
|
@end deftypefun
|
|
|
|
@comment stdio.h
|
|
@comment ISO
|
|
@deftypefun int ferror (FILE *@var{stream})
|
|
The @code{ferror} function returns nonzero if and only if the error
|
|
indicator for the stream @var{stream} is set, indicating that an error
|
|
has occurred on a previous operation on the stream.
|
|
@end deftypefun
|
|
|
|
In addition to setting the error indicator associated with the stream,
|
|
the functions that operate on streams also set @code{errno} in the same
|
|
way as the corresponding low-level functions that operate on file
|
|
descriptors. For example, all of the functions that perform output to a
|
|
stream---such as @code{fputc}, @code{printf}, and @code{fflush}---are
|
|
implemented in terms of @code{write}, and all of the @code{errno} error
|
|
conditions defined for @code{write} are meaningful for these functions.
|
|
For more information about the descriptor-level I/O functions, see
|
|
@ref{Low-Level I/O}.
|
|
|
|
@node Binary Streams
|
|
@section Text and Binary Streams
|
|
|
|
The GNU system and other POSIX-compatible operating systems organize all
|
|
files as uniform sequences of characters. However, some other systems
|
|
make a distinction between files containing text and files containing
|
|
binary data, and the input and output facilities of @w{ISO C} provide for
|
|
this distinction. This section tells you how to write programs portable
|
|
to such systems.
|
|
|
|
@cindex text stream
|
|
@cindex binary stream
|
|
When you open a stream, you can specify either a @dfn{text stream} or a
|
|
@dfn{binary stream}. You indicate that you want a binary stream by
|
|
specifying the @samp{b} modifier in the @var{opentype} argument to
|
|
@code{fopen}; see @ref{Opening Streams}. Without this
|
|
option, @code{fopen} opens the file as a text stream.
|
|
|
|
Text and binary streams differ in several ways:
|
|
|
|
@itemize @bullet
|
|
@item
|
|
The data read from a text stream is divided into @dfn{lines} which are
|
|
terminated by newline (@code{'\n'}) characters, while a binary stream is
|
|
simply a long series of characters. A text stream might on some systems
|
|
fail to handle lines more than 254 characters long (including the
|
|
terminating newline character).
|
|
@cindex lines (in a text file)
|
|
|
|
@item
|
|
On some systems, text files can contain only printing characters,
|
|
horizontal tab characters, and newlines, and so text streams may not
|
|
support other characters. However, binary streams can handle any
|
|
character value.
|
|
|
|
@item
|
|
Space characters that are written immediately preceding a newline
|
|
character in a text stream may disappear when the file is read in again.
|
|
|
|
@item
|
|
More generally, there need not be a one-to-one mapping between
|
|
characters that are read from or written to a text stream, and the
|
|
characters in the actual file.
|
|
@end itemize
|
|
|
|
Since a binary stream is always more capable and more predictable than a
|
|
text stream, you might wonder what purpose text streams serve. Why not
|
|
simply always use binary streams? The answer is that on these operating
|
|
systems, text and binary streams use different file formats, and the
|
|
only way to read or write ``an ordinary file of text'' that can work
|
|
with other text-oriented programs is through a text stream.
|
|
|
|
In the GNU library, and on all POSIX systems, there is no difference
|
|
between text streams and binary streams. When you open a stream, you
|
|
get the same kind of stream regardless of whether you ask for binary.
|
|
This stream can handle any file content, and has none of the
|
|
restrictions that text streams sometimes have.
|
|
|
|
@node File Positioning
|
|
@section File Positioning
|
|
@cindex file positioning on a stream
|
|
@cindex positioning a stream
|
|
@cindex seeking on a stream
|
|
|
|
The @dfn{file position} of a stream describes where in the file the
|
|
stream is currently reading or writing. I/O on the stream advances the
|
|
file position through the file. In the GNU system, the file position is
|
|
represented as an integer, which counts the number of bytes from the
|
|
beginning of the file. @xref{File Position}.
|
|
|
|
During I/O to an ordinary disk file, you can change the file position
|
|
whenever you wish, so as to read or write any portion of the file. Some
|
|
other kinds of files may also permit this. Files which support changing
|
|
the file position are sometimes referred to as @dfn{random-access}
|
|
files.
|
|
|
|
You can use the functions in this section to examine or modify the file
|
|
position indicator associated with a stream. The symbols listed below
|
|
are declared in the header file @file{stdio.h}.
|
|
@pindex stdio.h
|
|
|
|
@comment stdio.h
|
|
@comment ISO
|
|
@deftypefun {long int} ftell (FILE *@var{stream})
|
|
This function returns the current file position of the stream
|
|
@var{stream}.
|
|
|
|
This function can fail if the stream doesn't support file positioning,
|
|
or if the file position can't be represented in a @code{long int}, and
|
|
possibly for other reasons as well. If a failure occurs, a value of
|
|
@code{-1} is returned.
|
|
@end deftypefun
|
|
|
|
@comment stdio.h
|
|
@comment ISO
|
|
@deftypefun int fseek (FILE *@var{stream}, long int @var{offset}, int @var{whence})
|
|
The @code{fseek} function is used to change the file position of the
|
|
stream @var{stream}. The value of @var{whence} must be one of the
|
|
constants @code{SEEK_SET}, @code{SEEK_CUR}, or @code{SEEK_END}, to
|
|
indicate whether the @var{offset} is relative to the beginning of the
|
|
file, the current file position, or the end of the file, respectively.
|
|
|
|
This function returns a value of zero if the operation was successful,
|
|
and a nonzero value to indicate failure. A successful call also clears
|
|
the end-of-file indicator of @var{stream} and discards any characters
|
|
that were ``pushed back'' by the use of @code{ungetc}.
|
|
|
|
@code{fseek} either flushes any buffered output before setting the file
|
|
position or else remembers it so it will be written later in its proper
|
|
place in the file.
|
|
@end deftypefun
|
|
|
|
@strong{Portability Note:} In non-POSIX systems, @code{ftell} and
|
|
@code{fseek} might work reliably only on binary streams. @xref{Binary
|
|
Streams}.
|
|
|
|
The following symbolic constants are defined for use as the @var{whence}
|
|
argument to @code{fseek}. They are also used with the @code{lseek}
|
|
function (@pxref{I/O Primitives}) and to specify offsets for file locks
|
|
(@pxref{Control Operations}).
|
|
|
|
@comment stdio.h
|
|
@comment ISO
|
|
@deftypevr Macro int SEEK_SET
|
|
This is an integer constant which, when used as the @var{whence}
|
|
argument to the @code{fseek} function, specifies that the offset
|
|
provided is relative to the beginning of the file.
|
|
@end deftypevr
|
|
|
|
@comment stdio.h
|
|
@comment ISO
|
|
@deftypevr Macro int SEEK_CUR
|
|
This is an integer constant which, when used as the @var{whence}
|
|
argument to the @code{fseek} function, specifies that the offset
|
|
provided is relative to the current file position.
|
|
@end deftypevr
|
|
|
|
@comment stdio.h
|
|
@comment ISO
|
|
@deftypevr Macro int SEEK_END
|
|
This is an integer constant which, when used as the @var{whence}
|
|
argument to the @code{fseek} function, specifies that the offset
|
|
provided is relative to the end of the file.
|
|
@end deftypevr
|
|
|
|
@comment stdio.h
|
|
@comment ISO
|
|
@deftypefun void rewind (FILE *@var{stream})
|
|
The @code{rewind} function positions the stream @var{stream} at the
|
|
begining of the file. It is equivalent to calling @code{fseek} on the
|
|
@var{stream} with an @var{offset} argument of @code{0L} and a
|
|
@var{whence} argument of @code{SEEK_SET}, except that the return
|
|
value is discarded and the error indicator for the stream is reset.
|
|
@end deftypefun
|
|
|
|
These three aliases for the @samp{SEEK_@dots{}} constants exist for the
|
|
sake of compatibility with older BSD systems. They are defined in two
|
|
different header files: @file{fcntl.h} and @file{sys/file.h}.
|
|
|
|
@table @code
|
|
@comment sys/file.h
|
|
@comment BSD
|
|
@item L_SET
|
|
@vindex L_SET
|
|
An alias for @code{SEEK_SET}.
|
|
|
|
@comment sys/file.h
|
|
@comment BSD
|
|
@item L_INCR
|
|
@vindex L_INCR
|
|
An alias for @code{SEEK_CUR}.
|
|
|
|
@comment sys/file.h
|
|
@comment BSD
|
|
@item L_XTND
|
|
@vindex L_XTND
|
|
An alias for @code{SEEK_END}.
|
|
@end table
|
|
|
|
@node Portable Positioning
|
|
@section Portable File-Position Functions
|
|
|
|
On the GNU system, the file position is truly a character count. You
|
|
can specify any character count value as an argument to @code{fseek} and
|
|
get reliable results for any random access file. However, some @w{ISO C}
|
|
systems do not represent file positions in this way.
|
|
|
|
On some systems where text streams truly differ from binary streams, it
|
|
is impossible to represent the file position of a text stream as a count
|
|
of characters from the beginning of the file. For example, the file
|
|
position on some systems must encode both a record offset within the
|
|
file, and a character offset within the record.
|
|
|
|
As a consequence, if you want your programs to be portable to these
|
|
systems, you must observe certain rules:
|
|
|
|
@itemize @bullet
|
|
@item
|
|
The value returned from @code{ftell} on a text stream has no predictable
|
|
relationship to the number of characters you have read so far. The only
|
|
thing you can rely on is that you can use it subsequently as the
|
|
@var{offset} argument to @code{fseek} to move back to the same file
|
|
position.
