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Use cached offset in ftell when reliable

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The cached offset is reliable to use in ftell when the stream handle
is active.  We can consider a stream as being active when there is
unflushed data.  However, even in this case, we can use the cached
offset only when the stream is not being written to in a+ mode,
because this case may have unflushed data and a stale offset; the
previous read could have sent it off somewhere other than the end of
the file.

There were a couple of adjustments necessary to get this to work.
Firstly, fdopen now ceases to use _IO_attach_fd because it sets the
offset cache to the current file position.  This is not correct
because there could be changes to the file descriptor before the
stream handle is activated, which would not get reflected.

A similar offset caching action is done in _IO_fwide, claiming that
wide streams have 'problems' with the file offsets.  There don't seem
to be any obvious problems with not having the offset cache available,
other than that it will have to be queried in a subsequent
read/write/seek.  I have removed this as well.

The testsuite passes successfully with these changes on x86_64.
This commit is contained in:
Siddhesh Poyarekar
2014-03-04 12:23:27 +05:30
parent 000232b9bc
commit fa3cd24827
5 changed files with 65 additions and 23 deletions
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16
libio/iofdopen.c
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@ -141,9 +141,6 @@ _IO_new_fdopen (fd, mode)
#ifdef _IO_MTSAFE_IO
new_f->fp.file._lock = &new_f->lock;
#endif
/* Set up initially to use the `maybe_mmap' jump tables rather than using
__fopen_maybe_mmap to do it, because we need them in place before we
call _IO_file_attach or else it will allocate a buffer immediately. */
_IO_no_init (&new_f->fp.file, 0, 0, &new_f->wd,
#ifdef _G_HAVE_MMAP
(use_mmap && (read_write & _IO_NO_WRITES))
@ -159,13 +156,12 @@ _IO_new_fdopen (fd, mode)
#if !_IO_UNIFIED_JUMPTABLES
new_f->fp.vtable = NULL;
#endif
if (_IO_file_attach ((_IO_FILE *) &new_f->fp, fd) == NULL)
{
_IO_setb (&new_f->fp.file, NULL, NULL, 0);
_IO_un_link (&new_f->fp);
free (new_f);
return NULL;
}
/* We only need to record the fd because _IO_file_init will have unset the
offset. It is important to unset the cached offset because the real
offset in the file could change between now and when the handle is
activated and we would then mislead ftell into believing that we have a
valid offset. */
new_f->fp.file._fileno = fd;
new_f->fp.file._flags &= ~_IO_DELETE_DONT_CLOSE;
_IO_mask_flags (&new_f->fp.file, read_write,