- wait for child process to change state
#include <wait.h> int waitid(idtype_t idtype, id_t id, siginfo_t *infop, int options);
The waitid() function suspends the calling process until one of its child processes changes state. It records the current state of a child in the structure pointed to by infop. It returns immediately if a child process changed state prior to the call.
The idtype and id arguments specify which children waitid() is to wait for, as follows:
If idtype is P_PID, waitid() waits for the child with a process ID equal to (pid_t)id.
If idtype is P_PGID, waitid() waits for any child with a process group ID equal to (pid_t)id.
If idtype is P_ALL, waitid() waits for any child and id is ignored.
The options argument is used to specify which state changes waitid() is to wait for. It is formed by bitwise OR operation of any of the following flags:
Return the status for any child that was stopped and has been continued.
Wait for process(es) to exit.
Keep the process in a waitable state.
Wait for and return the process status of any child that has stopped upon receipt of a signal.
Wait for traced process(es) to become trapped or reach a breakpoint (see ptrace(3C)).
The infop argument must point to a siginfo_t structure, as defined in siginfo.h(3HEAD). If waitid() returns because a child process was found that satisfies the conditions indicated by the arguments idtype and options, then the structure pointed to by infop will be filled by the system with the status of the process. The si_signo member will always be equal to SIGCHLD.
One instance of a SIGCHLD signal is queued for each child process whose status has changed. If waitid() 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 waitid() returns due to a change of state of one of its children and WNOHANG was not used, 0 is returned. Otherwise, -1 is returned and errno is set to indicate the error. If WNOHANG was used, 0 can be returned (indicating no error); however, no children may have changed state if info->si_pid is 0.
The waitid() function will fail if:
The set of processes specified by idtype and id does not contain any unwaited processes.
The infop argument points to an illegal address.
The waitid() function was interrupted due to the receipt of a signal by the calling process.
An invalid value was specified for options, or idtype and id specify an invalid set of processes.
See attributes(5) for descriptions of the following attributes: