1092 lines
36 KiB
Plaintext
1092 lines
36 KiB
Plaintext
@node System Configuration, Language Features, System Information, Top
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@chapter System Configuration Parameters
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The functions and macros listed in this chapter give information about
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configuration parameters of the operating system---for example, capacity
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limits, presence of optional POSIX features, and the default path for
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executable files (@pxref{String Parameters}).
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@menu
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* General Limits:: Constants and functions that describe
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various process-related limits that have
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one uniform value for any given machine.
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* System Options:: Optional POSIX features.
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* Version Supported:: Version numbers of POSIX.1 and POSIX.2.
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* Sysconf:: Getting specific configuration values
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of general limits and system options.
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* Minimums:: Minimum values for general limits.
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* Limits for Files:: Size limitations that pertain to individual files.
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These can vary between file systems
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or even from file to file.
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* Options for Files:: Optional features that some files may support.
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* File Minimums:: Minimum values for file limits.
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* Pathconf:: Getting the limit values for a particular file.
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* Utility Limits:: Capacity limits of some POSIX.2 utility programs.
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* Utility Minimums:: Minimum allowable values of those limits.
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* String Parameters:: Getting the default search path.
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@end menu
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@node General Limits
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@section General Capacity Limits
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@cindex POSIX capacity limits
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@cindex limits, POSIX
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@cindex capacity limits, POSIX
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The POSIX.1 and POSIX.2 standards specify a number of parameters that
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describe capacity limitations of the system. These limits can be fixed
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constants for a given operating system, or they can vary from machine to
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machine. For example, some limit values may be configurable by the
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system administrator, either at run time or by rebuilding the kernel,
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and this should not require recompiling application programs.
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@pindex limits.h
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Each of the following limit parameters has a macro that is defined in
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@file{limits.h} only if the system has a fixed, uniform limit for the
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parameter in question. If the system allows different file systems or
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files to have different limits, then the macro is undefined; use
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@code{sysconf} to find out the limit that applies at a particular time
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on a particular machine. @xref{Sysconf}.
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Each of these parameters also has another macro, with a name starting
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with @samp{_POSIX}, which gives the lowest value that the limit is
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allowed to have on @emph{any} POSIX system. @xref{Minimums}.
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@cindex limits, program argument size
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@comment limits.h
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@comment POSIX.1
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@deftypevr Macro int ARG_MAX
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If defined, the unvarying maximum combined length of the @var{argv} and
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@var{environ} arguments that can be passed to the @code{exec} functions.
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@end deftypevr
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@cindex limits, number of processes
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@comment limits.h
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@comment POSIX.1
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@deftypevr Macro int CHILD_MAX
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If defined, the unvarying maximum number of processes that can exist
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with the same real user ID at any one time. In BSD and GNU, this is
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controlled by the @code{RLIMIT_NPROC} resource limit; @pxref{Limits on
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Resources}.
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@end deftypevr
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@cindex limits, number of open files
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@comment limits.h
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@comment POSIX.1
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@deftypevr Macro int OPEN_MAX
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If defined, the unvarying maximum number of files that a single process
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can have open simultaneously. In BSD and GNU, this is controlled
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by the @code{RLIMIT_NOFILE} resource limit; @pxref{Limits on Resources}.
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@end deftypevr
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@comment limits.h
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@comment POSIX.1
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@deftypevr Macro int STREAM_MAX
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If defined, the unvarying maximum number of streams that a single
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process can have open simultaneously. @xref{Opening Streams}.
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@end deftypevr
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@cindex limits, time zone name length
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@comment limits.h
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@comment POSIX.1
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@deftypevr Macro int TZNAME_MAX
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If defined, the unvarying maximum length of a time zone name.
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@xref{Time Zone Functions}.
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@end deftypevr
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These limit macros are always defined in @file{limits.h}.
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@cindex limits, number of supplementary group IDs
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@comment limits.h
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@comment POSIX.1
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@deftypevr Macro int NGROUPS_MAX
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The maximum number of supplementary group IDs that one process can have.
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The value of this macro is actually a lower bound for the maximum. That
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is, you can count on being able to have that many supplementary group
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IDs, but a particular machine might let you have even more. You can use
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@code{sysconf} to see whether a particular machine will let you have
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more (@pxref{Sysconf}).
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@end deftypevr
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@comment limits.h
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@comment POSIX.1
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@deftypevr Macro int SSIZE_MAX
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The largest value that can fit in an object of type @code{ssize_t}.
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Effectively, this is the limit on the number of bytes that can be read
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or written in a single operation.
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This macro is defined in all POSIX systems because this limit is never
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configurable.
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@end deftypevr
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@comment limits.h
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@comment POSIX.2
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@deftypevr Macro int RE_DUP_MAX
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The largest number of repetitions you are guaranteed is allowed in the
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construct @samp{\@{@var{min},@var{max}\@}} in a regular expression.
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The value of this macro is actually a lower bound for the maximum. That
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is, you can count on being able to have that many repetitions, but a
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particular machine might let you have even more. You can use
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@code{sysconf} to see whether a particular machine will let you have
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more (@pxref{Sysconf}). And even the value that @code{sysconf} tells
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you is just a lower bound---larger values might work.
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This macro is defined in all POSIX.2 systems, because POSIX.2 says it
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should always be defined even if there is no specific imposed limit.
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@end deftypevr
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@node System Options
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@section Overall System Options
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@cindex POSIX optional features
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@cindex optional POSIX features
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POSIX defines certain system-specific options that not all POSIX systems
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support. Since these options are provided in the kernel, not in the
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library, simply using the GNU C library does not guarantee any of these
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features is supported; it depends on the system you are using.
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@pindex unistd.h
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You can test for the availability of a given option using the macros in
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this section, together with the function @code{sysconf}. The macros are
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defined only if you include @file{unistd.h}.
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For the following macros, if the macro is defined in @file{unistd.h},
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then the option is supported. Otherwise, the option may or may not be
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supported; use @code{sysconf} to find out. @xref{Sysconf}.
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@comment unistd.h
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@comment POSIX.1
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@deftypevr Macro int _POSIX_JOB_CONTROL
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If this symbol is defined, it indicates that the system supports job
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control. Otherwise, the implementation behaves as if all processes
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within a session belong to a single process group. @xref{Job Control}.
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@end deftypevr
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@comment unistd.h
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@comment POSIX.1
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@deftypevr Macro int _POSIX_SAVED_IDS
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If this symbol is defined, it indicates that the system remembers the
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effective user and group IDs of a process before it executes an
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executable file with the set-user-ID or set-group-ID bits set, and that
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explicitly changing the effective user or group IDs back to these values
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is permitted. If this option is not defined, then if a nonprivileged
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process changes its effective user or group ID to the real user or group
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ID of the process, it can't change it back again. @xref{Enable/Disable
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Setuid}.
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@end deftypevr
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For the following macros, if the macro is defined in @file{unistd.h},
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then its value indicates whether the option is supported. A value of
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@code{-1} means no, and any other value means yes. If the macro is not
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defined, then the option may or may not be supported; use @code{sysconf}
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to find out. @xref{Sysconf}.
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@comment unistd.h
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@comment POSIX.2
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@deftypevr Macro int _POSIX2_C_DEV
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If this symbol is defined, it indicates that the system has the POSIX.2
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C compiler command, @code{c89}. The GNU C library always defines this
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as @code{1}, on the assumption that you would not have installed it if
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you didn't have a C compiler.
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@end deftypevr
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@comment unistd.h
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@comment POSIX.2
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@deftypevr Macro int _POSIX2_FORT_DEV
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If this symbol is defined, it indicates that the system has the POSIX.2
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Fortran compiler command, @code{fort77}. The GNU C library never
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defines this, because we don't know what the system has.
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@end deftypevr
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@comment unistd.h
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@comment POSIX.2
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@deftypevr Macro int _POSIX2_FORT_RUN
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If this symbol is defined, it indicates that the system has the POSIX.2
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@code{asa} command to interpret Fortran carriage control. The GNU C
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library never defines this, because we don't know what the system has.
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@end deftypevr
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@comment unistd.h
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@comment POSIX.2
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@deftypevr Macro int _POSIX2_LOCALEDEF
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If this symbol is defined, it indicates that the system has the POSIX.2
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@code{localedef} command. The GNU C library never defines this, because
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we don't know what the system has.
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@end deftypevr
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@comment unistd.h
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@comment POSIX.2
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@deftypevr Macro int _POSIX2_SW_DEV
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If this symbol is defined, it indicates that the system has the POSIX.2
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commands @code{ar}, @code{make}, and @code{strip}. The GNU C library
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always defines this as @code{1}, on the assumption that you had to have
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@code{ar} and @code{make} to install the library, and it's unlikely that
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@code{strip} would be absent when those are present.
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@end deftypevr
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@node Version Supported
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@section Which Version of POSIX is Supported
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@comment unistd.h
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@comment POSIX.1
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@deftypevr Macro {long int} _POSIX_VERSION
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This constant represents the version of the POSIX.1 standard to which
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the implementation conforms. For an implementation conforming to the
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1990 POSIX.1 standard, the value is the integer @code{199009L}.
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@code{_POSIX_VERSION} is always defined (in @file{unistd.h}) in any
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POSIX system.
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@strong{Usage Note:} Don't try to test whether the system supports POSIX
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by including @file{unistd.h} and then checking whether
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@code{_POSIX_VERSION} is defined. On a non-POSIX system, this will
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probably fail because there is no @file{unistd.h}. We do not know of
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@emph{any} way you can reliably test at compilation time whether your
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target system supports POSIX or whether @file{unistd.h} exists.
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The GNU C compiler predefines the symbol @code{__POSIX__} if the target
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system is a POSIX system. Provided you do not use any other compilers
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on POSIX systems, testing @code{defined (__POSIX__)} will reliably
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detect such systems.
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@end deftypevr
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@comment unistd.h
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@comment POSIX.2
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@deftypevr Macro {long int} _POSIX2_C_VERSION
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This constant represents the version of the POSIX.2 standard which the
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library and system kernel support. We don't know what value this will
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be for the first version of the POSIX.2 standard, because the value is
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based on the year and month in which the standard is officially adopted.
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The value of this symbol says nothing about the utilities installed on
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the system.
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@strong{Usage Note:} You can use this macro to tell whether a POSIX.1
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system library supports POSIX.2 as well. Any POSIX.1 system contains
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@file{unistd.h}, so include that file and then test @code{defined
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(_POSIX2_C_VERSION)}.
