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WAIT(2) System Calls Manual WAIT(2)

wait, waitpid, wait4, wait3wait for process termination

library “libc”

#include <sys/wait.h>

pid_t
wait(int *status);

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

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() 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().

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 status parameter is defined below.

The options parameter contains the bitwise OR of any of the following options:

This option is used to indicate that the call should not block if there are no processes that wish to report status.
If this 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 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.
This is an alias for WALTSIG. It is provided for compatibility with the Linux clone(2) API.
If this option is specified, the call will wait for all children regardless of what exit signal they post.
This is an alias for WALLSIG. It is provided for compatibility with the Linux clone(2) API .

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 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)).

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.

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 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:

[]
The calling process has no existing unwaited-for child processes.
[]
The status or rusage arguments point 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.

In addition, wait3(), wait4(), and waitpid() will fail and return immediately if:

[]
An invalid value was specified for options.

_exit(2), sigaction(2)

The wait() and waitpid() functions conform to IEEE Std 1003.1-1990 (“POSIX.1”); the wait3() function conforms to X/Open Portability Guide Issue 4 (“XPG4”); wait4() is an extension. The WCOREDUMP() macro and the ability to restart a pending wait() call are extensions to the POSIX interface.

A wait() function call appeared in Version 2 AT&T UNIX.

April 3, 2010 NetBSD-7.0.1