|
|
|
|
@item
|
|
In a call to @code{fseek} on a text stream, either the @var{offset} must
|
|
either be zero; or @var{whence} must be @code{SEEK_SET} and the
|
|
@var{offset} must be the result of an earlier call to @code{ftell} on
|
|
the same stream.
|
|
|
|
@item
|
|
The value of the file position indicator of a text stream is undefined
|
|
while there are characters that have been pushed back with @code{ungetc}
|
|
that haven't been read or discarded. @xref{Unreading}.
|
|
@end itemize
|
|
|
|
But even if you observe these rules, you may still have trouble for long
|
|
files, because @code{ftell} and @code{fseek} use a @code{long int} value
|
|
to represent the file position. This type may not have room to encode
|
|
all the file positions in a large file.
|
|
|
|
So if you do want to support systems with peculiar encodings for the
|
|
file positions, it is better to use the functions @code{fgetpos} and
|
|
@code{fsetpos} instead. These functions represent the file position
|
|
using the data type @code{fpos_t}, whose internal representation varies
|
|
from system to system.
|
|
|
|
These symbols are declared in the header file @file{stdio.h}.
|
|
@pindex stdio.h
|
|
|
|
@comment stdio.h
|
|
@comment ISO
|
|
@deftp {Data Type} fpos_t
|
|
This is the type of an object that can encode information about the
|
|
file position of a stream, for use by the functions @code{fgetpos} and
|
|
@code{fsetpos}.
|
|
|
|
In the GNU system, @code{fpos_t} is equivalent to @code{off_t} or
|
|
@code{long int}. In other systems, it might have a different internal
|
|
representation.
|
|
@end deftp
|
|
|
|
@comment stdio.h
|
|
@comment ISO
|
|
@deftypefun int fgetpos (FILE *@var{stream}, fpos_t *@var{position})
|
|
This function stores the value of the file position indicator for the
|
|
stream @var{stream} in the @code{fpos_t} object pointed to by
|
|
@var{position}. If successful, @code{fgetpos} returns zero; otherwise
|
|
it returns a nonzero value and stores an implementation-defined positive
|
|
value in @code{errno}.
|
|
@end deftypefun
|
|
|
|
@comment stdio.h
|
|
@comment ISO
|
|
@deftypefun int fsetpos (FILE *@var{stream}, const fpos_t @var{position})
|
|
This function sets the file position indicator for the stream @var{stream}
|
|
to the position @var{position}, which must have been set by a previous
|
|
call to @code{fgetpos} on the same stream. If successful, @code{fsetpos}
|
|
clears the end-of-file indicator on the stream, discards any characters
|
|
that were ``pushed back'' by the use of @code{ungetc}, and returns a value
|
|
of zero. Otherwise, @code{fsetpos} returns a nonzero value and stores
|
|
an implementation-defined positive value in @code{errno}.
|
|
@end deftypefun
|
|
|
|
@node Stream Buffering
|
|
@section Stream Buffering
|
|
|
|
@cindex buffering of streams
|
|
Characters that are written to a stream are normally accumulated and
|
|
transmitted asynchronously to the file in a block, instead of appearing
|
|
as soon as they are output by the application program. Similarly,
|
|
streams often retrieve input from the host environment in blocks rather
|
|
than on a character-by-character basis. This is called @dfn{buffering}.
|
|
|
|
If you are writing programs that do interactive input and output using
|
|
streams, you need to understand how buffering works when you design the
|
|
user interface to your program. Otherwise, you might find that output
|
|
(such as progress or prompt messages) doesn't appear when you intended
|
|
it to, or other unexpected behavior.
|
|
|
|
This section deals only with controlling when characters are transmitted
|
|
between the stream and the file or device, and @emph{not} with how
|
|
things like echoing, flow control, and the like are handled on specific
|
|
classes of devices. For information on common control operations on
|
|
terminal devices, see @ref{Low-Level Terminal Interface}.
|
|
|
|
You can bypass the stream buffering facilities altogether by using the
|
|
low-level input and output functions that operate on file descriptors
|
|
instead. @xref{Low-Level I/O}.
|
|
|
|
@menu
|
|
* Buffering Concepts:: Terminology is defined here.
|
|
* Flushing Buffers:: How to ensure that output buffers are flushed.
|
|
* Controlling Buffering:: How to specify what kind of buffering to use.
|
|
@end menu
|
|
|
|
@node Buffering Concepts
|
|
@subsection Buffering Concepts
|
|
|
|
There are three different kinds of buffering strategies:
|
|
|
|
@itemize @bullet
|
|
@item
|
|
Characters written to or read from an @dfn{unbuffered} stream are
|
|
transmitted individually to or from the file as soon as possible.
|
|
@cindex unbuffered stream
|
|
|
|
@item
|
|
Characters written to a @dfn{line buffered} stream are transmitted to
|
|
the file in blocks when a newline character is encountered.