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@end deftypevr
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@node Sysconf
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@section Using @code{sysconf}
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When your system has configurable system limits, you can use the
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@code{sysconf} function to find out the value that applies to any
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particular machine. The function and the associated @var{parameter}
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constants are declared in the header file @file{unistd.h}.
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@menu
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* Sysconf Definition:: Detailed specifications of @code{sysconf}.
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* Constants for Sysconf:: The list of parameters @code{sysconf} can read.
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* Examples of Sysconf:: How to use @code{sysconf} and the parameter
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macros properly together.
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@end menu
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@node Sysconf Definition
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@subsection Definition of @code{sysconf}
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@comment unistd.h
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@comment POSIX.1
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@deftypefun {long int} sysconf (int @var{parameter})
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This function is used to inquire about runtime system parameters. The
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@var{parameter} argument should be one of the @samp{_SC_} symbols listed
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below.
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The normal return value from @code{sysconf} is the value you requested.
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A value of @code{-1} is returned both if the implementation does not
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impose a limit, and in case of an error.
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The following @code{errno} error conditions are defined for this function:
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@table @code
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@item EINVAL
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The value of the @var{parameter} is invalid.
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@end table
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@end deftypefun
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@node Constants for Sysconf
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@subsection Constants for @code{sysconf} Parameters
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Here are the symbolic constants for use as the @var{parameter} argument
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to @code{sysconf}. The values are all integer constants (more
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specifically, enumeration type values).
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@table @code
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@comment unistd.h
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@comment POSIX.1
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@item _SC_ARG_MAX
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Inquire about the parameter corresponding to @code{ARG_MAX}.
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@comment unistd.h
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@comment POSIX.1
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@item _SC_CHILD_MAX
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Inquire about the parameter corresponding to @code{CHILD_MAX}.
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@comment unistd.h
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@comment POSIX.1
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@item _SC_OPEN_MAX
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Inquire about the parameter corresponding to @code{OPEN_MAX}.
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@comment unistd.h
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@comment POSIX.1
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@item _SC_STREAM_MAX
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Inquire about the parameter corresponding to @code{STREAM_MAX}.
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@comment unistd.h
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@comment POSIX.1
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@item _SC_TZNAME_MAX
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Inquire about the parameter corresponding to @code{TZNAME_MAX}.
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@comment unistd.h
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@comment POSIX.1
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@item _SC_NGROUPS_MAX
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Inquire about the parameter corresponding to @code{NGROUPS_MAX}.
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@comment unistd.h
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@comment POSIX.1
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@item _SC_JOB_CONTROL
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Inquire about the parameter corresponding to @code{_POSIX_JOB_CONTROL}.
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@comment unistd.h
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@comment POSIX.1
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@item _SC_SAVED_IDS
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Inquire about the parameter corresponding to @code{_POSIX_SAVED_IDS}.
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@comment unistd.h
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@comment POSIX.1
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@item _SC_VERSION
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Inquire about the parameter corresponding to @code{_POSIX_VERSION}.
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@comment unistd.h
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@comment POSIX.1
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@item _SC_CLK_TCK
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Inquire about the parameter corresponding to @code{CLOCKS_PER_SEC};
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@pxref{Basic CPU Time}.
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@comment unistd.h
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@comment POSIX.2
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@item _SC_2_C_DEV
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Inquire about whether the system has the POSIX.2 C compiler command,
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@code{c89}.
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@comment unistd.h
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@comment POSIX.2
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@item _SC_2_FORT_DEV
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Inquire about whether the system has the POSIX.2 Fortran compiler
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command, @code{fort77}.
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@comment unistd.h
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@comment POSIX.2
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@item _SC_2_FORT_RUN
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Inquire about whether the system has the POSIX.2 @code{asa} command to
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interpret Fortran carriage control.
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@comment unistd.h
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@comment POSIX.2
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@item _SC_2_LOCALEDEF
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Inquire about whether the system has the POSIX.2 @code{localedef}
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command.
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@comment unistd.h
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@comment POSIX.2
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@item _SC_2_SW_DEV
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Inquire about whether the system has the POSIX.2 commands @code{ar},
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@code{make}, and @code{strip}.
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@comment unistd.h
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@comment POSIX.2
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@item _SC_BC_BASE_MAX
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Inquire about the maximum value of @code{obase} in the @code{bc}
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utility.
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@comment unistd.h
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@comment POSIX.2
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@item _SC_BC_DIM_MAX
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Inquire about the maximum size of an array in the @code{bc}
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utility.
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@comment unistd.h
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@comment POSIX.2
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@item _SC_BC_SCALE_MAX
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Inquire about the maximum value of @code{scale} in the @code{bc}
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utility.
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@comment unistd.h
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@comment POSIX.2
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@item _SC_BC_STRING_MAX
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Inquire about the maximum size of a string constant in the
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@code{bc} utility.
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@comment unistd.h
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@comment POSIX.2
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@item _SC_COLL_WEIGHTS_MAX
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Inquire about the maximum number of weights that can necessarily
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be used in defining the collating sequence for a locale.
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@comment unistd.h
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@comment POSIX.2
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@item _SC_EXPR_NEST_MAX
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Inquire about the maximum number of expressions nested within
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parentheses when using the @code{expr} utility.
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@comment unistd.h
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@comment POSIX.2
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@item _SC_LINE_MAX
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Inquire about the maximum size of a text line that the POSIX.2 text
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utilities can handle.
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@comment unistd.h
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@comment POSIX.2
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@item _SC_EQUIV_CLASS_MAX
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Inquire about the maximum number of weights that can be assigned to an
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entry of the @code{LC_COLLATE} category @samp{order} keyword in a locale
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definition. The GNU C library does not presently support locale
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definitions.
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@comment unistd.h
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@comment POSIX.2
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@item _SC_VERSION
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Inquire about the version number of POSIX.1 that the library and kernel
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support.
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@comment unistd.h
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@comment POSIX.2
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@item _SC_2_VERSION
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Inquire about the version number of POSIX.2 that the system utilities
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support.
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@comment unistd.h
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@comment GNU
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@item _SC_PAGESIZE
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Inquire about the virtual memory page size of the machine.
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@code{getpagesize} returns the same value.
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@c @xref{XXX getpagesize}. !!! ???
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@end table
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@node Examples of Sysconf
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@subsection Examples of @code{sysconf}
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We recommend that you first test for a macro definition for the
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parameter you are interested in, and call @code{sysconf} only if the
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macro is not defined. For example, here is how to test whether job
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control is supported:
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@smallexample
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@group
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int
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have_job_control (void)
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@{
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#ifdef _POSIX_JOB_CONTROL
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return 1;
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#else
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int value = sysconf (_SC_JOB_CONTROL);
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if (value < 0)
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/* @r{If the system is that badly wedged,}
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@r{there's no use trying to go on.} */
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fatal (strerror (errno));
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return value;
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#endif
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@}
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@end group
|
|
@end smallexample
|
|
|
|
Here is how to get the value of a numeric limit:
|
|
|
|
@smallexample
|
|
int
|
|
get_child_max ()
|
|
@{
|
|
#ifdef CHILD_MAX
|
|
return CHILD_MAX;
|
|
#else
|
|
int value = sysconf (_SC_CHILD_MAX);
|
|
if (value < 0)
|
|
fatal (strerror (errno));
|
|
return value;
|
|
#endif
|
|
@}
|
|
@end smallexample
|
|
|
|
@node Minimums
|
|
@section Minimum Values for General Capacity Limits
|
|
|
|
Here are the names for the POSIX minimum upper bounds for the system
|
|
limit parameters. The significance of these values is that you can
|
|
safely push to these limits without checking whether the particular
|
|
system you are using can go that far.
|
|
|
|
@table @code
|
|
@comment limits.h
|
|
@comment POSIX.1
|
|
@item _POSIX_ARG_MAX
|
|
The value of this macro is the most restrictive limit permitted by POSIX
|
|
for the maximum combined length of the @var{argv} and @var{environ}
|
|
arguments that can be passed to the @code{exec} functions.
|
|
Its value is @code{4096}.
|
|
|
|
@comment limits.h
|
|
@comment POSIX.1
|
|
@item _POSIX_CHILD_MAX
|
|
The value of this macro is the most restrictive limit permitted by POSIX
|
|
for the maximum number of simultaneous processes per real user ID. Its
|
|
value is @code{6}.
|
|
|
|
@comment limits.h
|
|
@comment POSIX.1
|
|
@item _POSIX_NGROUPS_MAX
|
|
The value of this macro is the most restrictive limit permitted by POSIX
|
|
for the maximum number of supplementary group IDs per process. Its
|
|
value is @code{0}.
|
|
|
|
@comment limits.h
|
|
@comment POSIX.1
|
|
@item _POSIX_OPEN_MAX
|
|
The value of this macro is the most restrictive limit permitted by POSIX
|
|
for the maximum number of files that a single process can have open
|
|
simultaneously. Its value is @code{16}.
|
|
|
|
@comment limits.h
|
|
@comment POSIX.1
|
|
@item _POSIX_SSIZE_MAX
|
|
The value of this macro is the most restrictive limit permitted by POSIX
|
|
for the maximum value that can be stored in an object of type
|
|
@code{ssize_t}. Its value is @code{32767}.
|
|
|
|
@comment limits.h
|
|
@comment POSIX.1
|
|
@item _POSIX_STREAM_MAX
|
|
The value of this macro is the most restrictive limit permitted by POSIX
|
|
for the maximum number of streams that a single process can have open
|
|
simultaneously. Its value is @code{8}.
|
|
|
|
@comment limits.h
|
|
@comment POSIX.1
|
|
@item _POSIX_TZNAME_MAX
|
|
The value of this macro is the most restrictive limit permitted by POSIX
|
|
for the maximum length of a time zone name. Its value is @code{3}.
|
|
|
|
@comment limits.h
|
|
@comment POSIX.2
|
|
@item _POSIX2_RE_DUP_MAX
|
|
The value of this macro is the most restrictive limit permitted by POSIX
|
|
for the numbers used in the @samp{\@{@var{min},@var{max}\@}} construct
|
|
in a regular expression. Its value is @code{255}.
|
|
@end table
|
|
|
|
@node Limits for Files
|
|
@section Limits on File System Capacity
|
|
|
|
The POSIX.1 standard specifies a number of parameters that describe the
|
|
limitations of the file system. It's possible for the system to have a
|
|
fixed, uniform limit for a parameter, but this isn't the usual case. On
|
|
most systems, it's possible for different file systems (and, for some
|
|
parameters, even different files) to have different maximum limits. For
|
|
example, this is very likely if you use NFS to mount some of the file
|
|
systems from other machines.
