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waitid — wait for a child process to change state
int waitid(idtype_t idtype, id_t id, siginfo_t *infop, int options);
The waitid() function shall suspend the calling thread until one child of the process containing the calling thread changes state. It records the current state of a child in the structure pointed to by infop. The fields of the structure pointed to by infop are filled in as described for the SIGCHLD signal in <signal.h>. If a child process changed state prior to the call to waitid(), waitid() shall return immediately. If more than one thread is suspended in wait(), waitid(), or waitpid() waiting for termination of the same process, exactly one thread shall return the process status at the time of the target process termination.
The idtype and id arguments are used to specify which children waitid() waits for.
If idtype is P_PID, waitid() shall wait for the child with a process ID equal to ( pid_t)id.
If idtype is P_PGID, waitid() shall wait for any child with a process group ID equal to ( pid_t)id.
If idtype is P_ALL, waitid() shall wait for any children and id is ignored.
The options argument is used to specify which state changes waitid() shall wait for. It is formed by OR'ing together the following flags:
- Status shall be returned for any continued child process whose status either has not been reported since it continued from a job control stop or has been reported only by calls to waitid() with the WNOWAIT flag set.
- Wait for processes that have exited.
- Do not hang if no status is available; return immediately.
- Keep the process whose status is returned in infop in a waitable state. This shall not affect the state of the process; the process may be waited for again after this call completes.
Applications shall specify at least one of the flags WEXITED, WSTOPPED, or WCONTINUED to be OR'ed in with the options argument.
The application shall ensure that the infop argument points to a siginfo_t structure. If waitid() returns because a child process was found that satisfied the conditions indicated by the arguments idtype and options, then the structure pointed to by infop shall be filled in by the system with the status of the process; the si_signo member shall be set equal to SIGCHLD. If waitid() returns because WNOHANG was specified and status is not available for any process specified by idtype and id, then the si_signo and si_pid members of the structure pointed to by infop shall be set to zero and the values of other members of the structure are unspecified.
If WNOHANG was specified and status is not available for any process specified by idtype and id, 0 shall be returned. If waitid() returns due to the change of state of one of its children, 0 shall be returned. Otherwise, −1 shall be returned and errno set to indicate the error.
The waitid() function shall fail if:
- Status shall be returned for any child that has stopped upon receipt of a signal, and whose status either has not been reported since it stopped or has been reported only by calls to waitid() with the WNOWAIT flag set.
- The calling process has no existing unwaited-for child processes.
- The waitid() function was interrupted by a signal.
The following sections are informative.
Calls to waitid() with idtype equal to P_ALL will collect information about any child process. This may result in interactions with other interfaces that may be waiting for their own children (such as by use of system()). For this reason it is recommended that portable applications not use waitid() with idtype of P_ALL. See also APPLICATION USAGE for wait().
exec, exit(), wait()
The Base Definitions volume of POSIX.1‐2008, <signal.h>, <sys_wait.h>
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, Copyright (C) 2013 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.unix.org/online.html .
- An invalid value was specified for options, or idtype and id specify an invalid set of processes.
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