Program with SIGFPE exception behaves differently under gdb

Question

I have a simple C program which behaves differently when debugged with gdb and not. The program is this:

#include <stdio.h>
#include <signal.h>

int main() {
    kill(getpid(), SIGFPE);
    printf("I'm happy.\n");
    return 0;
}

When run by itself, I get this very strange result:

ezyang@javelin:~$ ./mini
I'm happy.
ezyang@javelin:~$ echo $?
0

No error! That is not to say that the signal is not being fired, it is:

ezyang@javelin:~$ strace -e signal ./mini
kill(31950, SIGFPE)                     = 0
--- SIGFPE (Floating point exception) @ 0 (0) ---
I'm happy

When in GDB, things proceed differently:

ezyang@javelin:~/Dev/ghc-build-sandbox/libraries/unix/tests/libposix$ gdb ./mini
GNU gdb (GDB) 7.5.91.20130417-cvs-ubuntu
Copyright (C) 2013 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later 
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "x86_64-linux-gnu".
For bug reporting instructions, please see:
...
Reading symbols from /srv/code/ghc-build-sandbox/libraries/unix/tests/libposix/mini...(no debugging symbols found)...done.
(gdb) r
Starting program: /srv/code/ghc-build-sandbox/libraries/unix/tests/libposix/mini 
warning: no loadable sections found in added symbol-file system-supplied DSO at 0x7ffff7ffa000

Program received signal SIGFPE, Arithmetic exception.
0x00007ffff7a49317 in kill () at ../sysdeps/unix/syscall-template.S:81
81  ../sysdeps/unix/syscall-template.S: No such file or directory.
(gdb) c
Continuing.

Program terminated with signal SIGFPE, Arithmetic exception.
The program no longer exists.

Asking GDB to not stop makes no difference

(gdb) handle SIGFPE nostop
Signal        Stop  Print   Pass to program Description
SIGFPE        No    Yes Yes     Arithmetic exception
(gdb) r
Starting program: /srv/code/ghc-build-sandbox/libraries/unix/tests/libposix/mini 
warning: no loadable sections found in added symbol-file system-supplied DSO at 0x7ffff7ffa000

Program received signal SIGFPE, Arithmetic exception.

Program terminated with signal SIGFPE, Arithmetic exception.
The program no longer exists.

What's going on?! For one thing, why isn't the SIGFPE killing the program; for the second thing, why is GDB behaving differently?

Update. One thought is that the child process is inheriting the signal masks of the parent. However, as can be seen in this transcript, that clearly is not the case: This analysis was not correct, see below.

ezyang@javelin:~$ trap - SIGFPE
ezyang@javelin:~$ ./mini
I'm happy.

Update 2. A friend of mine points out that trap only reports signals as set by the shell itself, and not by any parent processes. So we tracked down the ignore masks of all the parents, and lo and behold, rxvt-unicode had SIGFPE masked. A friend confirmed he could reproduce when he ran the executable using rxvt-unicode.

Answer 1

Ignored signals are inherited across fork() and exec*():

$ ./mini
Floating point exception (core dumped)
$ trap '' SIGFPE
$ ./mini
I'm happy.
$ trap - SIGFPE
$ ./mini
Floating point exception (core dumped)

I discussed this privately with the question author. Debugging was complicated by the fact that bash saves and restores the signal mask from its parent process, and that the trap builtin only reports signals that were handled or ignored in the current shell, even though ignored signals inherited from the parent process will still take effect.

It turns out the root problem was that he was running the test inside urxvt, which links libperl, which unconditionally ignores SIGFPE.

Answer 2

Signal masks and signal dispositions of SIG_IGN are inherited by child processes. The only possibility I can think of is that your shell has SIGFPE masked or ignored for some reason and is not clearing this status before starting your program.