wait3(2)

NAME

wait3(), wait4() — wait for child process to change state

SYNOPSIS

#include <sys/wait.h> 

pid_t wait3 (int *stat_loc, int options, struct rusage *resource_usage); 

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

DESCRIPTION

The wait3() and wait4() functions allow the calling process to obtain various status information for a caller's child process based on the options specified. If status information is available for two or more child processes, the order of which process to report status on is not defined.

The wait4() function is similar to wait3(), except that wait4() waits for a specific child as indicated by the pid parameter.

Note that the following call

wait3(stat_loc, options, resource_usage); 

is equivalent to the call:

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

Note that the following call

wait4(pid, stat_loc, options, resource_usage); 

is equivalent to the call:

waitpid(pid, stat_loc, options); 

In both of the previous prototypes, on successful completion, if the resource_usage argument to wait3() or wait4() is not a null pointer, the rusage structure that the resource_usage argument points to is filled in for the child process identified by the return value.

The pid argument specifies a child process for which status is requested. The following rules define which status information is returned:

If pid is equal to (pid_t) -1, status is requested for any child process.
If pid is greater than 0, it specifies the process ID of a single child process for which status is requested.
If pid is 0, status is requested for any child process whose process group ID is equal to that of the calling process.
If pid is less than (pid_t) -1, status is requested for any child process whose process group ID is equal to the absolute value of pid.

The stat_loc argument is the address where status about the specified child process is placed.

The options argument is constructed from the bitwise-inclusive OR of zero or more of the following flags defined in the <sys/wait.h> header file:

WCONTINUED: The status of any continued child process specified by pid that has not been reported since it continued is reported to the requesting process.
WNOHANG: The wait3() and wait4() functions will not suspend execution of the calling process if status is not immediately available for one of the child processes specified by pid.
WNOWAIT: This causes the wait not to be registered. This means that the registered process that is being waited on, can be waited on again with identical results, provided that the status of the child does not change in the meantime.
WUNTRACED: The status of any child processes specified by pid that are stopped and whose status has not yet been reported since they stopped, will also be reported to the requesting process.

The resource_usage argument points to the resource utilization structure.

APPLICATION USAGE

Threads Considerations

In a multi-threaded application, only the calling thread is suspended by wait3() and wait4().

The wait3() and wait4() functions will not return until all threads in the process have reached the desired state. For example, wait3() and wait4() will not return until all threads have terminated. If the WUNTRACED or WCONTINUED options are specified, wait3() and wait4() will not return until all threads have stopped or continued respectively.

RETURN VALUE

If wait3(), wait4(), or waitpid() returns because the status of a child process is available, the return value is the process ID of that child process. If wait3(), wait4(), or waitpid() returns due to the receipt of a signal, the return value receives a -1 and errno is set to EINTR.

If wait3() or wait4() was called with the WNOHANG options argument where status is not available for any process specified by the pid argument, 0 will be returned.

Otherwise, (pid_t) -1 will be returned and errno will be set to indicate the error.

ERRORS

If wait3() or wait4() fails, errno is set to one of the following values:

ECHILD: The calling process has no existing unwaited-for child processes; or the states specified by the options argument are invalid for the set of processes specified by the pid argument.
EFAULT: Problems were encountered in the retrieval of status information for the specified child process.
EINTR: The wait3() or wait4() function has been interrupted by a signal. The value in the location pointed to by the stat_loc argument is undefined.
EINVAL: The options argument to wait3(), wait4(), or waitpid() is invalid.

WARNINGS

The behavior of wait3() and wait4() is affected if the SIGCLD signal is set to SIG_IGN. See the WARNINGS section of signal(5). Signal handlers that cause system calls to be restarted can affect the EINTR condition described above (see bsdproc(3C) and sigaction(2)).

AUTHOR

The wait3() and wait4() functions were developed by HP, AT&T, and the University of California, Berkeley.