shell - C++: system(0) Returns 0

Question

When I call system(0) it returns 0, which means shell is not available.

When a command using system (calls gcc to compile a hello world program), it works just fine but returns -1 (I assume it's the same cause as for system(0) returning 0). What causes it to be "not available" and why does it work anyway?

Compiler: gcc version 4.6.1 (Ubuntu/Linaro 4.6.1-9ubuntu3)

OS: Ubuntu 11.10

EDIT

Errno says No Child Processes.

I have also compiled another test program that does not do anything but calls system(0) and it returns nonzero value. Can it be affected by Code::Blocks?

EDIT

So far I have figured out that system(0) only fails after I start my first pthread.

SOLVED

I used fork in my early implementation of one of this piece of code:

signal(SIGCHLD,SIG_IGN);

I used it to "handle" zombies. I just found out that this caused the error.

Answer 1

This could happen if you are ignoring the SIGCHLD signal using code like :

signal(SIGCHLD, SIG_IGN);

This would cause system to return -1 when all children have ended, setting errno to ECHILD.

Refer to http://pubs.opengroup.org/onlinepubs/009695399/functions/wait.html. Specifically :

If the calling process has SA_NOCLDWAIT set or has SIGCHLD set to SIG_IGN, and the process has no unwaited-for children that were transformed into zombie processes, the calling thread shall block until all of the children of the process containing the calling thread terminate, and wait() and waitpid() shall fail and set errno to [ECHILD].