A recent change to libio/tst-ftell-active-handler.c (postdating my
last check for warnings on x86) introduced a format warning from a
long int variable used with a %zu format. This patch fixes it by
using %ld for the format to match the variable.
Tested for x86.
* libio/tst-ftell-active-handler.c (do_ftruncate_test): Use %ld
format for long int variable.
On 'do_ftell_test' the code:
365 if (test_modes[i].fd_mode != O_WRONLY)
366 {
367 char tmpbuf[data_len];
368
369 rewind (fp);
370
371 while (fgets_func (tmpbuf, sizeof (tmpbuf), fp) && !feof (fp));
The 'data_len' is calculated with wsclen and allocated as 'char'. The
subsequent fgetws will then try to write at most 'data_len' wchar_t
in a buffer with just data_len 'char'. This patch fixes it by
allocating the tmpbuf using 'wchar_t' * data_len bytes.
POSIX allows applications to switch file handles when a read results
in an end of file. Unset the cached offset at this point so that it
is queried again.
The test case fails to truncate the file when a file is intended to be
opened in w or w+ mode. Add O_TRUNC to fix this. The test still
succeeds with this change.
Currently we seek to end of file if there are unflushed writes or the
stream is in write mode, to get the current offset for writing in
append mode, which is the end of file. The latter case (i.e. stream
is in write mode, but no unflushed writes) is unnecessary since it
will only happen when the stream has just been flushed, in which case
the recorded offset ought to be reliable.
Removing that case lets ftell give the correct offset when it follows
an ftruncate. The latter truncates the file, but does not change the
file position, due to which it is permissible to call ftell without an
intervening fseek call.
Tested on x86_64 to verify that the added test case fails without the
patch and succeeds with it, and that there are no additional
regressions due to it.
[BZ #17647]
* libio/fileops.c (do_ftell): Seek only when there are
unflushed writes.
* libio/wfileops.c (do_ftell_wide): Likewise.
* libio/tst-ftell-active-handler.c (do_ftruncate_test): New
test case.
(do_one_test): Call it.
fdopen should only be allowed to change the offset in the file it
attaches to if it is setting O_APPEND. If O_APPEND is already set, it
should not change the state of the handle.
The ftell implementation was made conservative to ensure that
incorrectly cached offsets never affect it. However, this causes
problems for append mode when a file stream is rewound. Additionally,
the 'clever' trick of using stat to get position for append mode files
caused more problems than it solved and broke old behavior. I have
described the various problems that it caused and then finally the
solution.
For a and a+ mode files, rewinding the stream should result in ftell
returning 0 as the offset, but the stat() trick caused it to
(incorrectly) always return the end of file. Now I couldn't find
anything in POSIX that specifies the stream position after rewind()
for a file opened in 'a' mode, but for 'a+' mode it should be set to
0. For 'a' mode too, it probably makes sense to keep it set to 0 in
the interest of retaining old behavior.
The initial file position for append mode files is implementation
defined, so the implementation could either retain the current file
position or move the position to the end of file. The earlier ftell
implementation would move the offset to end of file for append-only
mode, but retain the old offset for a+ mode. It would also cache the
offset (this detail is important). My patch broke this and would set
the initial position to end of file for both append modes, thus
breaking old behavior. I was ignorant enough to write an incorrect
test case for it too.
The Change:
I have now brought back the behavior of seeking to end of file for
append-only streams, but with a slight difference. I don't cache the
offset though, since we would want ftell to query the current file
position through lseek while the stream is not active. Since the
offset is moved to the end of file, we can rely on the file position
reported by lseek and we don't need to resort to the stat() nonsense.
Finally, the cache is always reliable, except when there are unflished
writes in an append mode stream (i.e. both a and a+). In the latter
case, it is safe to just do an lseek to SEEK_END. The value can be
safely cached too, since the file handle is already active at this
point. Incidentally, this is the only state change we affect in the
file handle (apart from taking locks of course).
I have also updated the test case to correct my impression of the
initial file position for a+ streams to the initial behavior. I have
verified that this does not break any existing tests in the testsuite
and also passes with the new tests.
The test functions used a variable ret to store failure codes for
individual tests, but the variable was incorrectly used to record
other failure codes too, resulting in overwriting of the tests status.
This is now fixed by making sure that the ret variable is used only
for recording test failures.
* libio/tst-ftell-active-handler.c (do_ftell_test): Don't mix
up test status with function return status.
(do_write_test): Likewise.
(do_append_test): Likewise.
ftell semantics are distinct from fseek(SEEK_CUR) especially when it
is called on a file handler that is not yet active. Due to this
caveat, much care needs to be taken while modifying the handler data
and hence, this first iteration on separating out ftell focusses on
maintaining handler data integrity at all times while it figures out
the current stream offset. The result is that it makes a syscall for
every offset request.
There is scope for optimizing this by caching offsets when we know
that the handler is active. A simple way to find out is when the
buffers have data. It is not so simple to find this out when the
buffer is empty without adding some kind of flag.
Joseph Myers
H.J. Lu
Adhemerval Zanella
Siddhesh Poyarekar