|
|
|
|
@pindex limits.h
|
|
Each of the following macros is defined in @file{limits.h} only if the
|
|
system has a fixed, uniform limit for the parameter in question. If the
|
|
system allows different file systems or files to have different limits,
|
|
then the macro is undefined; use @code{pathconf} or @code{fpathconf} to
|
|
find out the limit that applies to a particular file. @xref{Pathconf}.
|
|
|
|
Each parameter also has another macro, with a name starting with
|
|
@samp{_POSIX}, which gives the lowest value that the limit is allowed to
|
|
have on @emph{any} POSIX system. @xref{File Minimums}.
|
|
|
|
@cindex limits, link count of files
|
|
@comment limits.h
|
|
@comment POSIX.1
|
|
@deftypevr Macro int LINK_MAX
|
|
The uniform system limit (if any) for the number of names for a given
|
|
file. @xref{Hard Links}.
|
|
@end deftypevr
|
|
|
|
@cindex limits, terminal input queue
|
|
@comment limits.h
|
|
@comment POSIX.1
|
|
@deftypevr Macro int MAX_CANON
|
|
The uniform system limit (if any) for the amount of text in a line of
|
|
input when input editing is enabled. @xref{Canonical or Not}.
|
|
@end deftypevr
|
|
|
|
@comment limits.h
|
|
@comment POSIX.1
|
|
@deftypevr Macro int MAX_INPUT
|
|
The uniform system limit (if any) for the total number of characters
|
|
typed ahead as input. @xref{I/O Queues}.
|
|
@end deftypevr
|
|
|
|
@cindex limits, file name length
|
|
@comment limits.h
|
|
@comment POSIX.1
|
|
@deftypevr Macro int NAME_MAX
|
|
The uniform system limit (if any) for the length of a file name component.
|
|
@end deftypevr
|
|
|
|
@comment limits.h
|
|
@comment POSIX.1
|
|
@deftypevr Macro int PATH_MAX
|
|
The uniform system limit (if any) for the length of an entire file name (that
|
|
is, the argument given to system calls such as @code{open}).
|
|
@end deftypevr
|
|
|
|
@cindex limits, pipe buffer size
|
|
@comment limits.h
|
|
@comment POSIX.1
|
|
@deftypevr Macro int PIPE_BUF
|
|
The uniform system limit (if any) for the number of bytes that can be
|
|
written atomically to a pipe. If multiple processes are writing to the
|
|
same pipe simultaneously, output from different processes might be
|
|
interleaved in chunks of this size. @xref{Pipes and FIFOs}.
|
|
@end deftypevr
|
|
|
|
These are alternative macro names for some of the same information.
|
|
|
|
@comment dirent.h
|
|
@comment BSD
|
|
@deftypevr Macro int MAXNAMLEN
|
|
This is the BSD name for @code{NAME_MAX}. It is defined in
|
|
@file{dirent.h}.
|
|
@end deftypevr
|
|
|
|
@comment stdio.h
|
|
@comment ISO
|
|
@deftypevr Macro int FILENAME_MAX
|
|
The value of this macro is an integer constant expression that
|
|
represents the maximum length of a file name string. It is defined in
|
|
@file{stdio.h}.
|
|
|
|
Unlike @code{PATH_MAX}, this macro is defined even if there is no actual
|
|
limit imposed. In such a case, its value is typically a very large
|
|
number. @strong{This is always the case on the GNU system.}
|
|
|
|
@strong{Usage Note:} Don't use @code{FILENAME_MAX} as the size of an
|
|
array in which to store a file name! You can't possibly make an array
|
|
that big! Use dynamic allocation (@pxref{Memory Allocation}) instead.
|
|
@end deftypevr
|
|
|
|
@node Options for Files
|
|
@section Optional Features in File Support
|
|
|
|
POSIX defines certain system-specific options in the system calls for
|
|
operating on files. Some systems support these options and others do
|
|
not. Since these options are provided in the kernel, not in the
|
|
library, simply using the GNU C library does not guarantee any of these
|
|
features is supported; it depends on the system you are using. They can
|
|
also vary between file systems on a single machine.
|
|
|
|
@pindex unistd.h
|
|
This section describes the macros you can test to determine whether a
|
|
particular option is supported on your machine. If a given macro is
|
|
defined in @file{unistd.h}, then its value says whether the
|
|
corresponding feature is supported. (A value of @code{-1} indicates no;
|
|
any other value indicates yes.) If the macro is undefined, it means
|
|
particular files may or may not support the feature.
|
|
|
|
Since all the machines that support the GNU C library also support NFS,
|
|
one can never make a general statement about whether all file systems
|
|
support the @code{_POSIX_CHOWN_RESTRICTED} and @code{_POSIX_NO_TRUNC}
|
|
features. So these names are never defined as macros in the GNU C
|
|
library.
|
|
|
|
@comment unistd.h
|
|
@comment POSIX.1
|
|
@deftypevr Macro int _POSIX_CHOWN_RESTRICTED
|
|
If this option is in effect, the @code{chown} function is restricted so
|
|
that the only changes permitted to nonprivileged processes is to change
|
|
the group owner of a file to either be the effective group ID of the
|
|
process, or one of its supplementary group IDs. @xref{File Owner}.
