libc/sys/wait.2

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.\"     @(#)wait.2	8.2 (Berkeley) 4/19/94
.\" $FreeBSD$
.\"
.Dd November 12, 2005
.Dt WAIT 2
.Os
.Sh NAME
.Nm wait ,
.Nm waitpid ,
.Nm wait4 ,
.Nm wait3
.Nd wait for process termination
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In sys/types.h
.In sys/wait.h
.Ft pid_t
.Fn wait "int *status"
.Ft pid_t
.Fn waitpid "pid_t wpid" "int *status" "int options"
.In sys/time.h
.In sys/resource.h
.Ft pid_t
.Fn wait3 "int *status" "int options" "struct rusage *rusage"
.Ft pid_t
.Fn wait4 "pid_t wpid" "int *status" "int options" "struct rusage *rusage"
.Sh DESCRIPTION
The
.Fn wait
function suspends execution of its calling process until
.Fa status
information is available for a terminated child process,
or a signal is received.
On return from a successful
.Fn wait
call,
the
.Fa status
area contains termination information about the process that exited
as defined below.
.Pp
The
.Fn wait4
system call provides a more general interface for programs
that need to wait for certain child processes,
that need resource utilization statistics accumulated by child processes,
or that require options.
The other wait functions are implemented using
.Fn wait4 .
.Pp
The
.Fa wpid
argument specifies the set of child processes for which to wait.
If
.Fa wpid
is -1, the call waits for any child process.
If
.Fa wpid
is 0,
the call waits for any child process in the process group of the caller.
If
.Fa wpid
is greater than zero, the call waits for the process with process id
.Fa wpid .
If
.Fa wpid
is less than -1, the call waits for any process whose process group id
equals the absolute value of
.Fa wpid .
.Pp
The
.Fa status
argument is defined below.
.Pp
The
.Fa options
argument contains the bitwise OR of any of the following options.
The
.Dv WCONTINUED
option indicates that children of the current process that
have continued from a job control stop, by receiving a
.Dv SIGCONT
signal, should also have their status reported.
The
.Dv WNOHANG
option
is used to indicate that the call should not block if
there are no processes that wish to report status.
If the
.Dv WUNTRACED
option is set,
children of the current process that are stopped
due to a
.Dv SIGTTIN , SIGTTOU , SIGTSTP ,
or
.Dv SIGSTOP
signal also have their status reported.
The
.Dv WSTOPPED
option is an alias for
.Dv WUNTRACED .
The
.Dv WNOWAIT
option keeps the process whose status is returned in a waitable state.
The process may be waited for again after this call completes.
.Pp
If
.Fa rusage
is non-zero, a summary of the resources used by the terminated
process and all its
children is returned (this information is currently not available
for stopped or continued processes).
.Pp
When the
.Dv WNOHANG
option is specified and no processes
wish to report status,
.Fn wait4
returns a
process id
of 0.
.Pp
The
.Fn waitpid
function is identical to
.Fn wait4
with an
.Fa rusage
value of zero.
The older
.Fn wait3
call is the same as
.Fn wait4
with a
.Fa wpid
value of -1.
.Pp
The following macros may be used to test the manner of exit of the process.
One of the first three macros will evaluate to a non-zero (true) value:
.Bl -tag -width Ds
.It Fn WIFCONTINUED status
True if the process has not terminated, and
has continued after a job control stop.
This macro can be true only if the wait call specified the
.Dv WCONTINUED
option).
.It Fn WIFEXITED status
True if the process terminated normally by a call to
.Xr _exit 2
or
.Xr exit 3 .
.It Fn WIFSIGNALED status
True if the process terminated due to receipt of a signal.
.It Fn WIFSTOPPED status
True if the process has not terminated, but has stopped and can be restarted.
This macro can be true only if the wait call specified the
.Dv WUNTRACED
option
or if the child process is being traced (see
.Xr ptrace 2 ) .
