lib/libsys/wait.2

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.Dd June 24, 2022
.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/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
.Fo wait6
.Fa "idtype_t idtype" "id_t id"
.Fa "int *status"
.Fa "int options"
.Fa "struct __wrusage *wrusage"
.Fa "siginfo_t *infop"
.Fc
.Sh DESCRIPTION
The
.Fn wait
function suspends execution of its calling thread until
.Fa status
information is available for a child process
or a signal is received.
On return from a successful
.Fn wait
call,
the
.Fa status
area contains information about the process that reported a status change
as defined below.
.Pp
The
.Fn wait4
and
.Fn wait6
system calls provide a more general interface for programs
that need to wait for specific child processes,
that need resource utilization statistics accumulated by child processes,
or that require options.
The other wait functions are implemented using either
.Fn wait4
or
.Fn wait6 .
.Pp
The
.Fn wait6
function is the most general function in this family and its distinct
features are:
.Pp
All of the desired process statuses to be waited on must be explicitly
specified in
.Fa options .
The
.Fn wait ,
.Fn waitpid ,
.Fn wait3 ,
and
.Fn wait4
functions all implicitly wait for exited and trapped processes,
but the
.Fn waitid
and
.Fn wait6
functions require the corresponding
.Dv WEXITED
and
.Dv WTRAPPED
flags to be explicitly specified.
This allows waiting for processes which have experienced other
status changes without having to also handle the exit status from
terminated processes.
.Pp
The
.Fn wait6
function accepts a
.Fa wrusage
argument which points to a structure defined as:
.Bd -literal
struct __wrusage {
	struct rusage   wru_self;
	struct rusage   wru_children;
};
.Ed
.Pp
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 .
.Pp
The last argument
.Fa infop
must be either
.Dv NULL
or a pointer to a
.Fa siginfo_t
structure.
If
.Pf non- Dv NULL ,
the structure is filled with the same data as for a
.Dv SIGCHLD
signal delivered when the process changed state.
.Pp
The set of child processes to be queried is specified by the arguments
.Fa idtype
and
.Fa id .
The separate
.Fa idtype
and
.Fa id
arguments support many other types of
identifiers in addition to process IDs and process group IDs.
.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 identifier types supported by this
implementation of
.Fn waitid
and
.Fn wait6
are:
.Bl -tag -width P_JAILID
.It Dv P_UID
Wait for processes whose effective user ID is equal to
.Dv (uid_t) Fa id .
.It Dv P_GID
Wait for processes whose effective group ID is equal to
.Dv (gid_t) Fa id .
.It Dv P_SID
Wait for processes whose session ID is equal to
.Fa id .
.\" This is just how sessions work, not sure this needs to be documented here
If the child process started its own session,
its session ID will be the same as its process ID.
Otherwise the session ID of a child process will match the caller's session ID.
.It Dv P_JAILID
Waits for processes within a jail whose jail identifier is equal to
.Fa id .
.El
.Pp
For the
.Fn waitpid
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 WCONTINUED
.It Dv WCONTINUED
Report the status of selected processes that
have continued from a job control stop by receiving a
.Dv SIGCONT
signal.
.It Dv WNOHANG
Do not block when
there are no processes wishing to report status.
.It Dv WUNTRACED
Report the status of selected processes which are stopped due to a
.Dv SIGTTIN , SIGTTOU , SIGTSTP ,
or
.Dv SIGSTOP
signal.
.It Dv WSTOPPED
An alias for
.Dv WUNTRACED .
.It Dv WTRAPPED
Report the status of selected processes which are being traced via
.Xr ptrace 2
and have trapped or reached a breakpoint.
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
.Fa options
if status reports from trapped processes are expected.
.It Dv WEXITED
Report the status of selected processes which have terminated.
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
.Fa options
if status reports from terminated processes are expected.
.It Dv WNOWAIT
Keep 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
is non-NULL, separate summaries are returned for the resources used
by the terminated process and the resources used by all its children.
.Pp
If
.Fa infop
is non-NULL, a
.Dv siginfo_t
structure is returned with the
.Fa si_signo
field set to
.Dv SIGCHLD
and the
.Fa si_pid
field set to the process ID of the process reporting status.
For the exited process, the
.Fa si_status
field of the
.Dv siginfo_t
structure contains the full 32 bit exit status passed to
.Xr _exit 2 ;
the
.Fa status
argument of other calls only returns 8 lowest bits of the exit status.
.Pp
When the
.Dv WNOHANG
option is specified and no processes
wish to report status,
.Fn waitid
sets the
.Fa si_signo
and
.Fa si_pid
fields in
.Fa infop
to zero.
Checking these fields is the only way to know if a status change was reported.
.Pp
When the
.Dv WNOHANG
option is specified and no processes
wish to report status,
.Fn wait4
and
.Fn wait6
return a
process id
of 0.
.Pp
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
.Dv NULL .
The
.Fn waitpid
function is identical to
.Fn wait4
with an
.Fa rusage
value of
.Dv NULL .
The older
.Fn wait3
call is the same as
.Fn wait4
with a
.Fa wpid
value of -1.
The
.Fn wait4
function is identical to
.Fn wait6
with the flags
.Dv WEXITED
and
.Dv WTRAPPED
set in
.Fa options
and
.Fa infop
set to
.Dv NULL .
.Pp
The following macros may be used to test the current status of the process.
Exactly one of the following four macros will evaluate to a non-zero
.Pq 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 re-assigned to the reaper
of the exiting process as the parent, see
.Xr procctl 2
.Dv PROC_REAP_ACQUIRE .
If no specific reaper was assigned, the process with ID 1, the init process,
becomes the parent of the orphaned children by default.
.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 and
.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.
If an error is detected,
a value of -1
is returned and
.Va errno
is set to indicate the error.
If
.Dv WNOHANG
is specified and there are
no stopped, continued or exited children,
0 is returned.
The
.Fa si_signo
and
.Fa si_pid
fields of
.Fa infop
must be checked against zero to determine if a process reported status.
.Pp
The
.Fn wait
family of functions will not return a child process created with
.Xr pdfork 2
unless specifically directed to do so by specifying its process ID.
.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 procctl 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 .
.Sh HISTORY
The
.Fn wait
function appeared in
.At v1 .