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manual/llio.texi: Add Linux-specific comments for write().

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Add Linux-specific comments about the atomicity of write() and
the POSIX requirements.

2014-10-29  Carlos O'Donell  <carlos@redhat.com>

	* manual/llio.texi: Add comments discussing why write() may be
	considered MT-unsafe on Linux.
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Carlos O'Donell 2014-10-29 20:39:07 -04:00
parent cc00cecef5
commit 0c6891a003
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ChangeLog
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@ -1,3 +1,8 @@
2014-10-29 Carlos O'Donell <carlos@redhat.com>
* manual/llio.texi: Add comments discussing why write() may be
considered MT-unsafe on Linux.
2014-10-28 Carlos O'Donell <carlos@redhat.com>
* dl-load.c (local_strdup): Remove.

25
manual/llio.texi
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@ -466,6 +466,31 @@ When the source file is compiled with @code{_FILE_OFFSET_BITS == 64} on a
@comment POSIX.1
@deftypefun ssize_t write (int @var{filedes}, const void *@var{buffer}, size_t @var{size})
@safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
@c Some say write is thread-unsafe on Linux without O_APPEND. In the VFS layer
@c the vfs_write() does no locking around the acquisition of a file offset and
@c therefore multiple threads / kernel tasks may race and get the same offset
@c resulting in data loss.
@c
@c See:
@c http://thread.gmane.org/gmane.linux.kernel/397980
@c http://lwn.net/Articles/180387/
@c
@c The counter argument is that POSIX only says that the write starts at the
@c file position and that the file position is updated *before* the function
@c returns. What that really means is that any expectation of atomic writes is
@c strictly an invention of the interpretation of the reader. Data loss could
@c happen if two threads start the write at the same time. Only writes that
@c come after the return of another write are guaranteed to follow the other
@c write.
@c
@c The other side of the coin is that POSIX goes on further to say in
@c "2.9.7 Thread Interactions with Regular File Operations" that threads
@c should never see interleaving sets of file operations, but it is insane
@c to do anything like that because it kills performance, so you don't get
@c those guarantees in Linux.
@c
@c So we mark it thread safe, it doesn't blow up, but you might loose
@c data, and we don't strictly meet the POSIX requirements.
The @code{write} function writes up to @var{size} bytes from
@var{buffer} to the file with descriptor @var{filedes}. The data in
@var{buffer} is not necessarily a character string and a null character is