'\" et
.TH WAITID "3P" 2017 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\"
.SH 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.
.\"
.SH NAME
waitid
\(em wait for a child process to change state
.SH SYNOPSIS
.LP
.nf
#include <sys/wait.h>
.P
int waitid(idtype_t \fIidtype\fP, id_t \fIid\fP, siginfo_t *\fIinfop\fP, int \fIoptions\fP);
.fi
.SH DESCRIPTION
The
\fIwaitid\fR()
function shall obtain status information (see
.IR "Section 2.13" ", " "Status Information")
pertaining to termination, stop, and/or continue events in one of the
caller's child processes.
.P
The
\fIwaitid\fR()
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''):
.IP " *" 4
The status information is from one of the child processes in the set
of child processes specified by the
.IR idtype
and
.IR id
arguments.
.IP " *" 4
The state change in the status information matches one of the state
change flags set in the
.IR options
argument.
.P
If matching status information is available prior to the call to
\fIwaitid\fR(),
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.
.P
As described in
.IR "Section 2.13" ", " "Status Information",
the
\fIwaitid\fR()
function consumes the status information it obtains unless the
WNOWAIT flag is set in the
.IR options
argument.
.P
The behavior when multiple threads are blocked in
\fIwait\fR(),
\fIwaitid\fR(),
or
\fIwaitpid\fR()
is described in
.IR "Section 2.13" ", " "Status Information".
.P
The
\fIwaitid\fR()
function shall record the obtained status information in the structure
pointed to by
.IR infop .
The fields of the structure pointed to by
.IR infop
shall be filled in as described under ``Pointer to a Function'' in
.IR "Section 2.4.3" ", " "Signal Actions".
.P
The
.IR idtype
and
.IR id
arguments are used to specify which children
\fIwaitid\fR()
waits for.
.P
If
.IR idtype
is P_PID,
\fIwaitid\fR()
shall wait for the child with a process ID equal to
(\fBpid_t\fP)\fIid\fR.
.P
If
.IR idtype
is P_PGID,
\fIwaitid\fR()
shall wait for any child with a process group ID equal to
(\fBpid_t\fP)\fIid\fR.
.P
If
.IR idtype
is P_ALL,
\fIwaitid\fR()
shall wait for any children and
.IR id
is ignored.
.P
The
.IR options
argument is used to specify which state changes
\fIwaitid\fR()
shall wait for. It is formed by OR'ing together the following flags:
.IP WCONTINUED 12
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
\fIwaitid\fR()
with the WNOWAIT flag set.
.IP WEXITED 12
Wait for processes that have exited.
.IP WNOHANG 12
Do not hang if no status is available; return immediately.
.IP WNOWAIT 12
Keep the process whose status is returned in
.IR 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.
.IP WSTOPPED 12
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
\fIwaitid\fR()
with the WNOWAIT flag set.
.P
Applications shall specify at least one of the flags WEXITED, WSTOPPED,
or WCONTINUED to be OR'ed in with the
.IR options
argument.
.P
The application shall ensure that the
.IR infop
argument points to a
.BR siginfo_t
structure. If
\fIwaitid\fR()
returns because a child process was found that satisfied the conditions
indicated by the arguments
.IR idtype
and
.IR options ,
then the structure pointed to by
.IR infop
shall be filled in by the system with the status of the process; the
.IR si_signo
member shall be set equal to SIGCHLD.
If
\fIwaitid\fR()
returns because WNOHANG was specified and status is not available for
any process specified by
.IR idtype
and
.IR id ,
then the
.IR si_signo
and
.IR si_pid
members of the structure pointed to by
.IR infop
shall be set to zero and the values of other members of the structure
are unspecified.
.SH "RETURN VALUE"
If WNOHANG was specified and status is not available for any process
specified by
.IR idtype
and
.IR id ,
0 shall be returned. If
\fIwaitid\fR()
returns due to the change of state of one of its children, 0 shall be
returned. Otherwise, \-1 shall be returned and
.IR errno
set to indicate the error.
.SH ERRORS
The
\fIwaitid\fR()
function shall fail if:
.TP
.BR ECHILD
The calling process has no existing unwaited-for child processes.
.TP
.BR EINTR
The
\fIwaitid\fR()
function was interrupted by a signal.
.TP
.BR EINVAL
An invalid value was specified for
.IR options ,
or
.IR idtype
and
.IR id
specify an invalid set of processes.
.LP
.IR "The following sections are informative."
.SH EXAMPLES
None.
.SH "APPLICATION USAGE"
Calls to
\fIwaitid\fR()
with
.IR 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
\fIsystem\fR()).
For this reason it is recommended that portable applications not use
\fIwaitid\fR()
with idtype of P_ALL. See also APPLICATION USAGE for
\fIwait\fR().
.P
As specified in
.IR "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
\fIwait\fR(),
\fIwaitid\fR(),
or
\fIwaitpid\fR().
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
.BR [ECHILD] 
failure due to no remaining children in the set of waited-for children).
.SH RATIONALE
None.
.SH "FUTURE DIRECTIONS"
None.
.SH "SEE ALSO"
.IR "Section 2.4.3" ", " "Signal Actions",
.IR "Section 2.13" ", " "Status Information",
.IR "\fIexec\fR\^",
.IR "\fIexit\fR\^(\|)",
.IR "\fIwait\fR\^(\|)"
.P
The Base Definitions volume of POSIX.1\(hy2017,
.IR "\fB<signal.h>\fP",
.IR "\fB<sys_wait.h>\fP"
.\"
.SH 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 .
.PP
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 .