- wait for child process to stop or terminate
#include <sys/types.h> #include <sys/wait.h> pid_t wait(int *stat_loc);
The wait() function will suspend execution of the calling thread until status information for one of its terminated child processes is available, or until delivery of a signal whose action is either to execute a signal-catching function or to terminate the process. If more than one thread is suspended in wait(), waitpid(3C), or waitid(2) awaiting termination of the same process, exactly one thread will return the process status at the time of the target process termination. If status information is available prior to the call to wait(), return will be immediate.
If wait() returns because the status of a child process is available, it returns the process ID of the child process. If the calling process specified a non-zero value for stat_loc, the status of the child process is stored in the location pointed to by stat_loc. That status can be evaluated with the macros described on the wait.h(3HEAD) manual page.
In the following, status is the object pointed to by stat_loc:
If the child process terminated due to an _exit() call, the low order 8 bits of status will be 0 and the high order 8 bits will contain the low order 7 bits of the argument that the child process passed to _exit(); see exit(2).
If the child process terminated due to a signal, the high order 8 bits of status will be 0 and the low order 7bits will contain the number of the signal that caused the termination. In addition, if WCOREFLG is set, a “core image” will have been produced; see signal.h(3HEAD) and wait.h(3HEAD).
One instance of a SIGCHLD signal is queued for each child process whose status has changed. If wait() returns because the status of a child process is available, any pending SIGCHLD signal associated with the process ID of that child process is discarded. Any other pending SIGCHLD signals remain pending.
If the calling process has SA_NOCLDWAIT set or has SIGCHLD set to SIG_IGN, and the process has no unwaited children that were transformed into zombie processes, it will block until all of its children terminate, and wait() will fail and set errno to ECHILD.
If a parent process terminates without waiting for its child processes to terminate, the parent process ID of each child process is set to 1, with the initialization process inheriting the child processes; see Intro(2).
When wait() returns due to a terminated child process, the process ID of the child is returned to the calling process. Otherwise, -1 is returned and errno is set to indicate the error.
The wait() function will fail if:
The calling process has no existing unwaited-for child processes.
The function was interrupted by a signal.
Since wait() blocks on a stopped child, a calling process wanting to see the return results of such a call should use waitpid(3C) or waitid(2) instead of wait(). The wait() function is implemented as a call to waitpid(-1, stat_loc, 0).
See attributes(5) for descriptions of the following attributes: