process_id

This returns the process number of the child process that exited. If no child exited, it returns 0. If there are no child processes, or if all child processes were already ended before the function was called, it returns -1.

option

This can have one of the following values:

• WTHANG: If the parent process ignores child signals (see signal()), the function waits until all children have exited. The process_id argument returns the process ID of the last child process to exit. If the parent process does not ignore child signals (see signal()), the function waits until one child exits. The process_id argument returns the process ID of that child process.
• WTNOHANG: If the parent process ignores child signals (see signal()), the function does not block. If one or more child processes have previously exited and are in a zombie state, the process_id argument returns the process of the last child process that exited. If the parent process does not ignore child signals (see signal()), the function waits until one child exits. The process_id argument returns the process ID of that child process.