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wait, waitid,

Standard C Library (libc, -lc)

#include <sys/types.h>
#include <sys/wait.h>

pid_t
wait(int *status);

pid_t
waitpid(pid_t wpid, int *status, int options);

#include <sys/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);

The () 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 wait() call is the same as wait4() with a wpid value of -1, with an options value of zero, and a rusage value of zero.

The () 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.

The broadest interface of all functions in this family is () which is otherwise very much like wait4() but with a few very important distinctions. To wait for exited processes, the option flag 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 rusage argument, a pointer to a new structure

struct __wrusage {
	struct rusage   wru_self;
	struct rusage   wru_children;
};
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 NULL. The last argument infop must be either NULL or a pointer to a siginfo_t structure. When specified, the structure is filled the same as for SIGNCHLD signal, delivered at the process state change.
The process, which state is queried, is specified by two arguments idtype and id. The separate idtype and id arguments allows to support many other types of IDs as well in addition to PID and PGID.
  • 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 specifiers for the process to wait for, supported by this implementation of () and wait6(), are:

  • The idtype value P_UID waits for processes which effective UID is equal to (uid_t)id.
  • The idtype value P_GID waits for processes which effective GID is equal to (gid_t)id.
  • The idtype value P_SID waits for processes which session ID is equal to 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.
  • The idtype value P_JAILID waits for processes within a jail which jail identifier is equal to id.

For (), (), 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.

indicates that children of the current process that have continued from a job control stop, by receiving a SIGCONT signal, should also have their status reported.
is used to indicate that the call should not block when there are no processes wishing to report status.
indicates that children of the current process which are stopped due to a SIGTTIN, SIGTTOU, SIGTSTP, or SIGSTOP signal shall have their status reported.
is an alias for WUNTRACED.
allows waiting for processes which have trapped or reached a breakpoint.
indicates that the caller is wants to receive status reports from terminated processes. This flag is implicitly set for the functions (), waitpid(), wait3(), and wait4().
For the waitid() and wait6() functions, the flag has to be explicitly included in the options, if status reports from terminated processes are expected.
keeps 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 argument is non-NULL, a resource usage statistics from both its own child process as well as from its grand children is returned.

If infop is non-NULL, it must point to a siginfo_t structure which is filled on return such that the si_signo field is always SIGCHLD and the field 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 () function with the 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 waitid() is be zero as it is for any successful return from waitid().

When the WNOHANG option is specified and no processes wish to report status, () returns a process id of 0.

The () function 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 wait6() call, with the bits WEXITED and WTRAPPED set in the options and with infop set to NULL, is similar to wait4().

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:

(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).
(status)
True if the process terminated normally by a call to _exit(2) or exit(3).
(status)
True if the process terminated due to receipt of a signal.
(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:

(status)
If (status) is true, evaluates to the low-order 8 bits of the argument passed to _exit(2) or exit(3) by the child.
(status)
If WIFSIGNALED(status) is true, evaluates to the number of the signal that caused the termination of the process.
(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.
(status)
If WIFSTOPPED(status) is true, evaluates to the number of the signal that caused the process to stop.

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 () 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 () 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, () 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.

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. To indicate an error, -1 will be returned and errno set to an appropriate value. If 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.

The wait() function will fail and return immediately if:

[]
The calling process has no existing unwaited-for child processes.
[]
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.
[]
The status or rusage argument points to an illegal address. (May not be detected before exit of a child process.)
[]
The call was interrupted by a caught signal, or the signal did not have the SA_RESTART flag set.
[]
An invalid value was specified for options, or idtype and id do not specify a valid set of processes.

_exit(2), ptrace(2), sigaction(2), exit(3), siginfo(3)

The wait(), waitpid(), and waitid() functions are defined by POSIX; wait6(), wait4(), and wait3() are not specified by POSIX. The WCOREDUMP() macro and the ability to restart a pending wait() call are extensions to the POSIX interface.

The wait() function appeared in Version 6 AT&T UNIX.