[issue12545] Incorrect handling of return codes in the posix_lseek function in posixmodule.c

[Charles-François Natali]

Charles-François Natali <neologix at free.fr> added the comment:

Patch attached.

> For lseek, we can rely on errno. Try something like that:
>
> errno = 0;
> offset = lseek(...);
> if (offset == (off_t)-1 && errno) /* error */
>

It's a little bit overkill :-) (for mktime, time_t can overflow easily
on 32-bit).

> We can write a test using a sparse file... Or maybe a mmap object?
>

I'm not sure it's easily testable, because it's really a corner case
not addressed by POSIX. On my Linux box, I can't get lseek to return a
negative value, I get EINVAL - which does make sense (the Linux kernel
doesn't accept or return negative file offsets, see
http://lwn.net/Articles/138063/).

Kuberan, on what operating system did you notice this problem? Solaris?

----------
keywords: +patch
Added file: http://bugs.python.org/file22719/lseek_negative.diff

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<http://bugs.python.org/issue12545>
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-------------- next part --------------
diff -r d3f0f72c31f8 Modules/_io/fileio.c
--- a/Modules/_io/fileio.c	Fri Jul 22 11:10:43 2011 -0500
+++ b/Modules/_io/fileio.c	Fri Jul 22 22:33:54 2011 +0200
@@ -823,7 +823,7 @@
         Py_END_ALLOW_THREADS
     } else
         res = -1;
-    if (res < 0)
+    if (res == -1)
         return PyErr_SetFromErrno(PyExc_IOError);
 
 #if defined(HAVE_LARGEFILE_SUPPORT)
diff -r d3f0f72c31f8 Modules/posixmodule.c
--- a/Modules/posixmodule.c	Fri Jul 22 11:10:43 2011 -0500
+++ b/Modules/posixmodule.c	Fri Jul 22 22:33:54 2011 +0200
@@ -6257,7 +6257,7 @@
     res = lseek(fd, pos, how);
 #endif
     Py_END_ALLOW_THREADS
-    if (res < 0)
+    if (res == -1)
         return posix_error();
 
 #if !defined(HAVE_LARGEFILE_SUPPORT)