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.
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
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
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
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.
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 waitid() function shall fail if:
ECHILD The calling process has no existing unwaited-for child
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.
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().
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 .
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
IEEE/The Open Group 2013 WAITID(3P)