|
|
@cindex line buffered stream
|
|
|
|
@item
|
|
Characters written to or read from a @dfn{fully buffered} stream are
|
|
transmitted to or from the file in blocks of arbitrary size.
|
|
@cindex fully buffered stream
|
|
@end itemize
|
|
|
|
Newly opened streams are normally fully buffered, with one exception: a
|
|
stream connected to an interactive device such as a terminal is
|
|
initially line buffered. @xref{Controlling Buffering}, for information
|
|
on how to select a different kind of buffering. Usually the automatic
|
|
selection gives you the most convenient kind of buffering for the file
|
|
or device you open.
|
|
|
|
The use of line buffering for interactive devices implies that output
|
|
messages ending in a newline will appear immediately---which is usually
|
|
what you want. Output that doesn't end in a newline might or might not
|
|
show up immediately, so if you want them to appear immediately, you
|
|
should flush buffered output explicitly with @code{fflush}, as described
|
|
in @ref{Flushing Buffers}.
|
|
|
|
@node Flushing Buffers
|
|
@subsection Flushing Buffers
|
|
|
|
@cindex flushing a stream
|
|
@dfn{Flushing} output on a buffered stream means transmitting all
|
|
accumulated characters to the file. There are many circumstances when
|
|
buffered output on a stream is flushed automatically:
|
|
|
|
@itemize @bullet
|
|
@item
|
|
When you try to do output and the output buffer is full.
|
|
|
|
@item
|
|
When the stream is closed. @xref{Closing Streams}.
|
|
|
|
@item
|
|
When the program terminates by calling @code{exit}.
|
|
@xref{Normal Termination}.
|
|
|
|
@item
|
|
When a newline is written, if the stream is line buffered.
|
|
|
|
@item
|
|
Whenever an input operation on @emph{any} stream actually reads data
|
|
from its file.
|
|
@end itemize
|
|
|
|
If you want to flush the buffered output at another time, call
|
|
@code{fflush}, which is declared in the header file @file{stdio.h}.
|
|
@pindex stdio.h
|
|
|
|
@comment stdio.h
|
|
@comment ISO
|
|
@deftypefun int fflush (FILE *@var{stream})
|
|
This function causes any buffered output on @var{stream} to be delivered
|
|
to the file. If @var{stream} is a null pointer, then
|
|
@code{fflush} causes buffered output on @emph{all} open output streams
|
|
to be flushed.
|
|
|
|
This function returns @code{EOF} if a write error occurs, or zero
|
|
otherwise.
|
|
@end deftypefun
|
|
|
|
@strong{Compatibility Note:} Some brain-damaged operating systems have
|
|
been known to be so thoroughly fixated on line-oriented input and output
|
|
that flushing a line buffered stream causes a newline to be written!
|
|
Fortunately, this ``feature'' seems to be becoming less common. You do
|
|
not need to worry about this in the GNU system.
|
|
|
|
|
|
@node Controlling Buffering
|
|
@subsection Controlling Which Kind of Buffering
|
|
|
|
After opening a stream (but before any other operations have been
|
|
performed on it), you can explicitly specify what kind of buffering you
|
|
want it to have using the @code{setvbuf} function.
|
|
@cindex buffering, controlling
|
|
|
|
The facilities listed in this section are declared in the header
|
|
file @file{stdio.h}.
|
|
@pindex stdio.h
|
|
|
|
@comment stdio.h
|
|
@comment ISO
|
|
@deftypefun int setvbuf (FILE *@var{stream}, char *@var{buf}, int @var{mode}, size_t @var{size})
|
|
This function is used to specify that the stream @var{stream} should
|
|
have the buffering mode @var{mode}, which can be either @code{_IOFBF}
|
|
(for full buffering), @code{_IOLBF} (for line buffering), or
|
|
@code{_IONBF} (for unbuffered input/output).
|
|
|
|
If you specify a null pointer as the @var{buf} argument, then @code{setvbuf}
|
|
allocates a buffer itself using @code{malloc}. This buffer will be freed
|
|
when you close the stream.
|
|
|
|
Otherwise, @var{buf} should be a character array that can hold at least
|
|
@var{size} characters. You should not free the space for this array as
|
|
long as the stream remains open and this array remains its buffer. You
|
|
should usually either allocate it statically, or @code{malloc}
|
|
(@pxref{Unconstrained Allocation}) the buffer. Using an automatic array
|
|
is not a good idea unless you close the file before exiting the block
|
|
that declares the array.
|
|
|
|
While the array remains a stream buffer, the stream I/O functions will
|
|
use the buffer for their internal purposes. You shouldn't try to access
|
|
the values in the array directly while the stream is using it for
|
|
buffering.
|
|
|
|
The @code{setvbuf} function returns zero on success, or a nonzero value
|
|
if the value of @var{mode} is not valid or if the request could not
|
|
be honored.
