wait and waitpid - 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 – block until a child changes state (terminated, stopped, continued) and reap it. wait(&status) == waitpid(-1, &status, 0). pid selection: -1 any child (WAIT_ANY), 0 any child in caller’s process group (WAIT_MYPGRP), > 0 that child, < -1 process group -pid. Options: WNOHANG (return 0 instead of blocking when no status is available – the SIGCHLD-handler polling idiom), WUNTRACED, and WCONTINUED (with WIFCONTINUED).

Status macros – decode status only through: WIFEXITED, WEXITSTATUS (low 8 bits), WIFSIGNALED, WTERMSIG, WCOREDUMP (BSD, POSIX.1-2024), WIFSTOPPED, WSTOPSIG, WIFCONTINUED. The encoding is deliberately opaque – never bit-twiddle it.

Return and errors – PID of the reported child (0 with WNOHANG and nothing ready); -1 with EINTR (an unblocked signal or caught SIGCHLD interrupted the wait; with SA_RESTART the call may instead restart), ECHILD (no children left – the classic “all reaped” loop terminator), EINVAL.

glibc layer – cancellation points in multithreaded programs; wrap long waits in cleanup handlers. glibc’s wait is documented as a call to wait4(2); the raw waitid(2) syscall’s fifth rusage argument is hidden by the wrapper.

Example

int status;
pid_t w;
do { w = waitpid(pid, &status, WUNTRACED | WCONTINUED); }
while (w == -1 && errno == EINTR);
if (WIFEXITED(status))       printf("exit=%d\n", WEXITSTATUS(status));
else if (WIFSIGNALED(status)) printf("sig=%d\n", WTERMSIG(status));

References – wait(2); syscall view: wait4.