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posix_fallocate.3: Mention glibc emulation caveats 

When the glibc implementation of posix_fallocate detects
that the underlying filesystem does not support fallocate()
it uses an emulation function to attempt to allocate the
space requested. The most common case is calling
posix_fallocatei() for a file that is on NFS where the
NFS server is not new enough to support the recent fallocate
extensions. This emulation has various serious caveats that
must be understood in order to use posix_fallocate robustly
on all filesystems. The change documents the caveats in the
glibc implementation.

Lastly, we expand the meaning of EINVAL to match POSIX
2013 (Issue 7). If the underlying filesystem doesn't support
posix_fallocate()i, the implementation can return EINVAL, but
glibc does not do this, it emulates the operation instead.

Signed-off-by: Carlos O'Donell <carlos@redhat.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
This commit is contained in:
Carlos O'Donell 2015-10-02 00:17:43 -04:00 committed by Michael Kerrisk
parent 422dd47fd8
commit a74bc4c9e4
1 changed files with 26 additions and 1 deletions
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27
man3/posix_fallocate.3
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@ -83,7 +83,8 @@ exceeds the maximum file size.
.I offset
was less than 0, or
.I len
was less than or equal to 0.
was less than or equal to 0, or the underlying filesystem does not
support the operation.
.TP
.B ENODEV
.I fd
@ -142,6 +143,30 @@ In the glibc implementation,
.BR posix_fallocate ()
is implemented using
.BR fallocate (2).
If the underlying filesystem does not support the
.BR fallocate (2)
syscall then the operation is emulated with the following caveats:
.IP * 2
The emulation is inefficient.
.IP *
There is a race condition where concurrent writes from another thread or
process could be overwritten with null bytes.
.IP *
There is a race condition where concurrent file size increase by
another thread or process could result in a file whose size is smaller
than expected.
.IP *
If fd has been opened with the O_APPEND or O_WRONLY flags the function
will fail with
.B EBADF.
.PP
In general the emulation is not MT-safe. On Linux, applications may use
.BR fallocate (2)
if they cannot work around the emulation caveats. In general this is
only recommended if the application plans to terminate the operation if
.B EOPNOTSUPP
is returned, otherwise the application itself will need to implement an
fallback with all the same problems as the emulation provided by glibc.
.SH SEE ALSO
.BR fallocate (1),
.BR fallocate (2),