#include <sys/types.h> #include <sys/wait.h> pid_t waitpid(pid_t pid, int *stat_loc, int options);
The waitpid() 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 waitpid(), wait(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 waitpid(), return will be immediate.
The pid argument specifies a set of child processes for which status is requested, as follows:
One instance of a SIGCHLD signal is queued for each child process whose status has changed. If waitpid() returns because the status of a child process is available, and WNOWAIT was not specified in options, 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 waitpid() will fail and set errno to ECHILD.
If waitpid() returns because the status of a child process is available, then that status may be evaluated with the macros defined by wait.h(3HEAD) If the calling process had specified a non-zero value of stat_loc, the status of the child process will be stored in the location pointed to by stat_loc.
The options argument is constructed from the bitwise-inclusive OR of zero or more of the following flags, defined in the header <sys/wait.h>:
If waitpid() returns because the status of a child process is available, it returns a value equal to the process ID of the child process for which status is reported. If waitpid() returns due to the delivery of a signal to the calling process, -1 is returned and errno is set to EINTR. If waitpid() was invoked with WNOHANG set in options, it has at least one child process specified by pid for which status is not available, and status is not available for any process specified by pid, then 0 is returned. Otherwise, -1 is returned and errno is set to indicate the error.
The waitpid() function will fail if:
With options equal to 0 and pid equal to (pid_t)-1, waitpid() is identical to wait(3C). The waitpid() function is implemented as a call to the more general waitid(2) function.
See attributes(5) for descriptions of the following attributes:
intro(2), exec(2), exit(2), fork(2), pause(2), sigaction(2), ptrace(3C), signal(3C), siginfo.h(3HEAD), wait(3C), wait.h(3HEAD), attributes(5), standards(5)