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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. |
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| WUR-REPORT | ||
This directory contains the sources of the GNU C Library. See the file "version.h" for what release version you have. The GNU C Library is the standard system C library for all GNU systems, and is an important part of what makes up a GNU system. It provides the system API for all programs written in C and C-compatible languages such as C++ and Objective C; the runtime facilities of other programming languages use the C library to access the underlying operating system. In GNU/Linux systems, the C library works with the Linux kernel to implement the operating system behavior seen by user applications. In GNU/Hurd systems, it works with a microkernel and Hurd servers. The GNU C Library implements much of the POSIX.1 functionality in the GNU/Hurd system, using configurations i[4567]86-*-gnu. The current GNU/Hurd support requires out-of-tree patches that will eventually be incorporated into an official GNU C Library release. When working with Linux kernels, this version of the GNU C Library requires Linux kernel version 2.6.16 or later. Also note that the shared version of the libgcc_s library must be installed for the pthread library to work correctly. The GNU C Library supports these configurations for using Linux kernels: aarch64*-*-linux-gnu alpha*-*-linux-gnu arm-*-linux-gnueabi i[4567]86-*-linux-gnu x86_64-*-linux-gnu Can build either x86_64 or x32 ia64-*-linux-gnu m68k-*-linux-gnu microblaze*-*-linux-gnu mips-*-linux-gnu mips64-*-linux-gnu powerpc-*-linux-gnu Hardware or software floating point, BE only. powerpc64*-*-linux-gnu Big-endian and little-endian. s390-*-linux-gnu s390x-*-linux-gnu sh[34]-*-linux-gnu sparc*-*-linux-gnu sparc64*-*-linux-gnu tilegx-*-linux-gnu tilepro-*-linux-gnu The code for other CPU configurations supported by volunteers outside of the core glibc maintenance effort is contained in the `ports' add-on, located in the `ports' subdirectory of the source tree. hppa-*-linux-gnu Not currently functional without patches. If you are interested in doing a port, please contact the glibc maintainers; see http://www.gnu.org/software/libc/ for more information. See the file INSTALL to find out how to configure, build, and install the GNU C Library. You might also consider reading the WWW pages for the C library at http://www.gnu.org/software/libc/. The GNU C Library is (almost) completely documented by the Texinfo manual found in the `manual/' subdirectory. The manual is still being updated and contains some known errors and omissions; we regret that we do not have the resources to work on the manual as much as we would like. For corrections to the manual, please file a bug in the `manual' component, following the bug-reporting instructions below. Please be sure to check the manual in the current development sources to see if your problem has already been corrected. Please see http://www.gnu.org/software/libc/bugs.html for bug reporting information. We are now using the Bugzilla system to track all bug reports. This web page gives detailed information on how to report bugs properly. The GNU C Library is free software. See the file COPYING.LIB for copying conditions, and LICENSES for notices about a few contributions that require these additional notices to be distributed. License copyright years may be listed using range notation, e.g., 2000-2013, indicating that every year in the range, inclusive, is a copyrightable year that would otherwise be listed individually.