|
|
@end deftypefun
|
|
|
|
@comment stdio.h
|
|
@comment ISO
|
|
@deftypevr Macro int _IOFBF
|
|
The value of this macro is an integer constant expression that can be
|
|
used as the @var{mode} argument to the @code{setvbuf} function to
|
|
specify that the stream should be fully buffered.
|
|
@end deftypevr
|
|
|
|
@comment stdio.h
|
|
@comment ISO
|
|
@deftypevr Macro int _IOLBF
|
|
The value of this macro is an integer constant expression that can be
|
|
used as the @var{mode} argument to the @code{setvbuf} function to
|
|
specify that the stream should be line buffered.
|
|
@end deftypevr
|
|
|
|
@comment stdio.h
|
|
@comment ISO
|
|
@deftypevr Macro int _IONBF
|
|
The value of this macro is an integer constant expression that can be
|
|
used as the @var{mode} argument to the @code{setvbuf} function to
|
|
specify that the stream should be unbuffered.
|
|
@end deftypevr
|
|
|
|
@comment stdio.h
|
|
@comment ISO
|
|
@deftypevr Macro int BUFSIZ
|
|
The value of this macro is an integer constant expression that is good
|
|
to use for the @var{size} argument to @code{setvbuf}. This value is
|
|
guaranteed to be at least @code{256}.
|
|
|
|
The value of @code{BUFSIZ} is chosen on each system so as to make stream
|
|
I/O efficient. So it is a good idea to use @code{BUFSIZ} as the size
|
|
for the buffer when you call @code{setvbuf}.
|
|
|
|
Actually, you can get an even better value to use for the buffer size
|
|
by means of the @code{fstat} system call: it is found in the
|
|
@code{st_blksize} field of the file attributes. @xref{Attribute Meanings}.
|
|
|
|
Sometimes people also use @code{BUFSIZ} as the allocation size of
|
|
buffers used for related purposes, such as strings used to receive a
|
|
line of input with @code{fgets} (@pxref{Character Input}). There is no
|
|
particular reason to use @code{BUFSIZ} for this instead of any other
|
|
integer, except that it might lead to doing I/O in chunks of an
|
|
efficient size.
|
|
@end deftypevr
|
|
|
|
@comment stdio.h
|
|
@comment ISO
|
|
@deftypefun void setbuf (FILE *@var{stream}, char *@var{buf})
|
|
If @var{buf} is a null pointer, the effect of this function is
|
|
equivalent to calling @code{setvbuf} with a @var{mode} argument of
|
|
@code{_IONBF}. Otherwise, it is equivalent to calling @code{setvbuf}
|
|
with @var{buf}, and a @var{mode} of @code{_IOFBF} and a @var{size}
|
|
argument of @code{BUFSIZ}.
|
|
|
|
The @code{setbuf} function is provided for compatibility with old code;
|
|
use @code{setvbuf} in all new programs.
|
|
@end deftypefun
|
|
|
|
@comment stdio.h
|
|
@comment BSD
|
|
@deftypefun void setbuffer (FILE *@var{stream}, char *@var{buf}, size_t @var{size})
|
|
If @var{buf} is a null pointer, this function makes @var{stream} unbuffered.
|
|
Otherwise, it makes @var{stream} fully buffered using @var{buf} as the
|
|
buffer. The @var{size} argument specifies the length of @var{buf}.
|
|
|
|
This function is provided for compatibility with old BSD code. Use
|
|
@code{setvbuf} instead.
|
|
@end deftypefun
|
|
|
|
@comment stdio.h
|
|
@comment BSD
|
|
@deftypefun void setlinebuf (FILE *@var{stream})
|
|
This function makes @var{stream} be line buffered, and allocates the
|
|
buffer for you.
|
|
|
|
This function is provided for compatibility with old BSD code. Use
|
|
@code{setvbuf} instead.
|
|
@end deftypefun
|
|
|
|
@node Other Kinds of Streams
|
|
@section Other Kinds of Streams
|
|
|
|
The GNU library provides ways for you to define additional kinds of
|
|
streams that do not necessarily correspond to an open file.
|
|
|
|
One such type of stream takes input from or writes output to a string.
|
|
These kinds of streams are used internally to implement the
|
|
@code{sprintf} and @code{sscanf} functions. You can also create such a
|
|
stream explicitly, using the functions described in @ref{String Streams}.
|
|
|
|
More generally, you can define streams that do input/output to arbitrary
|
|
objects using functions supplied by your program. This protocol is
|
|
discussed in @ref{Custom Streams}.
|
|
|
|
@strong{Portability Note:} The facilities described in this section are
|
|
specific to GNU. Other systems or C implementations might or might not
|
|
provide equivalent functionality.
|
|
|
|
@menu
|
|
* String Streams:: Streams that get data from or put data in
|
|
a string or memory buffer.
|
|
* Obstack Streams:: Streams that store data in an obstack.