|
|
@end deftypevr
|
|
|
|
@comment unistd.h
|
|
@comment POSIX.1
|
|
@deftypevr Macro int _POSIX_NO_TRUNC
|
|
If this option is in effect, file name components longer than
|
|
@code{NAME_MAX} generate an @code{ENAMETOOLONG} error. Otherwise, file
|
|
name components that are too long are silently truncated.
|
|
@end deftypevr
|
|
|
|
@comment unistd.h
|
|
@comment POSIX.1
|
|
@deftypevr Macro {unsigned char} _POSIX_VDISABLE
|
|
This option is only meaningful for files that are terminal devices.
|
|
If it is enabled, then handling for special control characters can
|
|
be disabled individually. @xref{Special Characters}.
|
|
@end deftypevr
|
|
|
|
@pindex unistd.h
|
|
If one of these macros is undefined, that means that the option might be
|
|
in effect for some files and not for others. To inquire about a
|
|
particular file, call @code{pathconf} or @code{fpathconf}.
|
|
@xref{Pathconf}.
|
|
|
|
@node File Minimums
|
|
@section Minimum Values for File System Limits
|
|
|
|
Here are the names for the POSIX minimum upper bounds for some of the
|
|
above parameters. The significance of these values is that you can
|
|
safely push to these limits without checking whether the particular
|
|
system you are using can go that far.
|
|
|
|
@table @code
|
|
@comment limits.h
|
|
@comment POSIX.1
|
|
@item _POSIX_LINK_MAX
|
|
The most restrictive limit permitted by POSIX for the maximum value of a
|
|
file's link count. The value of this constant is @code{8}; thus, you
|
|
can always make up to eight names for a file without running into a
|
|
system limit.
|
|
|
|
@comment limits.h
|
|
@comment POSIX.1
|
|
@item _POSIX_MAX_CANON
|
|
The most restrictive limit permitted by POSIX for the maximum number of
|
|
bytes in a canonical input line from a terminal device. The value of
|
|
this constant is @code{255}.
|
|
|
|
@comment limits.h
|
|
@comment POSIX.1
|
|
@item _POSIX_MAX_INPUT
|
|
The most restrictive limit permitted by POSIX for the maximum number of
|
|
bytes in a terminal device input queue (or typeahead buffer).
|
|
@xref{Input Modes}. The value of this constant is @code{255}.
|
|
|
|
@comment limits.h
|
|
@comment POSIX.1
|
|
@item _POSIX_NAME_MAX
|
|
The most restrictive limit permitted by POSIX for the maximum number of
|
|
bytes in a file name component. The value of this constant is
|
|
@code{14}.
|
|
|
|
@comment limits.h
|
|
@comment POSIX.1
|
|
@item _POSIX_PATH_MAX
|
|
The most restrictive limit permitted by POSIX for the maximum number of
|
|
bytes in a file name. The value of this constant is @code{255}.
|
|
|
|
@comment limits.h
|
|
@comment POSIX.1
|
|
@item _POSIX_PIPE_BUF
|
|
The most restrictive limit permitted by POSIX for the maximum number of
|
|
bytes that can be written atomically to a pipe. The value of this
|
|
constant is @code{512}.
|
|
@end table
|
|
|
|
@node Pathconf
|
|
@section Using @code{pathconf}
|
|
|
|
When your machine allows different files to have different values for a
|
|
file system parameter, you can use the functions in this section to find
|
|
out the value that applies to any particular file.
|
|
|
|
These functions and the associated constants for the @var{parameter}
|
|
argument are declared in the header file @file{unistd.h}.
|
|
|
|
@comment unistd.h
|
|
@comment POSIX.1
|
|
@deftypefun {long int} pathconf (const char *@var{filename}, int @var{parameter})
|
|
This function is used to inquire about the limits that apply to
|
|
the file named @var{filename}.
|
|
|
|
The @var{parameter} argument should be one of the @samp{_PC_} constants
|
|
listed below.
|
|
|
|
The normal return value from @code{pathconf} is the value you requested.
|
|
A value of @code{-1} is returned both if the implementation does not
|
|
impose a limit, and in case of an error. In the former case,
|
|
@code{errno} is not set, while in the latter case, @code{errno} is set
|
|
to indicate the cause of the problem. So the only way to use this
|
|
function robustly is to store @code{0} into @code{errno} just before
|
|
calling it.
|
|
|
|
Besides the usual file name errors (@pxref{File Name Errors}),
|
|
the following error condition is defined for this function:
|
|
|
|
@table @code
|
|
@item EINVAL
|
|
The value of @var{parameter} is invalid, or the implementation doesn't
|
|
support the @var{parameter} for the specific file.
|
|
@end table
|
|
@end deftypefun
|
|
|
|
@comment unistd.h
|
|
@comment POSIX.1
|
|
@deftypefun {long int} fpathconf (int @var{filedes}, int @var{parameter})
|
|
This is just like @code{pathconf} except that an open file descriptor
|
|
is used to specify the file for which information is requested, instead
|
|
of a file name.
|
|
|
|
The following @code{errno} error conditions are defined for this function:
|
|
|
|
@table @code
|
|
@item EBADF
|
|
The @var{filedes} argument is not a valid file descriptor.
