table of contents
WAIT(2) | System Calls Manual | WAIT(2) |
NAME¶
wait
, waitid
,
waitpid
, wait3
,
wait4
, wait6
—
LIBRARY¶
Standard C Library (libc, -lc)SYNOPSIS¶
#include <sys/types.h>
#include <sys/wait.h>
pid_t
wait
(int
*status);
pid_t
waitpid
(pid_t
wpid, int *status,
int options);
#include
<signal.h>
int
waitid
(idtype_t
idtype, id_t id,
siginfo_t *info,
int options);
#include
<sys/time.h>
#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
thread until status information is available for a child
process or a signal is received. On return from a successful
wait
() call, the status area
contains information about the process that reported a status change as
defined below.
The wait4
() and
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
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.
P_JAILID
- Waits for processes within a jail whose jail identifier is equal to id.
For the waitpid
() and
wait4
() functions, the single
wpid argument specifies the set of child processes for
which to wait.
- If wpid is -1, the call waits for any child process.
- If wpid is 0, 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.
WCONTINUED
- Report the status of selected processes that have continued from a job
control stop by receiving a
SIGCONT
signal. WNOHANG
- Do not block when there are no processes wishing to report status.
WUNTRACED
- Report the status of selected processes which are stopped due to a
SIGTTIN
,SIGTTOU
,SIGTSTP
, orSIGSTOP
signal. 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. 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. WNOWAIT
- Keep the process whose status is returned in a waitable state. The process may be waited for again after this call completes.
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
() and
wait6
() return a process id of 0.
The wait
() call is the same as
wait4
() with a wpid value of
-1, with an options value of zero, and a
rusage value of NULL
. The
waitpid
() function is identical to
wait4
() with an rusage value
of NULL
. The older wait3
()
call is the same as wait4
() with a
wpid value of -1. The wait4
()
function is identical to wait6
() with the flags
WEXITED
and WTRAPPED
set in
options and infop set to
NULL
.
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 (true) value:
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
WCONTINUED
option. 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)).
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 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
SA_RESTART
in sigaction(2).
The implementation queues one SIGCHLD
signal for each child process whose status has changed; if
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
SIGCHLD
signals remain pending.
If SIGCHLD
is blocked and
wait
() returns because the status of a child process
is available, the pending SIGCHLD
signal will be
cleared unless another status of the child process is available.
RETURN VALUES¶
Ifwait
() 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
errno is set to indicate the error.
If wait6
(),
wait4
(), wait3
(), or
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 errno set to
ECHILD
. Otherwise, if
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
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.
wait
() called with -1 to wait for any
child process will ignore a child that is referenced by a process descriptor
(see pdfork(2)). Specific processes can still be waited on
by specifying the process ID or descriptor (see
pdwait(4)).
ERRORS¶
Thewait
() function will fail and return immediately if:
- [
ECHILD
] - The calling process has no existing unwaited-for child processes.
- [
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
SIGCHLD
or setting the flagSA_NOCLDWAIT
for that signal. - [
EFAULT
] - The status or rusage argument points 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. - [
EINVAL
] - An invalid value was specified for options, or idtype and id do not specify a valid set of processes.
SEE ALSO¶
_exit(2), ptrace(2), sigaction(2), exit(3), siginfo(3)STANDARDS¶
Thewait
(), waitpid
(), and
waitid
() functions are defined by POSIX;
wait6
(), wait4
(), and
wait3
() are not specified by POSIX. The
WCOREDUMP
() macro is an extension to the POSIX
interface.
The ability to use the WNOWAIT
flag with
waitpid
() is an extension; POSIX only permits this
flag with waitid
().
HISTORY¶
Thewait
() function appeared in
Version 6 AT&T UNIX.
June 1, 2016 | Linux 4.9.0-9-amd64 |