|
|
* Custom Streams:: Defining your own streams with an arbitrary
|
|
input data source and/or output data sink.
|
|
@end menu
|
|
|
|
@node String Streams
|
|
@subsection String Streams
|
|
|
|
@cindex stream, for I/O to a string
|
|
@cindex string stream
|
|
The @code{fmemopen} and @code{open_memstream} functions allow you to do
|
|
I/O to a string or memory buffer. These facilities are declared in
|
|
@file{stdio.h}.
|
|
@pindex stdio.h
|
|
|
|
@comment stdio.h
|
|
@comment GNU
|
|
@deftypefun {FILE *} fmemopen (void *@var{buf}, size_t @var{size}, const char *@var{opentype})
|
|
This function opens a stream that allows the access specified by the
|
|
@var{opentype} argument, that reads from or writes to the buffer specified
|
|
by the argument @var{buf}. This array must be at least @var{size} bytes long.
|
|
|
|
If you specify a null pointer as the @var{buf} argument, @code{fmemopen}
|
|
dynamically allocates (as with @code{malloc}; @pxref{Unconstrained
|
|
Allocation}) an array @var{size} bytes long. This is really only useful
|
|
if you are going to write things to the buffer and then read them back
|
|
in again, because you have no way of actually getting a pointer to the
|
|
buffer (for this, try @code{open_memstream}, below). The buffer is
|
|
freed when the stream is open.
|
|
|
|
The argument @var{opentype} is the same as in @code{fopen}
|
|
(@xref{Opening Streams}). If the @var{opentype} specifies
|
|
append mode, then the initial file position is set to the first null
|
|
character in the buffer. Otherwise the initial file position is at the
|
|
beginning of the buffer.
|
|
|
|
When a stream open for writing is flushed or closed, a null character
|
|
(zero byte) is written at the end of the buffer if it fits. You
|
|
should add an extra byte to the @var{size} argument to account for this.
|
|
Attempts to write more than @var{size} bytes to the buffer result
|
|
in an error.
|
|
|
|
For a stream open for reading, null characters (zero bytes) in the
|
|
buffer do not count as ``end of file''. Read operations indicate end of
|
|
file only when the file position advances past @var{size} bytes. So, if
|
|
you want to read characters from a null-terminated string, you should
|
|
supply the length of the string as the @var{size} argument.
|
|
@end deftypefun
|
|
|
|
Here is an example of using @code{fmemopen} to create a stream for
|
|
reading from a string:
|
|
|
|
@smallexample
|
|
@include memopen.c.texi
|
|
@end smallexample
|
|
|
|
This program produces the following output:
|
|
|
|
@smallexample
|
|
Got f
|
|
Got o
|
|
Got o
|
|
Got b
|
|
Got a
|
|
Got r
|
|
@end smallexample
|
|
|
|
@comment stdio.h
|
|
@comment GNU
|
|
@deftypefun {FILE *} open_memstream (char **@var{ptr}, size_t *@var{sizeloc})
|
|
This function opens a stream for writing to a buffer. The buffer is
|
|
allocated dynamically (as with @code{malloc}; @pxref{Unconstrained
|
|
Allocation}) and grown as necessary.
|
|
|
|
When the stream is closed with @code{fclose} or flushed with
|
|
@code{fflush}, the locations @var{ptr} and @var{sizeloc} are updated to
|
|
contain the pointer to the buffer and its size. The values thus stored
|
|
remain valid only as long as no further output on the stream takes
|
|
place. If you do more output, you must flush the stream again to store
|
|
new values before you use them again.
|
|
|
|
A null character is written at the end of the buffer. This null character
|
|
is @emph{not} included in the size value stored at @var{sizeloc}.
|
|
|
|
You can move the stream's file position with @code{fseek} (@pxref{File
|
|
Positioning}). Moving the file position past the end of the data
|
|
already written fills the intervening space with zeroes.
|
|
@end deftypefun
|
|
|
|
Here is an example of using @code{open_memstream}:
|
|
|
|
@smallexample
|
|
@include memstrm.c.texi
|
|
@end smallexample
|
|
|
|
This program produces the following output:
|
|
|
|
@smallexample
|
|
buf = `hello', size = 5
|
|
buf = `hello, world', size = 12
|
|
@end smallexample
|
|
|
|
@c @group Invalid outside @example.
|
|
@node Obstack Streams
|
|
@subsection Obstack Streams
|
|
|
|
You can open an output stream that puts it data in an obstack.
|
|
@xref{Obstacks}.
|
|
|
|
@comment stdio.h
|
|
@comment GNU
|
|
@deftypefun {FILE *} open_obstack_stream (struct obstack *@var{obstack})
|
|
This function opens a stream for writing data into the obstack @var{obstack}.
|
|
This starts an object in the obstack and makes it grow as data is
|
|
written (@pxref{Growing Objects}).
|
|
@c @end group Doubly invalid because not nested right.
|
|
|
|
Calling @code{fflush} on this stream updates the current size of the
|
|
object to match the amount of data that has been written. After a call
|
|
to @code{fflush}, you can examine the object temporarily.
|
|
|
|
You can move the file position of an obstack stream with @code{fseek}
|
|
(@pxref{File Positioning}). Moving the file position past the end of
|
|
the data written fills the intervening space with zeros.
|
|
|
|
To make the object permanent, update the obstack with @code{fflush}, and
|
|
then use @code{obstack_finish} to finalize the object and get its address.
|
|
The following write to the stream starts a new object in the obstack,
|
|
and later writes add to that object until you do another @code{fflush}
|
|
and @code{obstack_finish}.
|
|
|
|
But how do you find out how long the object is? You can get the length
|
|
in bytes by calling @code{obstack_object_size} (@pxref{Status of an
|
|
Obstack}), or you can null-terminate the object like this:
|
|
|
|
@smallexample
|
|
obstack_1grow (@var{obstack}, 0);
|
|
@end smallexample
|
|
|
|
Whichever one you do, you must do it @emph{before} calling
|
|
@code{obstack_finish}. (You can do both if you wish.)
|
|
@end deftypefun
|
|
|
|
Here is a sample function that uses @code{open_obstack_stream}:
|
|
|
|
@smallexample
|
|
char *
|
|
make_message_string (const char *a, int b)
|
|
@{
|
|
FILE *stream = open_obstack_stream (&message_obstack);
|
|
output_task (stream);
|
|
fprintf (stream, ": ");
|
|
fprintf (stream, a, b);
|
|
fprintf (stream, "\n");
|
|
fclose (stream);
|
|
obstack_1grow (&message_obstack, 0);
|
|
return obstack_finish (&message_obstack);
|
|
@}
|
|
@end smallexample
|
|
|
|
@node Custom Streams
|
|
@subsection Programming Your Own Custom Streams
|
|
@cindex custom streams
|
|
@cindex programming your own streams
|
|
|
|
This section describes how you can make a stream that gets input from an
|
|
arbitrary data source or writes output to an arbitrary data sink
|
|
programmed by you. We call these @dfn{custom streams}.
|
|
|
|
@c !!! this does not talk at all about the higher-level hooks
|
|
|
|
@menu
|
|
* Streams and Cookies:: The @dfn{cookie} records where to fetch or
|
|
store data that is read or written.
|
|
* Hook Functions:: How you should define the four @dfn{hook
|
|
functions} that a custom stream needs.
|
|
@end menu
|
|
|
|
@node Streams and Cookies
|
|
@subsubsection Custom Streams and Cookies
|
|
@cindex cookie, for custom stream
|
|
|
|
Inside every custom stream is a special object called the @dfn{cookie}.
|
|
This is an object supplied by you which records where to fetch or store
|
|
the data read or written. It is up to you to define a data type to use
|
|
for the cookie. The stream functions in the library never refer
|
|
directly to its contents, and they don't even know what the type is;
|
|
they record its address with type @code{void *}.
|
|
|
|
To implement a custom stream, you must specify @emph{how} to fetch or
|
|
store the data in the specified place. You do this by defining
|
|
@dfn{hook functions} to read, write, change ``file position'', and close
|
|
the stream. All four of these functions will be passed the stream's
|
|
cookie so they can tell where to fetch or store the data. The library
|
|
functions don't know what's inside the cookie, but your functions will
|
|
know.
|
|
|
|
When you create a custom stream, you must specify the cookie pointer,
|
|
and also the four hook functions stored in a structure of type
|
|
@code{cookie_io_functions_t}.
|
|
|
|
These facilities are declared in @file{stdio.h}.
|
|
@pindex stdio.h
|
|
|
|
@comment stdio.h
|
|
@comment GNU
|
|
@deftp {Data Type} {cookie_io_functions_t}
|
|
This is a structure type that holds the functions that define the
|
|
communications protocol between the stream and its cookie. It has
|
|
the following members:
|
|
|
|
@table @code
|
|
@item cookie_read_function_t *read
|
|
This is the function that reads data from the cookie. If the value is a
|
|
null pointer instead of a function, then read operations on ths stream
|
|
always return @code{EOF}.
|
|
|
|
@item cookie_write_function_t *write
|
|
This is the function that writes data to the cookie. If the value is a
|
|
null pointer instead of a function, then data written to the stream is
|
|
discarded.
|
|
|
|
@item cookie_seek_function_t *seek
|
|
This is the function that performs the equivalent of file positioning on
|
|
the cookie. If the value is a null pointer instead of a function, calls
|
|
to @code{fseek} on this stream can only seek to locations within the
|
|
buffer; any attempt to seek outside the buffer will return an
|
|
@code{ESPIPE} error.
|
|
|
|
@item cookie_close_function_t *close
|
|
This function performs any appropriate cleanup on the cookie when
|
|
closing the stream. If the value is a null pointer instead of a
|
|
function, nothing special is done to close the cookie when the stream is
|
|
closed.
