manual: Move preadv/writev variants to Scatter-Gather section
Also remove a comment about performance. fwrite vs writev performance is a very complex topic and cannot be reduced to a simple advice based on transfer size.
This commit is contained in:
Florian Weimer
parent
5f9d19490b
commit
f6e965ee94
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@ -1,3 +1,9 @@
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2017-11-19 Florian Weimer <fweimer@redhat.com>
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* manual/llio.texi (I/O Primitives): Move preadv, preadv64,
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pwritev, pwritev64, preadv2, preadv64v2, pwritev2, pwritev64v2 ...
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(Scatter-Gather): ... to here. Remove misleading comment.
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2017-11-18 Christian Brauner <christian.brauner@ubuntu.com>
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* support/support_become_root.c (setup_uid_gid_mapping): Fix comment
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412
manual/llio.texi
412
manual/llio.texi
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@ -654,210 +654,6 @@ When the source file is compiled using @code{_FILE_OFFSET_BITS == 64} on a
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@code{pwrite} and so transparently replaces the 32 bit interface.
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@end deftypefun
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@deftypefun ssize_t preadv (int @var{fd}, const struct iovec *@var{iov}, int @var{iovcnt}, off_t @var{offset})
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@standards{BSD, sys/uio.h}
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@safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
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@c This is a syscall for Linux 3.2 for all architectures but microblaze
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@c (which was added on 3.15). The sysdeps/posix fallback emulation
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@c is also MT-Safe since it calls pread, and it is now a syscall on all
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@c targets.
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This function is similar to the @code{readv} function, with the difference
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it adds an extra @var{offset} parameter of type @code{off_t} similar to
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@code{pread}. The data is written to the file starting at position
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@var{offset}. The position of the file descriptor itself is not affected
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by the operation. The value is the same as before the call.
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When the source file is compiled with @code{_FILE_OFFSET_BITS == 64} the
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@code{preadv} function is in fact @code{preadv64} and the type
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@code{off_t} has 64 bits, which makes it possible to handle files up to
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@twoexp{63} bytes in length.
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The return value is a count of bytes (@emph{not} buffers) read, @math{0}
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indicating end-of-file, or @math{-1} indicating an error. The possible
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errors are the same as in @code{readv} and @code{pread}.
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@end deftypefun
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@deftypefun ssize_t preadv64 (int @var{fd}, const struct iovec *@var{iov}, int @var{iovcnt}, off64_t @var{offset})
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@standards{BSD, unistd.h}
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@safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
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@c This is a syscall for Linux 3.2 for all architectures but microblaze
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@c (which was added on 3.15). The sysdeps/posix fallback emulation
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@c is also MT-Safe since it calls pread64, and it is now a syscall on all
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@c targets.
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This function is similar to the @code{preadv} function with the difference
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is that the @var{offset} parameter is of type @code{off64_t} instead of
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@code{off_t}. It makes it possible on 32 bit machines to address
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files larger than @twoexp{31} bytes and up to @twoexp{63} bytes. The
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file descriptor @code{filedes} must be opened using @code{open64} since
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otherwise the large offsets possible with @code{off64_t} will lead to
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errors with a descriptor in small file mode.
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When the source file is compiled using @code{_FILE_OFFSET_BITS == 64} on a
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32 bit machine this function is actually available under the name
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@code{preadv} and so transparently replaces the 32 bit interface.
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@end deftypefun
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@deftypefun ssize_t pwritev (int @var{fd}, const struct iovec *@var{iov}, int @var{iovcnt}, off_t @var{offset})
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@standards{BSD, sys/uio.h}
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@safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
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@c This is a syscall for Linux 3.2 for all architectures but microblaze
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@c (which was added on 3.15). The sysdeps/posix fallback emulation
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@c is also MT-Safe since it calls pwrite, and it is now a syscall on all
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@c targets.
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This function is similar to the @code{writev} function, with the difference
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it adds an extra @var{offset} parameter of type @code{off_t} similar to
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@code{pwrite}. The data is written to the file starting at position
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@var{offset}. The position of the file descriptor itself is not affected
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by the operation. The value is the same as before the call.
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However, on Linux, if a file is opened with @code{O_APPEND}, @code{pwrite}
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appends data to the end of the file, regardless of the value of
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@code{offset}.
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When the source file is compiled with @code{_FILE_OFFSET_BITS == 64} the
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@code{pwritev} function is in fact @code{pwritev64} and the type
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@code{off_t} has 64 bits, which makes it possible to handle files up to
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@twoexp{63} bytes in length.
