libc/sys/wait.2

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.\"     @(#)wait.2	8.2 (Berkeley) 4/19/94
.\" $FreeBSD$
.\"
.Dd September 7, 2013
.Dt WAIT 2
.Os
.Sh NAME
.Nm wait ,
.Nm waitid ,
.Nm waitpid ,
.Nm wait3 ,
.Nm wait4 ,
.Nm wait6
.Nd wait for processes to change status
.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 signal.h
.Ft int
.Fn waitid "idtype_t idtype" "id_t id" "siginfo_t *info" "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"
.Ft pid_t
.Fn wait6 "idtype_t idtype" "id_t id" "int *status" "int options" "struct __wrusage *wrusage" "siginfo_t *infop"
.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.
The
.Fn wait
call is the same as
.Fn wait4
with a
.Fa wpid
value of -1,
with an
.Fa options
value of zero,
and a
.Fa rusage
value of zero.
.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.
.Pp
The broadest interface of all functions in this family is
.Fn wait6
which is otherwise very much like
.Fn wait4
but with a few very important distinctions.
To wait for exited processes, the option flag
.Dv WEXITED
need to be explicitly specified.
This allows for waiting for processes which have experienced other
status changes without having to handle also the exit status from
the terminated processes.
Instead of the traditional
.Dv rusage
argument, a pointer to a new structure
.Bd -literal
struct __wrusage {
	struct rusage   wru_self;
	struct rusage   wru_children;
};
.Ed
can be passed.
This allows the calling process to collect resource usage statistics
from both its own child process as well as from its grand children.
When no resource usage statistics are needed this pointer can be
.Dv NULL .
The last argument
.Fa infop
must be either
.Dv NULL
or a pointer to a
.Fa siginfo_t
structure.
When specified, the structure is filled the same as for
.Dv SIGNCHLD
signal, delivered at the process state change.
.br
The process, which state is queried, is specified by two arguments
.Fa idtype
and
.Fa id .
The separate
.Fa idtype
and
.Fa id
arguments allows to support many other types of
IDs  as well in addition to PID and PGID.
.Bl -bullet -offset indent
.It
If
.Fa idtype
is
.Dv P_PID ,
.Fn waitid
and
.Fn wait6
wait for the child process with a process ID equal to
.Dv (pid_t)id .
.It
If
.Fa idtype
is
.Dv P_PGID ,
.Fn waitid
and
.Fn wait6
wait for the child process with a process group ID equal to
.Dv (pid_t)id .
.It
If
.Fa idtype
is
.Dv P_ALL ,
.Fn waitid
and
.Fn wait6
wait for any child process and the
.Dv id
is ignored.
.It
If
.Fa idtype
is
.Dv P_PID
or
.Dv P_PGID
and the
.Dv id
is zero,
.Fn waitid
and
.Fn wait6
wait for any child process in the same process group as the caller.
.El
.Pp
Non-standard specifiers for the process to wait for, supported by this
implementation of
.Fn waitid
and
.Fn wait6 ,
are:
.Bl -bullet -offset indent
.It
The
.Fa idtype
value
.Dv P_UID
waits for processes which effective UID is equal to
.Dv (uid_t)id .
.It
The
.Fa idtype
value
.Dv P_GID
waits for processes which effective GID is equal to
.Dv (gid_t)id .
.It
The
.Fa idtype
value
.Dv P_SID
waits for processes which session ID is equal to
.Dv id .
In case the child process started its own new session,
SID will be the same as its own PID.
Otherwise the SID of a child process will match the caller's SID.
.It
The
.Fa idtype
value
.Dv P_JAILID
waits for processes within a jail which jail identifier is equal
to
.Dv id .
.El
.Pp
For
.Fn wait ,
.Fn wait3 ,
and
.Fn wait4
functions, the single
.Fa wpid
argument specifies the set of child processes for which to wait.
.Bl -bullet -offset indent
.It
If
.Fa wpid
is -1, the call waits for any child process.
.It
If
.Fa wpid
is 0,
the call waits for any child process in the process group of the caller.
.It
If
.Fa wpid
is greater than zero, the call waits for the process with process id
.Fa wpid .
.It
If
.Fa wpid
is less than -1, the call waits for any process whose process group id
equals the absolute value of
.Fa wpid .
