[forwarded from http://bugs.debian.org/195834] | troup@merulo:~/gdbm-1.8.3$ gcc -O2 -Wall -c gdbmopen.c | gdbmopen.c: In function `gdbm_open': | gdbmopen.c:15: warning: `lock_val' might be used uninitialized in this function | fstat (dbf->desc, &file_stat); | | if ((flags & GDBM_OPENMASK) == GDBM_READER) | { | if (dbf->file_locking) | { | struct flock flock; | flock.l_type = F_RDLCK; | flock.l_whence = SEEK_SET; | flock.l_start = flock.l_len = 0L; | lock_val = fcntl (dbf->desc, F_SETLK, &flock); | } | } | else if (dbf->file_locking) | { | struct flock flock; | flock.l_type = F_WRLCK; | flock.l_whence = SEEK_SET; | flock.l_start = flock.l_len = 0L; | lock_val = fcntl (dbf->desc, F_SETLK, &flock); | } | if (dbf->file_locking && (lock_val != 0)) ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ | { | return NULL; | } I think this warning is bogus because a) it only happens on ia64 (AFAICT, it doesn't happen on arm, i386 or hppa, at least), b) it only happens with optimization turned on (disappears with -O0) and c) if the (entirely unrelated) fstat call is commented out, the warning disappears. I also think that it's clear from the code above that lock_val couldn't be used uninitialized unless I'm missing something embarrassingly obvious. Unfortunately I haven't been able to determine if it's just a bogus warning or if the code's actually being mis-compiled. This isn't a regression, all previous versions of gcc (2.96, 3.0 and 3.2) for ia64 have the same problem and gcc-snapshot (20030531-2) doesn't fix it. http://people.debian.org/~troup/gcc/gdbm/ contains gdbmopen.{c,i}, fixed-gdbmopen.{c,i} (i.e. with the fstat commented out) and orig-gdbmopen.{c,i} (the unreduced original file from gdbm 1.8.3).
This is a know documented defect of gcc's warning about uninitialized variables. In fact gcc should warn about it on all platforms but it does not. Here is the sample from the documenation: { int save_y; if (change_y) save_y = y, y = new_y; ... if (change_y) y = save_y; } which is almost like your code but slightly different so this is not a bug.