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The return value is a count of bytes (@emph{not} buffers) written, @math{0}
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indicating end-of-file, or @math{-1} indicating an error. The possible
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errors are the same as in @code{writev} and @code{pwrite}.
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@end deftypefun
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@deftypefun ssize_t pwritev64 (int @var{fd}, const struct iovec *@var{iov}, int @var{iovcnt}, off64_t @var{offset})
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@standards{BSD, unistd.h}
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@safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
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@c This is a syscall for Linux 3.2 for all architectures but microblaze
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@c (which was added on 3.15). The sysdeps/posix fallback emulation
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@c is also MT-Safe since it calls pwrite64, and it is now a syscall on all
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@c targets.
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This function is similar to the @code{pwritev} function with the difference
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is that the @var{offset} parameter is of type @code{off64_t} instead of
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@code{off_t}. It makes it possible on 32 bit machines to address
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files larger than @twoexp{31} bytes and up to @twoexp{63} bytes. The
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file descriptor @code{filedes} must be opened using @code{open64} since
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otherwise the large offsets possible with @code{off64_t} will lead to
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errors with a descriptor in small file mode.
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When the source file is compiled using @code{_FILE_OFFSET_BITS == 64} on a
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32 bit machine this function is actually available under the name
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@code{pwritev} and so transparently replaces the 32 bit interface.
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@end deftypefun
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@deftypefun ssize_t preadv2 (int @var{fd}, const struct iovec *@var{iov}, int @var{iovcnt}, off_t @var{offset}, int @var{flags})
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@standards{GNU, sys/uio.h}
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@safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
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@c This is a syscall for Linux v4.6. The sysdeps/posix fallback emulation
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@c is also MT-Safe since it calls preadv.
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This function is similar to the @code{preadv} function, with the difference
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it adds an extra @var{flags} parameter of type @code{int}. The supported
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@var{flags} are dependent of the underlying system. For Linux it supports:
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@vtable @code
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@item RWF_HIPRI
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High priority request. This adds a flag that tells the file system that
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this is a high priority request for which it is worth to poll the hardware.
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The flag is purely advisory and can be ignored if not supported. The
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@var{fd} must be opened using @code{O_DIRECT}.
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@item RWF_DSYNC
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Per-IO synchronization as if the file was opened with @code{O_DSYNC} flag.
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@item RWF_SYNC
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Per-IO synchronization as if the file was opened with @code{O_SYNC} flag.
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@item RWF_NOWAIT
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Use nonblocking mode for this operation; that is, this call to @code{preadv2}
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will fail and set @code{errno} to @code{EAGAIN} if the operation would block.
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@end vtable
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When the source file is compiled with @code{_FILE_OFFSET_BITS == 64} the
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@code{preadv2} function is in fact @code{preadv64v2} and the type
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@code{off_t} has 64 bits, which makes it possible to handle files up to
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@twoexp{63} bytes in length.
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The return value is a count of bytes (@emph{not} buffers) read, @math{0}
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indicating end-of-file, or @math{-1} indicating an error. The possible
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errors are the same as in @code{preadv} with the addition of:
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@table @code
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@item EOPNOTSUPP
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@c The default sysdeps/posix code will return it for any flags value
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@c different than 0.
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An unsupported @var{flags} was used.
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@end table
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@end deftypefun
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@deftypefun ssize_t preadv64v2 (int @var{fd}, const struct iovec *@var{iov}, int @var{iovcnt}, off64_t @var{offset}, int @var{flags})
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@standards{GNU, unistd.h}
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@safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
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@c This is a syscall for Linux v4.6. The sysdeps/posix fallback emulation
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@c is also MT-Safe since it calls preadv.
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This function is similar to the @code{preadv2} function with the difference
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is that the @var{offset} parameter is of type @code{off64_t} instead of
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@code{off_t}. It makes it possible on 32 bit machines to address
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files larger than @twoexp{31} bytes and up to @twoexp{63} bytes. The
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file descriptor @code{filedes} must be opened using @code{open64} since
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otherwise the large offsets possible with @code{off64_t} will lead to
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errors with a descriptor in small file mode.
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When the source file is compiled using @code{_FILE_OFFSET_BITS == 64} on a
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32 bit machine this function is actually available under the name
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@code{preadv2} and so transparently replaces the 32 bit interface.
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@end deftypefun
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@deftypefun ssize_t pwritev2 (int @var{fd}, const struct iovec *@var{iov}, int @var{iovcnt}, off_t @var{offset}, int @var{flags})
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@standards{GNU, sys/uio.h}
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@safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
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@c This is a syscall for Linux v4.6. The sysdeps/posix fallback emulation
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@c is also MT-Safe since it calls pwritev.
