NAME
wait
, waitid
,
waitpid
, wait6
,
wait4
, wait3
—
wait for process termination
LIBRARY
library “libc”
SYNOPSIS
#include
<sys/wait.h>
pid_t
wait
(int
*status);
pid_t
waitpid
(pid_t
wpid, int *status,
int options);
int
waitid
(idtype_t
idtype, id_t id,
siginfo_t *info,
int options);
#include
<sys/resource.h>
pid_t
wait3
(int
*status, int
options, struct rusage
*rusage);
pid_t
wait4
(pid_t
wpid, int *status,
int options,
struct rusage
*rusage);
pid_t
wait6
(idtype_t
idtype, id_t id,
int *status,
int options,
struct wrusage *wrusage,
siginfo_t *infop);
DESCRIPTION
Thewait
()
function suspends execution of its calling process until
status information is available for a terminated child
process, or a signal is received. On return from a successful
wait
() call, the status area
contains termination information about the process that exited as defined
below.
The
wait4
() and
wait6
() 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
wait4
(). or wait6
().
The
wait6
()
function is the most general function in this family and its distinct
features are:
All of the desired process statuses to be waited on
must be explicitly specified in options. The
wait
(),
waitpid
(),
wait3
(),
and wait4
() functions all implicitly wait for exited
and trapped processes, but the waitid
() and
wait6
() functions require the corresponding
WEXITED
and 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.
The
wait6
()
function accepts a wrusage argument which points to a
structure defined as:
struct wrusage { struct rusage wru_self; struct rusage wru_children; };
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
NULL
.
The last argument infop must be either
NULL
or a pointer to a
siginfo_t structure. If
non-NULL
, the structure is filled with the same data
as for a SIGCHLD
signal delivered when the process
changed state.
The set of child processes to be queried is specified by the arguments idtype and id. The separate idtype and id arguments support many other types of identifiers in addition to process IDs and process group IDs.
- If idtype is
P_PID
,waitid
() andwait6
() wait for the child process with a process ID equal to(pid_t)id
. - If idtype is
P_PGID
,waitid
() andwait6
() wait for the child process with a process group ID equal to(pid_t)id
. - If idtype is
P_ALL
,waitid
() andwait6
() wait for any child process and theid
is ignored. - If idtype is
P_PID
orP_PGID
and theid
is zero,waitid
() andwait6
() wait for any child process in the same process group as the caller.
Non-standard identifier types supported by this
implementation of
waitid
()
and wait6
() are:
P_UID
- Wait for processes whose effective user ID is equal to
(uid_t)
id. P_GID
- Wait for processes whose effective group ID is equal to
(gid_t)
id. P_SID
- Wait for processes whose session ID is equal to id. 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.
For the
waitpid
()
and wait4
() functions, the single
wpid argument specifies the set of child processes for
which to wait. The following symbolic constants are defined in
<sys/wait.h>
#define WAIT_ANY (-1) /* any process */ #define WAIT_MYPGRP 0 /* any process in my process group */
- If wpid is
WAIT_ANY
, the call waits for any child process. - If wpid is
WAIT_MYPGRP
, the call waits for any child process in the process group of the caller. - If wpid is greater than zero, the call waits for the process with process ID wpid.
- If wpid is less than -1, the call waits for any process whose process group ID equals the absolute value of wpid.
The status argument is defined below.
The options argument contains the bitwise OR of any of the following options.
WALLSIG
- If this option is specified, the call will wait for all children regardless of what exit signal they post.
WALTSIG
- If this option is specified, the call will wait only for processes that
are configured to post a signal other than
SIGCHLD
when they exit. IfWALTSIG
is not specified, the call will wait only for processes that are configured to postSIGCHLD
. WCONTINUED
- Report the status of selected processes that have continued from a job
control stop by receiving a
SIGCONT
signal. WEXITED
- Report the status of selected processes which have terminated. This flag
is implicitly set for the functions
wait
(),waitpid
(),wait3
(), andwait4
().
For thewaitid
() andwait6
() functions, the flag has to be explicitly included in options if status reports from terminated processes are expected. WNOHANG
- Do not block when there are no processes wishing to report status.
WNOWAIT
- Keep the process whose status is returned in a waitable state. The process may be waited for again after this call completes.
WNOZOMBIE
- Exclude zombie processes from the child selection criteria.
WSTOPPED
- An alias for
WUNTRACED
. WTRAPPED
- Report the status of selected processes which are being traced via
ptrace(2) and have trapped or reached a breakpoint. This flag is
implicitly set for the functions
wait
(),waitpid
(),wait3
(), andwait4
().
