wait()

NAME

wait(), waitpid() - wait for process termination

SYNOPSIS

#include <sys/wait.h>


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

DESCRIPTION

The wait(2) function suspends execution of its calling process until status information is available for any terminated child process, or a signal is received. On return from a successful wait(2) call, the notification area contains termination information about the process that exited as defined below. waitpid(2) is a more general interface that allows you more control over the behavior.

The status parameter is a pointer to an integer description of the status of the child process, which can be interpreted using macros:

The following macros may be used to test the manner of exit of the process. 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.

Depending on the values of those macros, the following macros produce the remaining status information about the child process:

WEXITSTATUS(status)

If WIFEXITED() 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() is true, evaluates to the number of the signal that caused the termination of the process.

WCOREDUMP(status)

If WIFSIGNALED() 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() is true, evaluates to the number of the signal that caused the process to stop.

The waitpid(2) call also takes the pid argument, specifying which child processes will be waited for, and the options argument, a bitwise OR of flags controlling waitpid(2)'s behavior.

Values for pid can be:

pid=-1:

Status is requested for all child processes (this is identical to wait(2)).

pid>0:

Status is requested for the child process with process ID pid.

pid=0:

Status is requested for any child process whose process group ID is equal to that of the calling process.

pid<-1:

Status is requested for any child process whose process group ID is equal to the absolute value of pid.

The options parameter contains the bitwise OR of any of the following options, defined in <sys/wait.h>:

WNOHANG

the call should not block if there are no processes that want to report status.

WUNTRACED

children of the current process that are stopped due to a SIGTTIN, SIGTTOU, SIGTSTP, or SIGSTOP signal also have their status reported.

RETURN VALUES

If wait(2) 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 waitpid(2) 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.

ERRORS

The wait(2) call will fail and return immediately if:

[ECHILD]

The calling process has no existing unwaited-for child processes.

[EINTR]

The call was interrupted by a caught signal.

[EINVAL]

The options argument was invalid.