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This function is similar to the @code{pwritev} function, with the difference
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it adds an extra @var{flags} parameter of type @code{int}. The supported
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@var{flags} are dependent of the underlying system and for Linux it supports
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the same ones as for @code{preadv2}.
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When the source file is compiled with @code{_FILE_OFFSET_BITS == 64} the
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@code{pwritev2} function is in fact @code{pwritev64v2} and the type
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@code{off_t} has 64 bits, which makes it possible to handle files up to
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@twoexp{63} bytes in length.
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The return value is a count of bytes (@emph{not} buffers) write, @math{0}
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indicating end-of-file, or @math{-1} indicating an error. The possible
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errors are the same as in @code{preadv2}.
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@end deftypefun
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@deftypefun ssize_t pwritev64v2 (int @var{fd}, const struct iovec *@var{iov}, int @var{iovcnt}, off64_t @var{offset}, int @var{flags})
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@standards{GNU, unistd.h}
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@safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
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@c This is a syscall for Linux v4.6. The sysdeps/posix fallback emulation
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@c is also MT-Safe since it calls pwritev.
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This function is similar to the @code{pwritev2} function with the difference
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is that the @var{offset} parameter is of type @code{off64_t} instead of
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@code{off_t}. It makes it possible on 32 bit machines to address
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files larger than @twoexp{31} bytes and up to @twoexp{63} bytes. The
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file descriptor @code{filedes} must be opened using @code{open64} since
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otherwise the large offsets possible with @code{off64_t} will lead to
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errors with a descriptor in small file mode.
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When the source file is compiled using @code{_FILE_OFFSET_BITS == 64} on a
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32 bit machine this function is actually available under the name
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@code{pwritev2} and so transparently replaces the 32 bit interface.
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@end deftypefun
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@node File Position Primitive
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@section Setting the File Position of a Descriptor
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@ -1354,14 +1150,208 @@ error. The possible errors are the same as in @code{write}.
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@end deftypefun
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@c Note - I haven't read this anywhere. I surmised it from my knowledge
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@c of computer science. Thus, there could be subtleties I'm missing.
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@deftypefun ssize_t preadv (int @var{fd}, const struct iovec *@var{iov}, int @var{iovcnt}, off_t @var{offset})
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@standards{BSD, sys/uio.h}
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@safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
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@c This is a syscall for Linux 3.2 for all architectures but microblaze
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@c (which was added on 3.15). The sysdeps/posix fallback emulation
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@c is also MT-Safe since it calls pread, and it is now a syscall on all
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@c targets.
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Note that if the buffers are small (under about 1kB), high-level streams
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may be easier to use than these functions. However, @code{readv} and
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@code{writev} are more efficient when the individual buffers themselves
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(as opposed to the total output), are large. In that case, a high-level
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stream would not be able to cache the data efficiently.
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This function is similar to the @code{readv} function, with the difference
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it adds an extra @var{offset} parameter of type @code{off_t} similar to
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@code{pread}. The data is written to the file starting at position
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@var{offset}. The position of the file descriptor itself is not affected
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by the operation. The value is the same as before the call.
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When the source file is compiled with @code{_FILE_OFFSET_BITS == 64} the
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@code{preadv} function is in fact @code{preadv64} and the type
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@code{off_t} has 64 bits, which makes it possible to handle files up to
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@twoexp{63} bytes in length.
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The return value is a count of bytes (@emph{not} buffers) read, @math{0}
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indicating end-of-file, or @math{-1} indicating an error. The possible
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errors are the same as in @code{readv} and @code{pread}.
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@end deftypefun
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@deftypefun ssize_t preadv64 (int @var{fd}, const struct iovec *@var{iov}, int @var{iovcnt}, off64_t @var{offset})
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@standards{BSD, unistd.h}
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@safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
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@c This is a syscall for Linux 3.2 for all architectures but microblaze
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@c (which was added on 3.15). The sysdeps/posix fallback emulation
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@c is also MT-Safe since it calls pread64, and it is now a syscall on all
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@c targets.
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This function is similar to the @code{preadv} function with the difference
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is that the @var{offset} parameter is of type @code{off64_t} instead of
|
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@code{off_t}. It makes it possible on 32 bit machines to address
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files larger than @twoexp{31} bytes and up to @twoexp{63} bytes. The
|
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file descriptor @code{filedes} must be opened using @code{open64} since
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otherwise the large offsets possible with @code{off64_t} will lead to
|
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errors with a descriptor in small file mode.
