system - glibc

Split from the chapter glibc: Process, Threads and Signals into the per-function layout; see the method page and the glibc ledger for coverage status.

Semantics – run command through the shell and return its wait status. The documented signal dance in the caller for the duration of the command: SIGCHLD blocked, SIGINT and SIGQUIT ignored – set and restored via sigaction/sigprocmask internally, so the parent isn’t killed by Ctrl-C aimed at the child and doesn’t steal SIGCHLD from waitpid. The child gets default dispositions for those signals. POSIX.1 leaves it unspecified whether pthread_atfork handlers run; glibc does not run them.

Return encoding (interview favorite) –

• command == NULL: nonzero iff a shell is available (real /bin/sh check since glibc 2.1.3; before that always 1).
• -1 with errno: fork/waitpid failure.
• otherwise a wait status decode with WIFEXITED/WEXITSTATUS etc. – not an exit code. Shell-unavailable/exec-failure surfaces as if the shell called _exit(127); a command that legitimately exits 127 is indistinguishable from that.

Edge cases – never use from setuid/setgid programs (PATH and shell-environment subversion); system() is MT-Safe and a cancellation point; a command string starting with - is parsed by sh -c as an option (undefined; prepend a space); calls from a loop are uninterruptible unless you check WIFSIGNALED/WTERMSIG == SIGINT.

References – system(3), sh(1); glibc manual Running a Command.