.El
.Pp
The
.Fa status
argument is defined below.
.Pp
The
.Fa options
argument contains the bitwise OR of any of the following options.
.Bl -tag -width Ds
.It Dv WCONTINUED
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.
.It Dv WNOHANG
is used to indicate that the call should not block when
there are no processes wishing to report status.
.It Dv WUNTRACED
indicates that children of the current process which are stopped
due to a
.Dv SIGTTIN , SIGTTOU , SIGTSTP ,
or
.Dv SIGSTOP
signal shall have their status reported.
.It Dv WSTOPPED
is an alias for
.Dv WUNTRACED .
.It Dv WTRAPPED
allows waiting for processes which have trapped or reached a breakpoint.
.It Dv WEXITED
indicates that the caller is wants to receive status reports from
terminated processes.
This flag is implicitly set for the functions
.Fn wait ,
.Fn waitpid ,
.Fn wait3 ,
and
.Fn wait4 .
.br
For the
.Fn waitid
and
.Fn wait6
functions, the flag has to be explicitly included in the
.Fa options ,
if status reports from terminated processes are expected.
.It Dv WNOWAIT
keeps the process whose status is returned in a waitable state.
The process may be waited for again after this call completes.
.El
.sp
For the
.Fn waitid
and
.Fn wait6
functions, at least one of the options
.Dv WEXITED ,
.Dv WUNTRACED ,
.Dv WSTOPPED ,
.Dv WTRAPPED ,
or
.Dv WCONTINUED
must be specified.
Otherwise there will be no events for the call to report.
To avoid hanging indefinitely in such a case these functions
return -1 with
.Dv errno
set to
.Dv EINVAL .
.Pp
If
.Fa rusage
is non-NULL, a summary of the resources used by the terminated
process and all its children is returned.
.Pp
If
.Fa wrusage
argument is non-NULL, a resource usage statistics
from both its own child process as well as from its grand children
is returned.
.Pp
If
.Fa infop
is non-NULL, it must point to a
.Dv siginfo_t
structure which is filled on return such that the
.Dv si_signo
field is always
.Dv SIGCHLD
and the field
.Dv si_pid
if be non-zero, if there is a status change to report.
If there are no status changes to report and WNOHANG is applied,
both of these fields are returned zero.
When using the
.Fn waitid
function with the
.Dv WNOHANG
option set, checking these fields is the only way to know whether
there were any status changes to report, because the return value
from
.Fn waitid
is be zero as it is for any successful return from
.Fn waitid .
.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.
The
.Fn wait6
call, with the bits
.Dv WEXITED
and
.Dv WTRAPPED
set in the
.Fa options
and with
.Fa infop
set to
.Dv NULL ,
is similar to
.Fn wait4 .
.Pp
The following macros may be used to test the manner of exit of the process.
One of the first four 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 wait6 ,
.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.
.Pp
If
.Fn waitid
returns because one or more processes have a state change to report,
0 is returned.
To indicate an error, -1 will be returned and
.Dv errno
set to an appropriate value.
If
.Dv WNOHANG
was used, 0 can be returned indicating no error, but no processes
may have changed state either, if si_signo and/or si_pid are zero.
.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.
.It Bq Er EINVAL
An invalid value was specified for
.Fa options ,
or
.Fa idtype
and
.Fa id
do not specify a valid set of processes.
.El
.Sh SEE ALSO
.Xr _exit 2 ,
.Xr ptrace 2 ,
.Xr sigaction 2 ,
.Xr exit 3 ,
.Xr siginfo 3
.Sh STANDARDS
The
.Fn wait ,
.Fn waitpid ,
and
.Fn waitid
functions are defined by POSIX;
.Fn wait6 ,
.Fn wait4 ,
and
.Fn wait3
are not specified by POSIX.
The
.Fn WCOREDUMP
macro
is an extension to the POSIX interface.
.Pp
The ability to use the
.Dv WNOWAIT
flag with
.Fn waitpid
is an extension;
.Tn POSIX
only permits this flag with
.Fn waitid .
.Pp
.Tn POSIX
requires
.Fn waitid
to return the full 32 bits passed to
.Xr _exit 2 ;
this implementation only returns 8 bits like in the other calls.
.Sh HISTORY
The
.Fn wait
function appeared in
.At v6 .