libc/sys/wait.2

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.\"     @(#)wait.2	8.2 (Berkeley) 4/19/94
.\"
.Dd $Mdocdate: September 9 2014 $
.Dt WAIT 2
.Os
.Sh NAME
.Nm wait ,
.Nm waitpid ,
.Nm wait4 ,
.Nm wait3
.Nd wait for process termination
.Sh SYNOPSIS
.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/resource.h
.In sys/wait.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, if non-zero, is filled in with termination information about the
process that exited (see below).
.Pp
The
.Fn wait4
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
parameter specifies the set of child processes for which to wait.
The following symbolic constants are currently defined in
.In sys/wait.h :
.Bd -unfilled -offset indent
#define WAIT_ANY        (-1)    /* any process */
#define WAIT_MYPGRP     0       /* any process in my process group */
.Ed
.Pp
If
.Fa wpid
is set to
.Dv WAIT_ANY ,
the call waits for any child process.
If
.Fa wpid
is set to
.Dv WAIT_MYPGRP ,
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
parameter is defined below.
The
.Fa options
argument is the bitwise OR of zero or more of the following values:
.Bl -tag -width "WCONTINUED"
.It Dv WCONTINUED
Causes status to be reported for stopped child processes that have been
continued by receipt of a
.Dv SIGCONT
signal.
.It Dv WNOHANG
Indicates that the call should not block if there are no processes that wish
to report status.
.It Dv WUNTRACED
If 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.
.El
.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 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
call 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 is pending, the call may be interrupted or restarted when the
signal-catching routine returns, depending on the options in effect
for the signal; for further information, see
.Xr siginterrupt 3 .
.Sh RETURN VALUES
If
.Fn wait
returns due to a stopped
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 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
.Bq Er ECHILD .
Otherwise, if
.Dv WNOHANG
is specified and there are no stopped 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
.Fn wait
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 EFAULT
The
.Fa status
or
.Fa rusage
arguments point 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.
.It Bq Er EINVAL
Invalid or undefined flags were passed in the
.Fa options
argument.
.El
.Sh SEE ALSO
.Xr _exit 2 ,
.Xr sigaction 2 ,
.Xr exit 3
.Sh STANDARDS
The
.Fn wait
and
.Fn waitpid
functions conform to
.St -p1003.1-2008 .
.Pp
.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 that specification.
.Sh HISTORY
A
.Fn wait
system call first appeared in
.At v1 .
The
.Fa status
argument is accepted since
.At v2 .
A
.Fn wait3
system call first appeared in
.Bx 4 ,
but the final calling convention was only established in
.Bx 4.2 .
The
.Fn wait4
and
.Fn waitpid
function calls first appeared in
.Bx 4.3 Reno .