system executes a command specified in string by calling /bin/sh -c string, and returns after the command has been completed. During execution of the command, SIGCHLD will be blocked, and SIGINT and SIGQUIT will be ignored.
The value 127 returned if the execve call for /bin/sh fails, -1 if there was another error and the return code of the command otherwise.
If the value of string is NULL, system returns a non-ZERO value if the shell is available, and ZERO if not.
system does not affect the wait status of any other children.
The fact that system ignores interrupts is often not what a program wants. The Single UNIX Specification describes some of the consequences; an additional consequence is that a program calling system from a loop cannot be reliably interrupted. Many programs will want to use the exec(3) family of functions instead.
Do not use system from a program with suid or sgid privileges, because strange values for some environment variables might be used to subvert system integrity. Use the exec(3) family of functions instead, but not execlp(3) or execvp(3). system will not, in fact, work properly from programs with suid or sgid privileges on systems on which /bin/sh is bash version 2, since bash 2 drops privileges on startup. (Debian uses a modified bash which does not do this when invoked as sh.)
The check for the availability of /bin/sh is not actually performed; it is always assumed to be available. ISO C specifies the check, but POSIX.2 specifies that the return shall always be non-ZERO, since a system without the shell is not conforming, and it is this that is implemented.
It is possible for the shell command to return 127, so that code is not a sure indication that the execve call failed; check the global variable errno to make sure.