.El
.Pp
Depending on the values of those macros, the following macros
produce the remaining status information about the child process:
.Bl -tag -width Ds
.It Fn WEXITSTATUS status
If
.Fn WIFEXITED status
is true, evaluates to the low-order 8 bits
of the argument passed to
.Xr _exit 2
or
.Xr exit 3
by the child.
.It Fn WTERMSIG status
If
.Fn WIFSIGNALED status
is true, evaluates to the number of the signal
that caused the termination of the process.
.It Fn WCOREDUMP status
If
.Fn WIFSIGNALED status
is true, evaluates as true if the termination
of the process was accompanied by the creation of a core file
containing an image of the process when the signal was received.
.It Fn WSTOPSIG status
If
.Fn WIFSTOPPED status
is true, evaluates to the number of the signal
that caused the process to stop.
.El
.Sh NOTES
See
.Xr sigaction 2
for a list of termination signals.
A status of 0 indicates normal termination.
.Pp
If a parent process terminates without
waiting for all of its child processes to terminate,
the remaining child processes are assigned the parent
process 1 ID (the init process ID).
.Pp
If a signal is caught while any of the
.Fn wait
calls are pending,
the call may be interrupted or restarted when the signal-catching routine
returns,
depending on the options in effect for the signal;
see discussion of
.Dv SA_RESTART
in
.Xr sigaction 2 .
.Pp
The implementation queues one
.Dv SIGCHLD
signal for each child process whose
status has changed, if
.Fn wait
returns because the status of a child process is available, the pending
SIGCHLD signal associated with the process ID of the child process will
be discarded.
Any other pending
.Dv SIGCHLD
signals remain pending.
.Pp
If
.Dv SIGCHLD
is blocked,
.Fn wait
returns because the status of a child process is available, the pending
.Dv SIGCHLD
signal will be cleared unless another status of the child process
is available.
.Sh RETURN VALUES
If
.Fn wait
returns due to a stopped, continued,
or terminated child process, the process ID of the child
is returned to the calling process.
Otherwise, a value of \-1
is returned and
.Va errno
is set to indicate the error.
.Pp
If
.Fn wait4 ,
.Fn wait3 ,
or
.Fn waitpid
returns due to a stopped, continued,
or terminated child process, the process ID of the child
is returned to the calling process.
If there are no children not previously awaited,
-1 is returned with
.Va errno
set to
.Er ECHILD .
Otherwise, if
.Dv WNOHANG
is specified and there are
no stopped, continued or exited children,
0 is returned.
If an error is detected or a caught signal aborts the call,
a value of -1
is returned and
.Va errno
is set to indicate the error.
.Sh ERRORS
The
.Fn wait
function
will fail and return immediately if:
.Bl -tag -width Er
.It Bq Er ECHILD
The calling process has no existing unwaited-for
child processes.
.It Bq Er ECHILD
No status from the terminated child process is available
because the calling process has asked the system to discard
such status by ignoring the signal
.Dv SIGCHLD
or setting the flag
.Dv SA_NOCLDWAIT
for that signal.
.It Bq Er EFAULT
The
.Fa status
or
.Fa rusage
argument points to an illegal address.
(May not be detected before exit of a child process.)
.It Bq Er EINTR
The call was interrupted by a caught signal,
or the signal did not have the
.Dv SA_RESTART
flag set.
.El
.Sh SEE ALSO
.Xr _exit 2 ,
.Xr ptrace 2 ,
.Xr sigaction 2 ,
.Xr exit 3 ,
.Xr siginfo 3
.Sh STANDARDS
The
.Fn wait
and
.Fn waitpid
functions are defined by POSIX;
.Fn wait4
and
.Fn wait3
are not specified by POSIX.
The
.Fn WCOREDUMP
macro
and the ability to restart a pending
.Fn wait
call are extensions to the POSIX interface.
.Sh HISTORY
The
.Fn wait
function appeared in
.At v6 .