|
|
|
|
@item EINVAL
|
|
The value of @var{parameter} is invalid, or the implementation doesn't
|
|
support the @var{parameter} for the specific file.
|
|
@end table
|
|
@end deftypefun
|
|
|
|
Here are the symbolic constants that you can use as the @var{parameter}
|
|
argument to @code{pathconf} and @code{fpathconf}. The values are all
|
|
integer constants.
|
|
|
|
@table @code
|
|
@comment unistd.h
|
|
@comment POSIX.1
|
|
@item _PC_LINK_MAX
|
|
Inquire about the value of @code{LINK_MAX}.
|
|
|
|
@comment unistd.h
|
|
@comment POSIX.1
|
|
@item _PC_MAX_CANON
|
|
Inquire about the value of @code{MAX_CANON}.
|
|
|
|
@comment unistd.h
|
|
@comment POSIX.1
|
|
@item _PC_MAX_INPUT
|
|
Inquire about the value of @code{MAX_INPUT}.
|
|
|
|
@comment unistd.h
|
|
@comment POSIX.1
|
|
@item _PC_NAME_MAX
|
|
Inquire about the value of @code{NAME_MAX}.
|
|
|
|
@comment unistd.h
|
|
@comment POSIX.1
|
|
@item _PC_PATH_MAX
|
|
Inquire about the value of @code{PATH_MAX}.
|
|
|
|
@comment unistd.h
|
|
@comment POSIX.1
|
|
@item _PC_PIPE_BUF
|
|
Inquire about the value of @code{PIPE_BUF}.
|
|
|
|
@comment unistd.h
|
|
@comment POSIX.1
|
|
@item _PC_CHOWN_RESTRICTED
|
|
Inquire about the value of @code{_POSIX_CHOWN_RESTRICTED}.
|
|
|
|
@comment unistd.h
|
|
@comment POSIX.1
|
|
@item _PC_NO_TRUNC
|
|
Inquire about the value of @code{_POSIX_NO_TRUNC}.
|
|
|
|
@comment unistd.h
|
|
@comment POSIX.1
|
|
@item _PC_VDISABLE
|
|
Inquire about the value of @code{_POSIX_VDISABLE}.
|
|
@end table
|
|
|
|
@node Utility Limits
|
|
@section Utility Program Capacity Limits
|
|
|
|
The POSIX.2 standard specifies certain system limits that you can access
|
|
through @code{sysconf} that apply to utility behavior rather than the
|
|
behavior of the library or the operating system.
|
|
|
|
The GNU C library defines macros for these limits, and @code{sysconf}
|
|
returns values for them if you ask; but these values convey no
|
|
meaningful information. They are simply the smallest values that
|
|
POSIX.2 permits.
|
|
|
|
@comment limits.h
|
|
@comment POSIX.2
|
|
@deftypevr Macro int BC_BASE_MAX
|
|
The largest value of @code{obase} that the @code{bc} utility is
|
|
guaranteed to support.
|
|
@end deftypevr
|
|
|
|
@comment limits.h
|
|
@comment POSIX.2
|
|
@deftypevr Macro int BC_SCALE_MAX
|
|
The largest value of @code{scale} that the @code{bc} utility is
|
|
guaranteed to support.
|
|
@end deftypevr
|
|
|
|
@comment limits.h
|
|
@comment POSIX.2
|
|
@deftypevr Macro int BC_DIM_MAX
|
|
The largest number of elements in one array that the @code{bc} utility
|
|
is guaranteed to support.
|
|
@end deftypevr
|
|
|
|
@comment limits.h
|
|
@comment POSIX.2
|
|
@deftypevr Macro int BC_STRING_MAX
|
|
The largest number of characters in one string constant that the
|
|
@code{bc} utility is guaranteed to support.
|
|
@end deftypevr
|
|
|
|
@comment limits.h
|
|
@comment POSIX.2
|
|
@deftypevr Macro int BC_DIM_MAX
|
|
The largest number of elements in one array that the @code{bc} utility
|
|
is guaranteed to support.
|
|
@end deftypevr
|
|
|
|
@comment limits.h
|
|
@comment POSIX.2
|
|
@deftypevr Macro int COLL_WEIGHTS_MAX
|
|
The largest number of weights that can necessarily be used in defining
|
|
the collating sequence for a locale.
|
|
@end deftypevr
|
|
|
|
@comment limits.h
|
|
@comment POSIX.2
|
|
@deftypevr Macro int EXPR_NEST_MAX
|
|
The maximum number of expressions that can be nested within parenthesis
|
|
by the @code{expr} utility.
|
|
@end deftypevr
|
|
|
|
@comment limits.h
|
|
@comment POSIX.2
|
|
@deftypevr Macro int LINE_MAX
|
|
The largest text line that the text-oriented POSIX.2 utilities can
|
|
support. (If you are using the GNU versions of these utilities, then
|
|
there is no actual limit except that imposed by the available virtual
|
|
memory, but there is no way that the library can tell you this.)
|
|
@end deftypevr
|
|
|
|
@comment limits.h
|
|
@comment POSIX.2
|
|
@deftypevr Macro int EQUIV_CLASS_MAX
|
|
The maximum number of weights that can be assigned to an entry of the
|
|
@code{LC_COLLATE} category @samp{order} keyword in a locale definition.
|
|
The GNU C library does not presently support locale definitions.
|
|
@end deftypevr
|
|
|
|
@node Utility Minimums
|
|
@section Minimum Values for Utility Limits
|
|
|
|
@table @code
|
|
@comment limits.h
|
|
@comment POSIX.2
|
|
@item _POSIX2_BC_BASE_MAX
|
|
The most restrictive limit permitted by POSIX.2 for the maximum value of
|
|
@code{obase} in the @code{bc} utility. Its value is @code{99}.
|
|
|
|
@comment limits.h
|
|
@comment POSIX.2
|
|
@item _POSIX2_BC_DIM_MAX
|
|
The most restrictive limit permitted by POSIX.2 for the maximum size of
|
|
an array in the @code{bc} utility. Its value is @code{2048}.
|
|
|
|
@comment limits.h
|
|
@comment POSIX.2
|
|
@item _POSIX2_BC_SCALE_MAX
|
|
The most restrictive limit permitted by POSIX.2 for the maximum value of
|
|
@code{scale} in the @code{bc} utility. Its value is @code{99}.
|
|
|
|
@comment limits.h
|
|
@comment POSIX.2
|
|
@item _POSIX2_BC_STRING_MAX
|
|
The most restrictive limit permitted by POSIX.2 for the maximum size of
|
|
a string constant in the @code{bc} utility. Its value is @code{1000}.
|
|
|
|
@comment limits.h
|
|
@comment POSIX.2
|
|
@item _POSIX2_COLL_WEIGHTS_MAX
|
|
The most restrictive limit permitted by POSIX.2 for the maximum number
|
|
of weights that can necessarily be used in defining the collating
|
|
sequence for a locale. Its value is @code{2}.
|
|
|
|
@comment limits.h
|
|
@comment POSIX.2
|
|
@item _POSIX2_EXPR_NEST_MAX
|
|
The most restrictive limit permitted by POSIX.2 for the maximum number
|
|
of expressions nested within parenthesis when using the @code{expr} utility.
|
|
Its value is @code{32}.
|
|
|
|
@comment limits.h
|
|
@comment POSIX.2
|
|
@item _POSIX2_LINE_MAX
|
|
The most restrictive limit permitted by POSIX.2 for the maximum size of
|
|
a text line that the text utilities can handle. Its value is
|
|
@code{2048}.
|
|
|
|
@comment limits.h
|
|
@comment POSIX.2
|
|
@item _POSIX2_EQUIV_CLASS_MAX
|
|
The most restrictive limit permitted by POSIX.2 for the maximum number
|
|
of weights that can be assigned to an entry of the @code{LC_COLLATE}
|
|
category @samp{order} keyword in a locale definition. Its value is
|
|
@code{2}. The GNU C library does not presently support locale
|
|
definitions.
|
|
@end table
|
|
|
|
@node String Parameters
|
|
@section String-Valued Parameters
|
|
|
|
POSIX.2 defines a way to get string-valued parameters from the operating
|
|
system with the function @code{confstr}:
|
|
|
|
@comment unistd.h
|
|
@comment POSIX.2
|
|
@deftypefun size_t confstr (int @var{parameter}, char *@var{buf}, size_t @var{len})
|
|
This function reads the value of a string-valued system parameter,
|
|
storing the string into @var{len} bytes of memory space starting at
|
|
@var{buf}. The @var{parameter} argument should be one of the
|
|
@samp{_CS_} symbols listed below.
|
|
|
|
The normal return value from @code{confstr} is the length of the string
|
|
value that you asked for. If you supply a null pointer for @var{buf},
|
|
then @code{confstr} does not try to store the string; it just returns
|
|
its length. A value of @code{0} indicates an error.
|
|
|
|
If the string you asked for is too long for the buffer (that is, longer
|
|
than @code{@var{len} - 1}), then @code{confstr} stores just that much
|
|
(leaving room for the terminating null character). You can tell that
|
|
this has happened because @code{confstr} returns a value greater than or
|
|
equal to @var{len}.
|
|
|
|
The following @code{errno} error conditions are defined for this function:
|
|
|
|
@table @code
|
|
@item EINVAL
|
|
The value of the @var{parameter} is invalid.
|
|
@end table
|
|
@end deftypefun
|
|
|
|
Currently there is just one parameter you can read with @code{confstr}:
|
|
|
|
@table @code
|
|
@comment unistd.h
|
|
@comment POSIX.2
|
|
@item _CS_PATH
|
|
This parameter's value is the recommended default path for searching for
|
|
executable files. This is the path that a user has by default just
|
|
after logging in.
|
|
@end table
|
|
|
|
The way to use @code{confstr} without any arbitrary limit on string size
|
|
is to call it twice: first call it to get the length, allocate the
|
|
buffer accordingly, and then call @code{confstr} again to fill the
|
|
buffer, like this:
|
|
|
|
@smallexample
|
|
@group
|
|
char *
|
|
get_default_path (void)
|
|
@{
|
|
size_t len = confstr (_CS_PATH, NULL, 0);
|
|
char *buffer = (char *) xmalloc (len);
|
|
|
|
if (confstr (_CS_PATH, buf, len + 1) == 0)
|
|
@{
|
|
free (buffer);
|
|
return NULL;
|
|
@}
|
|
|
|
return buffer;
|
|
@}
|
|
@end group
|
|
@end smallexample
|