Issue 1461855: fdopen() not guaranteed to have Python semantics (original) (raw)

Issue1461855

Created on 2006-03-31 04:37 by movement, last changed 2022-04-11 14:56 by admin. This issue is now closed.

Messages (5)
msg27970 - (view)	Author: John Levon (movement)	Date: 2006-03-31 04:37
The specification for seek() says: seek( offset[, whence]) Note that if the file is opened for appending (mode 'a' or 'a+'), any seek() operations will be undone at the next write. Consider operating on an fdopen()ed file. The Python source simply calls into the OS-provided fdopen(): http://pxr.openlook.org/pxr/source/Modules/posixmodule.c#3530 However, the POSIX standard http://www.opengroup.org/onlinepubs/009695399/functions/fdopen.html says: "Although not explicitly required by this volume of IEEE Std 1003.1-2001, a good implementation of append (a) mode would cause the O_APPEND flag to be set." Thus, to ensure Python semantics, Python's fdopen() must perform an fcntl() to ensure O_APPEND is set. For example, on Solaris, this optional O_APPEND behaviour is not applied: http://cvs.opensolaris.org/source/xref/on/usr/src/lib/libc/port/stdio/fdopen.c?r=1.22#97 This has recently caused serious problems with the Mercurial SCM.
msg27971 - (view)	Author: Ralf Schmitt (titty)	Date: 2006-03-31 16:20
Logged In: YES user_id=17929 freebsd 4.11 shows the same behaviour. import fcntl import os def test_fdopen_append(): def is_append(fd): return bool(fcntl.fcntl(fd, fcntl.F_GETFL) & os.O_APPEND) fd = os.open("foo.txt", os.O_RDWR | os.O_CREAT) assert fd != -1 print "is_append:", is_append(fd) f=os.fdopen(fd, 'a') print "after fdopen is_append:", is_append(fd) assert is_append(fd) test_fdopen_append()
msg27972 - (view)	Author: Ralf Schmitt (titty)	Date: 2006-03-31 17:02
Logged In: YES user_id=17929 patch here: http://sourceforge.net/tracker/index.php?func=detail&aid=1462227&group_id=5470&atid=305470
msg27973 - (view)	Author: Georg Brandl (georg.brandl) *	Date: 2006-03-31 20:00
Logged In: YES user_id=849994 Applied patch in rev. 43501, 43502.
msg27974 - (view)	Author: John Levon (movement)	Date: 2006-03-31 20:15
Logged In: YES user_id=53034 Shouldn't the documentation now state this change in behaviour? i.e. if fdopen() fails, O_APPEND may still have become set. This behaviour seems a little odd to me in fact. Can't we set fcntl after a successful fdopen()[1]? Also wouldn't it be useful to point to the standards sentence I quote above? [1] not sure what the ALLOW_THREAD thing does?