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PROLOG
This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface
may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface
may not be implemented on Linux.
NAME
waitid — wait for a child process to change state
SYNOPSIS
#include <sys/wait.h>
int waitid(idtype_t idtype, id_t id, siginfo_t *infop, int options);
DESCRIPTION
The waitid() function shall obtain status information (see Section 2.13, Status Information) pertaining
to termination, stop, and/or continue events in one of the caller's child processes.
The waitid() function shall cause the calling thread to become blocked until an error occurs or status
information becomes available to the calling thread that satisfies all of the following properties
(``matching status information''):
* The status information is from one of the child processes in the set of child processes specified by
the idtype and id arguments.
* The state change in the status information matches one of the state change flags set in the options
argument.
If matching status information is available prior to the call to waitid(), return shall be immediate. If
matching status information is available for two or more child processes, the order in which their status
is reported is unspecified.
As described in Section 2.13, Status Information, the waitid() function consumes the status information
it obtains unless the WNOWAIT flag is set in the options argument.
The behavior when multiple threads are blocked in wait(), waitid(), or waitpid() is described in Section
2.13, Status Information.
The waitid() function shall record the obtained status information in the structure pointed to by infop.
The fields of the structure pointed to by infop shall be filled in as described under ``Pointer to a
Function'' in Section 2.4.3, Signal Actions.
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:
WCONTINUED 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.
WEXITED Wait for processes that have exited.
WNOHANG Do not hang if no status is available; return immediately.
WNOWAIT 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.
WSTOPPED 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.
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.
RETURN VALUE
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.
ERRORS
The waitid() function shall fail if:
ECHILD The calling process has no existing unwaited-for child processes.
EINTR The waitid() function was interrupted by a signal.
EINVAL An invalid value was specified for options, or idtype and id specify an invalid set of processes.
The following sections are informative.
EXAMPLES
None.
APPLICATION USAGE
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().
As specified in Consequences of Process Termination, if the calling process has SA_NOCLDWAIT set or has
SIGCHLD set to SIG_IGN, then the termination of a child process will not cause status information to
become available to a thread blocked in wait(), waitid(), or waitpid(). Thus, a thread blocked in one of
the wait functions will remain blocked unless some other condition causes the thread to resume execution
(such as an [ECHILD] failure due to no remaining children in the set of waited-for children).
RATIONALE
None.
FUTURE DIRECTIONS
None.
SEE ALSO
Section 2.4.3, Signal Actions, Section 2.13, Status Information, exec, exit(), wait()
The Base Definitions volume of POSIX.1‐2017, <signal.h>, <sys_wait.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard
for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics
Engineers, Inc and The Open Group. 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.opengroup.org/unix/online.html .
Any typographical or formatting errors that appear in this page are most likely to have been introduced
during the conversion of the source files to man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .