waitwaitpid

Waitwaitpid is not a single system call but a reference to the family of POSIX APIs used to reap state changes of child processes, typically involving the wait and waitpid functions. These calls let a parent process synchronize with its children and collect exit status information, helping to prevent zombie processes.

The wait function provides a simple, blocking mechanism. int wait(int *status) waits for any child process to terminate, blocking until one does. It returns the PID of the terminated child, or -1 on error. If status is non-null, it is filled with information about how the child terminated, such as normal exit, being killed by a signal, or a stop/continue event.

The waitpid function offers finer control. int waitpid(pid_t pid, int *status, int options) selects which child to wait for using pid. A positive pid waits for that specific child; pid == 0 waits for any child in the caller’s process group; pid < -1 waits for any child in the process group |pid|; pid == -1 waits for any child. The options parameter can include WNOHANG (nonblocking), WUNTRACED (report stopped children), WCONTINUED (report continued children), and, in some systems, WNOWAIT (don’t reap the child). The function returns the PID of the child that changed state, 0 if WNOHANG was used and no state change occurred, or -1 on error.

Status information is interpreted with macros such as WIFEXITED, WEXITSTATUS, WIFSIGNALED, WTERMSIG, WIFSTOPPED, WSTOPSIG, and WIFCONTINUED, which indicate how the child ended or changed state and any associated codes or signals.

Usage scenarios include implementing shells, daemons, and resource managers that need reliable child lifecycle handling. While waitpid is more flexible, wait remains a simple option for waiting on any child when fine-grained control is unnecessary.