ftruncate is missing from core::libc

[Dretch]

rust does not seem to have the ftruncate function (http://pubs.opengroup.org/onlinepubs/7908799/xsh/ftruncate.html).

Might it be possible to add such a function? Some trickery would be needed on windows, like using _chsize instead (http://msdn.microsoft.com/en-us/library/dk925tyb%28v=vs.80%29.aspx).

Is there any reason this function shouldn't exist? If not then I will try and submit a pull request.

[brson]

@graydon knows best.

Personally I think if the function doesn't exist on windows we should not try to emulate it in core::libc, but at a higher-level API.

[Dretch]

It seems that actually ftruncate is available on windows, using mingw.

So I think just adding a ftruncate binding should work, but that will break when rust abandons mingw in favour of msvc (#1768). Is that desirable (i guess not)?

[Dretch]

I think that ideally the cross-platform ftruncate would use ftruncate64 on linux, ftruncate on OS X (which doesn't have ftruncate64, but instead always has a 64-bit off_t so ftruncate always takes a 64 bit length) and _chsize_s on windows (which also takes a 64bit length).

Unfortunately mingw does not have either ftruncate64 or _chsize_s.

[thestinger]

@Dretch: There's no need to use ftruncate64 on Linux, as long as _FILE_OFFSET_BITS=64 is used on 32-bit (and it should be anyway for functions like stat). The man page for ftruncate64 confirms that it's just a leftover legacy function.

[Aatch]

There is no need for this, we have a trunc method implemented on floats that uses trunc[f] from libm.

There is also a push to remove dependency on cmath/libm so I'm closing this.

[thestinger]

@Aatch: ftruncate is for truncating files :)

[msullivan]

Still isn't implemented. Are we decided this is something we want? Does windows support it? Should this actually be milestoned?

[thestinger]

Still not implemented.

[Dretch]

It looks like this is now implemented in std::io::fs, thanks!