Red Hat Bugzilla – Bug 46446
fflush(0) not thread safe
Last modified: 2016-11-24 10:08:01 EST
Calling fflush(0) in a multi-threaded application can crash it.
I will attach a program that can reproduce this problem; I've tried it
on both Red Hat Linux v6.2 and v7.1, both fully updated,
both dual Pentium 3 machines.
By inspecting the GNU libc code, it seems no locking is done in the
_IO_flush_all() call, so I guess closing a file during fflush(0) can
break the list-iteration done in that function.
The attached example crashes much faster on the v7.1 machine (which is
also faster); the v7.1 is updated vanilla, the v6.2 one uses GCC v2.95.3
as compiler. On the v7.1 machine, it crashes in a few seconds, on the 6.2
machine, it can take a few minutes before it crashes.
All crashes are SIGSEGV.
I know ISO C does not say anything about thread safety in the IO
library, but GNU libio/libc seems to be implemented with thread safety
Created attachment 22086 [details]
Threaded fflush(0) example code
Should be fixed in glibc-2.2.3-17.
At least I run the test for more than an hour without any problems.
Jakub, I think this bug can be closed -- it has worked correctly for
ages now ;-)