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|
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When the source file is compiled using @code{_FILE_OFFSET_BITS == 64} on a
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32 bit machine this function is actually available under the name
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@code{preadv} and so transparently replaces the 32 bit interface.
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@end deftypefun
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@deftypefun ssize_t pwritev (int @var{fd}, const struct iovec *@var{iov}, int @var{iovcnt}, off_t @var{offset})
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@standards{BSD, sys/uio.h}
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@safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
|
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@c This is a syscall for Linux 3.2 for all architectures but microblaze
|
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@c (which was added on 3.15). The sysdeps/posix fallback emulation
|
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@c is also MT-Safe since it calls pwrite, and it is now a syscall on all
|
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@c targets.
|
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|
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This function is similar to the @code{writev} function, with the difference
|
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it adds an extra @var{offset} parameter of type @code{off_t} similar to
|
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@code{pwrite}. The data is written to the file starting at position
|
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@var{offset}. The position of the file descriptor itself is not affected
|
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by the operation. The value is the same as before the call.
|
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|
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However, on Linux, if a file is opened with @code{O_APPEND}, @code{pwrite}
|
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appends data to the end of the file, regardless of the value of
|
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@code{offset}.
|
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|
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When the source file is compiled with @code{_FILE_OFFSET_BITS == 64} the
|
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@code{pwritev} function is in fact @code{pwritev64} and the type
|
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@code{off_t} has 64 bits, which makes it possible to handle files up to
|
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@twoexp{63} bytes in length.
|
||||
|
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The return value is a count of bytes (@emph{not} buffers) written, @math{0}
|
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indicating end-of-file, or @math{-1} indicating an error. The possible
|
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errors are the same as in @code{writev} and @code{pwrite}.
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@end deftypefun
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@deftypefun ssize_t pwritev64 (int @var{fd}, const struct iovec *@var{iov}, int @var{iovcnt}, off64_t @var{offset})
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@standards{BSD, unistd.h}
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@safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
|
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@c This is a syscall for Linux 3.2 for all architectures but microblaze
|
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@c (which was added on 3.15). The sysdeps/posix fallback emulation
|
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@c is also MT-Safe since it calls pwrite64, and it is now a syscall on all
|
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@c targets.
|
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|
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This function is similar to the @code{pwritev} function with the difference
|
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is that the @var{offset} parameter is of type @code{off64_t} instead of
|
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@code{off_t}. It makes it possible on 32 bit machines to address
|
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files larger than @twoexp{31} bytes and up to @twoexp{63} bytes. The
|
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file descriptor @code{filedes} must be opened using @code{open64} since
|
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otherwise the large offsets possible with @code{off64_t} will lead to
|
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errors with a descriptor in small file mode.
|
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|
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When the source file is compiled using @code{_FILE_OFFSET_BITS == 64} on a
|
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32 bit machine this function is actually available under the name
|
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@code{pwritev} and so transparently replaces the 32 bit interface.
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@end deftypefun
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@deftypefun ssize_t preadv2 (int @var{fd}, const struct iovec *@var{iov}, int @var{iovcnt}, off_t @var{offset}, int @var{flags})
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@standards{GNU, sys/uio.h}
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@safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
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@c This is a syscall for Linux v4.6. The sysdeps/posix fallback emulation
|
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@c is also MT-Safe since it calls preadv.
|
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|
||||
This function is similar to the @code{preadv} function, with the difference
|
||||
it adds an extra @var{flags} parameter of type @code{int}. The supported
|
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@var{flags} are dependent of the underlying system. For Linux it supports:
|
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|
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@vtable @code
|
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@item RWF_HIPRI
|
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High priority request. This adds a flag that tells the file system that
|
||||
this is a high priority request for which it is worth to poll the hardware.
|
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The flag is purely advisory and can be ignored if not supported. The
|
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@var{fd} must be opened using @code{O_DIRECT}.
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@item RWF_DSYNC
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Per-IO synchronization as if the file was opened with @code{O_DSYNC} flag.
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@item RWF_SYNC
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Per-IO synchronization as if the file was opened with @code{O_SYNC} flag.
