Copyright (c) 1980, 1991 Regents of the University of California.
All rights reserved. The Berkeley software License Agreement
specifies the terms and conditions for redistribution.

@(#)wait.2 6.4 (Berkeley) 03/02/91

C 4 wait, waitpid, wait4, wait3 - wait for process to terminate

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

pid = wait(statp)
pid_t pid;
int *statp;

pid = waitpid(wpid, statp, options)
pid_t pid, wpid;
int *statp;
int options;

#include <sys/time.h>
#include <sys/resource.h>

pid = wait4(wpid, statp, options, rusage)
pid_t pid, wpid;
int *statp;
int options;
struct rusage *rusage;

pid = wait3(statp, options, rusage)
pid_t pid;
int *statp;
int options;
struct rusage *rusage;

Wait causes its caller to delay until a signal is received or one of its child processes terminates. If any child has died since the last wait , return is immediate, returning the process id and exit status of one of the terminated children. If there are no children, return is immediate with the value -1 returned.

On return from a successful wait call with a non-null statp pointer, the location to which statp points contains termination information about the process that exited as defined below.

The wait4 call provides a more general interface for programs that wish to wait for certain child processes, that wish resource utilization statistics accummulated by child processes, or that require options. The other wait functions are implemented using wait4 .

The wpid parameter 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 statp parameter is defined below. The options parameter contains the bitwise OR of any of the following options. The WNOHANG option is used to indicate that the call should not block if there are no processes that wish to report status. If the WUNTRACED option is set, children of the current process that are stopped due to a SIGTTIN, SIGTTOU, SIGTSTP, or SIGSTOP signal also have their status reported.

If rusage is non-zero, a summary of the resources used by the terminated process and all its children is returned (this information is currently not available for stopped processes).

When the WNOHANG option is specified and no processes wish to report status, wait4 returns a pid 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.

If the statp parameter is not null, the status of the process is placed in the location to which statp points. The following macros may be used with that status information to test the state and/or manner of exit of the process. One of the first three macros will evaluate to a non-zero (true) value:

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

If WIFEXITED(status) is true, evaluates to the low-order 8 bits of the argument passed to _exit (2) or exit (2) by the child. If WIFSIGNALED(status) is true, evaluates to the number of the signal that caused the termination of the process. 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. 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 the parent process terminates without waiting on its children, the initialization process (process ID = 1) inherits the children.

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 intro (2), System call restart.

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 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.

Wait will fail and return immediately if one or more of the following are true:

15 [ECHILD] The calling process has no existing unwaited-for child processes.

15 [EFAULT] The statp or rusage arguments point to an illegal address. (May not be detected before exit of a child process.)

15 [EINTR] The call was interrupted by a caught signal, or the signal did not have the SA_RESTART flag set.

The wait and waitpid functions are defined by POSIX; 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. exit(2), sigaction(2)