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NAME
wait, waitid, waitpid, wait3, wait4, wait6 — wait for processes to change status
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
The wait() 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() and wait6() wait for the child process with a process
ID equal to (pid_t)id.
• If idtype is P_PGID, waitid() and wait6() wait for the child process with a
process group ID equal to (pid_t)id.
• If idtype is P_ALL, waitid() and wait6() wait for any child process and the id is
ignored.
• If idtype is P_PID or P_PGID and the id is zero, waitid() and wait6() 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, or SIGSTOP 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(), and wait4().
For the waitid() and wait6() 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(), and wait4().
For the waitid() and wait6() 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
If 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
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
The wait() 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 flag SA_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
The wait(), 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
The wait() function appeared in Version 6 AT&T UNIX.