|
|
@end table
|
|
@end deftp
|
|
|
|
@comment stdio.h
|
|
@comment GNU
|
|
@deftypefun {FILE *} fopencookie (void *@var{cookie}, const char *@var{opentype}, cookie_io_functions_t @var{io-functions})
|
|
This function actually creates the stream for communicating with the
|
|
@var{cookie} using the functions in the @var{io-functions} argument.
|
|
The @var{opentype} argument is interpreted as for @code{fopen};
|
|
see @ref{Opening Streams}. (But note that the ``truncate on
|
|
open'' option is ignored.) The new stream is fully buffered.
|
|
|
|
The @code{fopencookie} function returns the newly created stream, or a null
|
|
pointer in case of an error.
|
|
@end deftypefun
|
|
|
|
@node Hook Functions
|
|
@subsubsection Custom Stream Hook Functions
|
|
@cindex hook functions (of custom streams)
|
|
|
|
Here are more details on how you should define the four hook functions
|
|
that a custom stream needs.
|
|
|
|
You should define the function to read data from the cookie as:
|
|
|
|
@smallexample
|
|
ssize_t @var{reader} (void *@var{cookie}, void *@var{buffer}, size_t @var{size})
|
|
@end smallexample
|
|
|
|
This is very similar to the @code{read} function; see @ref{I/O
|
|
Primitives}. Your function should transfer up to @var{size} bytes into
|
|
the @var{buffer}, and return the number of bytes read, or zero to
|
|
indicate end-of-file. You can return a value of @code{-1} to indicate
|
|
an error.
|
|
|
|
You should define the function to write data to the cookie as:
|
|
|
|
@smallexample
|
|
ssize_t @var{writer} (void *@var{cookie}, const void *@var{buffer}, size_t @var{size})
|
|
@end smallexample
|
|
|
|
This is very similar to the @code{write} function; see @ref{I/O
|
|
Primitives}. Your function should transfer up to @var{size} bytes from
|
|
the buffer, and return the number of bytes written. You can return a
|
|
value of @code{-1} to indicate an error.
|
|
|
|
You should define the function to perform seek operations on the cookie
|
|
as:
|
|
|
|
@smallexample
|
|
int @var{seeker} (void *@var{cookie}, fpos_t *@var{position}, int @var{whence})
|
|
@end smallexample
|
|
|
|
For this function, the @var{position} and @var{whence} arguments are
|
|
interpreted as for @code{fgetpos}; see @ref{Portable Positioning}. In
|
|
the GNU library, @code{fpos_t} is equivalent to @code{off_t} or
|
|
@code{long int}, and simply represents the number of bytes from the
|
|
beginning of the file.
|
|
|
|
After doing the seek operation, your function should store the resulting
|
|
file position relative to the beginning of the file in @var{position}.
|
|
Your function should return a value of @code{0} on success and @code{-1}
|
|
to indicate an error.
|
|
|
|
You should define the function to do cleanup operations on the cookie
|
|
appropriate for closing the stream as:
|
|
|
|
@smallexample
|
|
int @var{cleaner} (void *@var{cookie})
|
|
@end smallexample
|
|
|
|
Your function should return @code{-1} to indicate an error, and @code{0}
|
|
otherwise.
|
|
|
|
@comment stdio.h
|
|
@comment GNU
|
|
@deftp {Data Type} cookie_read_function
|
|
This is the data type that the read function for a custom stream should have.
|
|
If you declare the function as shown above, this is the type it will have.
|
|
@end deftp
|
|
|
|
@comment stdio.h
|
|
@comment GNU
|
|
@deftp {Data Type} cookie_write_function
|
|
The data type of the write function for a custom stream.
|
|
@end deftp
|
|
|
|
@comment stdio.h
|
|
@comment GNU
|
|
@deftp {Data Type} cookie_seek_function
|
|
The data type of the seek function for a custom stream.
|
|
@end deftp
|
|
|
|
@comment stdio.h
|
|
@comment GNU
|
|
@deftp {Data Type} cookie_close_function
|
|
The data type of the close function for a custom stream.
|
|
@end deftp
|
|
|
|
@ignore
|
|
Roland says:
|
|
|
|
@quotation
|
|
There is another set of functions one can give a stream, the
|
|
input-room and output-room functions. These functions must
|
|
understand stdio internals. To describe how to use these
|
|
functions, you also need to document lots of how stdio works
|
|
internally (which isn't relevant for other uses of stdio).
|
|
Perhaps I can write an interface spec from which you can write
|
|
good documentation. But it's pretty complex and deals with lots
|
|
of nitty-gritty details. I think it might be better to let this
|
|
wait until the rest of the manual is more done and polished.
|
|
@end quotation
|
|
@end ignore
|
|
|
|
@c ??? This section could use an example.
|