[RFC] fdopen("a"), O_APPEND and active handles

[Rich Felker]

On Wed, Feb 19, 2014 at 04:11:36PM +0530, Siddhesh Poyarekar wrote:
> Hi,
> 
> So while thinking over the third iteration of this fix, it occurred to
> be that even the act of adding O_APPEND to a file descriptor through
> fdopen will change the way subsequent write() calls behave, since they
> will suddenly start appending to the file instead of writing at the
> current location.  Given the fact that a handle's effect should not
> come into effect till it performs its first offset modifying act
> (i.e. fseek, read, write, etc.), this behaviour seems to be in
> violation of that expectation.

POSIX permits fdopen to set O_APPEND. See the text you seem to have
already found at:
http://pubs.opengroup.org/onlinepubs/9699919799/functions/fdopen.html

"RATIONALE

... Although not explicitly required by this volume of POSIX.1-2008, a
good implementation of append (a) mode would cause the O_APPEND flag
to be set."

Note that it's also conforming not to set O_APPEND at all, and to rely
on the application to have called fdopen with a mode matching the
existing state of the O_APPEND flag. Since an application cannot rely
on any particular behavior if the state of the O_APPEND flag did not
already match, I don't think there's any reason to worry about when
you set it; for correct applications, setting it will be a no-op
anyway.

Rich