WAIT(2)

NAME
wait, waitid, waitpid, wait6, wait4, wait3 - wait for process termination

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
The wait() 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() 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.

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. If WALTSIG is not specified, the call will wait only for processes that are configured to post SIGCHLD.

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(), 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.

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(), 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.

WUNTRACED
    Report the status of selected processes which are stopped due to a SIGTTIN, SIGTTOU, SIGTSTP, or SIGSTOP 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 the 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 the WCONTINUED 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 flag SA_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 p1003.1-90; the waitid() function conforms to p1003.1-2004; the wait3() function conforms to 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 AT&T UNIX.