WAITID
Section: POSIX Programmer's Manual (3P)
Updated: 2017
Page Index
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
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