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@item RWF_NOWAIT
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Use nonblocking mode for this operation; that is, this call to @code{preadv2}
|
||||
will fail and set @code{errno} to @code{EAGAIN} if the operation would block.
|
||||
@end vtable
|
||||
|
||||
When the source file is compiled with @code{_FILE_OFFSET_BITS == 64} the
|
||||
@code{preadv2} function is in fact @code{preadv64v2} and the type
|
||||
@code{off_t} has 64 bits, which makes it possible to handle files up to
|
||||
@twoexp{63} bytes in length.
|
||||
|
||||
The return value is a count of bytes (@emph{not} buffers) read, @math{0}
|
||||
indicating end-of-file, or @math{-1} indicating an error. The possible
|
||||
errors are the same as in @code{preadv} with the addition of:
|
||||
|
||||
@table @code
|
||||
|
||||
@item EOPNOTSUPP
|
||||
|
||||
@c The default sysdeps/posix code will return it for any flags value
|
||||
@c different than 0.
|
||||
An unsupported @var{flags} was used.
|
||||
|
||||
@end table
|
||||
|
||||
@end deftypefun
|
||||
|
||||
@deftypefun ssize_t preadv64v2 (int @var{fd}, const struct iovec *@var{iov}, int @var{iovcnt}, off64_t @var{offset}, int @var{flags})
|
||||
@standards{GNU, unistd.h}
|
||||
@safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
|
||||
@c This is a syscall for Linux v4.6. The sysdeps/posix fallback emulation
|
||||
@c is also MT-Safe since it calls preadv.
|
||||
|
||||
This function is similar to the @code{preadv2} function with the difference
|
||||
is that the @var{offset} parameter is of type @code{off64_t} instead of
|
||||
@code{off_t}. It makes it possible on 32 bit machines to address
|
||||
files larger than @twoexp{31} bytes and up to @twoexp{63} bytes. The
|
||||
file descriptor @code{filedes} must be opened using @code{open64} since
|
||||
otherwise the large offsets possible with @code{off64_t} will lead to
|
||||
errors with a descriptor in small file mode.
|
||||
|
||||
When the source file is compiled using @code{_FILE_OFFSET_BITS == 64} on a
|
||||
32 bit machine this function is actually available under the name
|
||||
@code{preadv2} and so transparently replaces the 32 bit interface.
|
||||
@end deftypefun
|
||||
|
||||
|
||||
@deftypefun ssize_t pwritev2 (int @var{fd}, const struct iovec *@var{iov}, int @var{iovcnt}, off_t @var{offset}, int @var{flags})
|
||||
@standards{GNU, sys/uio.h}
|
||||
@safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
|
||||
@c This is a syscall for Linux v4.6. The sysdeps/posix fallback emulation
|
||||
@c is also MT-Safe since it calls pwritev.
|
||||
|
||||
This function is similar to the @code{pwritev} function, with the difference
|
||||
it adds an extra @var{flags} parameter of type @code{int}. The supported
|
||||
@var{flags} are dependent of the underlying system and for Linux it supports
|
||||
the same ones as for @code{preadv2}.
|
||||
|
||||
When the source file is compiled with @code{_FILE_OFFSET_BITS == 64} the
|
||||
@code{pwritev2} function is in fact @code{pwritev64v2} and the type
|
||||
@code{off_t} has 64 bits, which makes it possible to handle files up to
|
||||
@twoexp{63} bytes in length.
|
||||
|
||||
The return value is a count of bytes (@emph{not} buffers) write, @math{0}
|
||||
indicating end-of-file, or @math{-1} indicating an error. The possible
|
||||
errors are the same as in @code{preadv2}.
|
||||
@end deftypefun
|
||||
|
||||
@deftypefun ssize_t pwritev64v2 (int @var{fd}, const struct iovec *@var{iov}, int @var{iovcnt}, off64_t @var{offset}, int @var{flags})
|
||||
@standards{GNU, unistd.h}
|
||||
@safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
|
||||
@c This is a syscall for Linux v4.6. The sysdeps/posix fallback emulation
|
||||
@c is also MT-Safe since it calls pwritev.
|
||||
|
||||
This function is similar to the @code{pwritev2} function with the difference
|
||||
is that the @var{offset} parameter is of type @code{off64_t} instead of
|
||||
@code{off_t}. It makes it possible on 32 bit machines to address
|
||||
files larger than @twoexp{31} bytes and up to @twoexp{63} bytes. The
|
||||
file descriptor @code{filedes} must be opened using @code{open64} since
|
||||
otherwise the large offsets possible with @code{off64_t} will lead to
|
||||
errors with a descriptor in small file mode.
|
||||
|
||||
When the source file is compiled using @code{_FILE_OFFSET_BITS == 64} on a
|
||||
32 bit machine this function is actually available under the name
|
||||
@code{pwritev2} and so transparently replaces the 32 bit interface.
|
||||
@end deftypefun
|
||||
|
||||
@node Memory-mapped I/O
|
||||
@section Memory-mapped I/O
|
||||
|
|
|
|||
Loading…
Reference in New Issue