tags 366487 fixed-upstream
thanks
Paul,
> The man page for ftruncate(2) says:
>
> If the file previously was shorter, it is extended,
>
> However this is not always true. The glibc documentation says:
>
> The POSIX standard leaves it implementation defined what happens
> if the specified new LENGTH of the file is bigger than the
> original size. The `ftruncate' function might simply leave the
> file alone and do nothing
Yes. But did you see the NOTES section in the man page:
NOTES
The above description is for XSI-compliant systems.
For non-XSI-compliant systems, the POSIX standard
allows two behaviours for ftruncate() when length
exceeds the file length (note that truncate() is not
specified at all in such an environment): either
returning an error, or extending the file. (Most
Unices follow the XSI requirement.)
I've now reworded this a little in the light of the specific
example you mention:
NOTES
The above description is for XSI-compliant systems.
For non-XSI-compliant systems, the POSIX standard
allows two behaviours for ftruncate() when length
exceeds the file length (note that truncate() is not
specified at all in such an environment): either
returning an error, or extending the file. Like most
Unix implementations, Linux follows the XSI require‐
ment when dealing with native file systems. However,
some non-native file systems do not permit truncate()
and ftruncate() to be used to extend a file beyond its
current length: a notable example on Linux is VFAT.
By the way, the way, I'm puzzled that this limitation exists,
since VFAT does let us do things like lseek() way past EOF
and then writing, which is analogous to using [f]truncate()
to extend a file.
> Empirical evidence shows that Linux returns EPERM and leaves the file
> unchanged when trying to extend files on a vfat filesystem.
Yep, I've replicated this on SUSE 10 (kernel 2.6.13).
I added the following error text for EPERM:
EPERM The underlying file system does not support
extending a file beyond its current size.
The new text will be in man-pages-2.32.
Thanks for your report.
Cheers,
Michael
--
Michael Kerrisk
maintainer of Linux man pages Sections 2, 3, 4, 5, and 7
Want to help with man page maintenance?
Grab the latest tarball at
ftp://ftp.win.tue.nl/pub/linux-local/manpages/,
read the HOWTOHELP file and grep the source
files for 'FIXME'.
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