For thewaitid
() andwait6
() functions, the flag has to be explicitly included in options if status reports from trapped processes are expected. WUNTRACED
- Report the status of selected processes which are stopped due to a
SIGTTIN
,SIGTTOU
,SIGTSTP
, orSIGSTOP
signal. __WALL
- This is an alias for
WALLSIG
. It is provided for compatibility with the Linux clone(2) API . __WCLONE
- This is an alias for
WALTSIG
. It is provided for compatibility with the Linux clone(2) API.
For the waitid
() and
wait6
() functions, at least one of the options
WEXITED
, WUNTRACED
,
WSTOPPED
, WTRAPPED
, or
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 errno
set to
EINVAL
.
If rusage is
non-NULL
, a summary of the resources used by the
terminated process and all its children is returned.
If wrusage is
non-NULL
, separate summaries are returned for the
resources used by the terminated process and the resources used by all its
children.
If infop is
non-NULL
, a siginfo_t
structure is returned with the si_signo field set to
SIGCHLD
and the si_pid field
set to the process ID of the process reporting status. For the exited
process, the si_status field of the
siginfo_t
structure contains the full 32 bit exit
status passed to _exit(2); the status argument of other
calls only returns 8 lowest bits of the exit status.
When the WNOHANG
option is
specified and no processes wish to report status,
waitid
()
sets the si_signo and si_pid
fields in infop to zero. Checking these fields is the
only way to know if a status change was reported.
When the WNOHANG
option is
specified and no processes wish to report status,
wait4
()
returns a process id of 0.
The
waitpid
()
call is identical to wait4
() with an
rusage value of zero. The older
wait3
()
call is the same as wait4
() with a
wpid value of -1.
The following macros may be used to test the manner of exit of the process. Note that these macros expect the status value itself, not a pointer to the status value. One of the first three macros will evaluate to a non-zero (true) value:
WIFEXITED
(status)- True if the process terminated normally by a call to _exit(2) or exit(3).
WIFSIGNALED
(status)- True if the process terminated due to receipt of a signal.
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
WUNTRACED
option or if the child process is being traced (see ptrace(2)). WIFCONTINUED
(status)- True if the process has not terminated, but has been continued via the
delivery of the
SIGCONT
signal. This macro can be true only if the wait call specified theWCONTINUED
option.
Depending on the values of those macros, the following macros produce the remaining status information about the child process:
WEXITSTATUS
(status)- If
WIFEXITED
(status) is true, evaluates to the low-order 8 bits of the argument passed to _exit(2) or exit(3) by the child. WTERMSIG
(status)- If
WIFSIGNALED
(status) is true, evaluates to the number of the signal that caused the termination of the process. WCOREDUMP
(status)- If
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. WSTOPSIG
(status)- If
WIFSTOPPED
(status) is true, evaluates to the number of the signal that caused the process to stop.
NOTES
See sigaction(2) for a list of termination signals. A status of 0 indicates normal termination.
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).
If a signal is caught while any of the
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; see
siginterrupt(3).
RETURN VALUES
If 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
errno is set to indicate the error.
If wait6
(),
wait4
(), wait3
() or
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 errno set to [ECHILD
].
Otherwise, if 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
errno is set to indicate the error.
If 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 errno is set
to indicate the error. If WNOHANG
is specified and
there are no stopped, continued or exited children, 0 is returned. The
si_signo and si_pid fields of
infop must be checked against zero to determine if a
process reported status.
ERRORS
wait
() will fail and return immediately
if:
- [
ECHILD
] - The calling process has no existing unwaited-for child processes; or 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
SIGCHLD
or setting the flagSA_NOCLDWAIT
for that signal. - [
EFAULT
] - The status or rusage arguments point to an illegal address. (May not be detected before exit of a child process.)
- [
EINTR
] - The call was interrupted by a caught signal, or the signal did not have
the
SA_RESTART
flag set.
In addition, wait6
(),
wait3
(), wait4
(),
waitid
(), and waitpid
() will
fail and return immediately if:
- [
EINVAL
] - An invalid value was specified for options.
SEE ALSO
_exit(2), fork(2), ptrace(2), sigaction(2), siginfo(2), exit(3), siginterrupt(3)
STANDARDS
The wait
() and
waitpid
() functions conform to IEEE
Std 1003.1-1990 (“POSIX.1”); the
waitid
() function conforms to IEEE
Std 1003.1-2004 (“POSIX.1”); the
wait3
() function conforms to X/Open
Portability Guide Issue 4 (“XPG4”);
wait4
() is an extension. The
WCOREDUMP
() macro and the ability to restart a
pending wait
() call are extensions to the POSIX
interface.
HISTORY
A wait
() function call appeared in
Version 1 AT&T UNIX.