fork - 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.

Processes

Semantics – duplicate the calling process: 0 in the child, child’s PID in the parent, -1/errno on failure. The child keeps copies of open file descriptors (sharing file offset and open-file status flags) and inherits the signal mask and dispositions, but starts with an empty pending-signal set, zeroed CPU-time counters, no inherited record locks (fcntl(2) locks; flock/OFD locks are inherited), no alarms, and – the part that matters for threads – only a single thread: the one that called fork.

glibc layer – the wrapper runs the pthread_atfork prepare handlers (reverse registration order), issues clone(SIGCHLD), then runs parent handlers in the parent and child handlers in the child (registration order), re-initializing internal locks so the child’s stdio/loader state is usable. That re-initialization is exactly what plain _Fork (GLIBC_2.34, verified via nm -D) skips: no atfork callbacks, no internal state reset – the child may call only async-signal-safe functions such as dup2 or execve.

Return and errors – EAGAIN (RLIMIT_NPROC, threads-max, pid_max, cgroup pids.max, or SCHED_DEADLINE without reset-on-fork), ENOMEM, ENOSYS (no-MMU platforms).

Edge cases – after fork in a multithreaded program the child may call only async-signal-safe functions until execve; mutexes locked by other threads stay locked forever in the child. vfork and (since 2.24) posix_spawn do not run atfork handlers.

Example

pid_t pid = fork();
if (pid == 0) {                 /* child: exec, don't linger */
   execl("/bin/ls", "ls", NULL);
   _exit(127);                 /* NOT exit(): would flush copied stdio */
}

References – fork(2), pthread_atfork(3), glibc manual Creating a Process; syscall view: clone.