wait4()

Wait for one or more child process to change its state

Synopsis:

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

pid_t wait4( pid_t pid, 
             int * stat_loc, 
             int options, 
             struct rusage * resource_usage );

Arguments:

pid

The set of child processes that you want to get status information for:

• less than -1 — any child process whose process group ID is equal to the absolute value of pid.
• -1 — any child process
• 0 — any child process whose process group ID is equal to that of the calling process.
• greater than 0 — the single child process with this ID.

stat_loc

NULL, or a pointer a location where the function can store the terminating status of the child process. For information about macros that extract information from this status, see “Status macros” in the documentation for wait().

options

A combination of zero or more of the following flags:

• WCONTINUED — return the status for any child that was stopped and has been continued.
• WEXITED — wait for the process(es) to exit.
• WNOHANG — return immediately if there are no children to wait for.
• WNOWAIT — keep the process in a waitable state. This doesn't affect the state of the process; the process may be waited for again after this call completion.
• WSTOPPED — wait for and return the process status of any child that has stopped because it received a signal.
• WUNTRACED — report the status of a stopped child process.

resource_usage

NULL, or a pointer to a rusage structure where the function can store information about resource usage. For information about this structure, see getrusage().

Library:

libc

Use the -l c option to qcc to link against this library. This library is usually included automatically.

Description:

The wait4() function suspends execution of the calling thread until status information from one of its terminated child processes is available, or until the delivery of a signal whose action is either to terminate the process or execute a signal handler. If status information is available prior to the call to wait4(), the return is immediate.

The wait4() function behaves the same as the wait() function when passed a pid argument of -1, and the options argument has a value of zero.

Only one of the WIFEXITED(stat_val) and WIFSIGNALED(stat_val) macros can evaluate to a nonzero value.

The following call:

wait3( stat_loc, options, resource_usage );

is equivalent to the call:

waitpid( (pid_t)-1, stat_loc, options );

except that on successful completion, if the resource_usage argument to wait3() isn't a NULL pointer, the rusage structure that the third argument points to is filled in for the child process identified by the return value.

It's also equivalent to:

wait4( (pid_t)-1, stat_loc, options, resource_usage );

Returns:

If successful, wait4() returns the process ID of the terminating child process. If wait4() was invoked with WNOHANG set in options, it has at least one child process specified by pid for which status is not available, and status is not available for any process specified by pid, a value of zero is returned. On delivery of a signal waitpid() returns -1, and errno is set to EINTR.

Errors:

ECHILD

The calling process has no existing unwaited-for child processes that meet the criteria set by pid.

EINTR

The function was interrupted by a signal. The value of the location pointed to by stat_loc is undefined.

EINVAL

The value of the options argument isn't valid.

Classification:

Unix

See also:

exit(), fork(), pause(), wait(), wait3(), waitid(), waitpid()