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DESCRIPTION

The wait4() and wait6() system calls provide a more general interface for programs that need to wait for specific child processes, that need resource utilization statistics accumulated by child processes, or that require options. The other wait functions are implemented using either wait4() or wait6().

The wait6() function is the most general function in this family and its distinct features are:

All of the desired process statuses to be waited on must be explicitly specified in options. The wait(), waitpid(), wait3(), and wait4() functions all implicitly wait for exited and trapped processes, but the waitid() and wait6() functions require the corresponding WEXITED and WTRAPPED flags to be explicitly specified. This allows waiting for processes which have experienced other status changes without having to also handle the exit status from terminated processes.

The wait6() function accepts a wrusage argument which points to a structure defined as:

struct __wrusage {
	struct rusage   wru_self;
	struct rusage   wru_children;
};

This allows the calling process to collect resource usage statistics from both its own child process as well as from its grand children. When no resource usage statistics are needed this pointer can be NULL.

The last argument infop must be either NULL or a pointer to a siginfo_t structure. If non-NULL, the structure is filled with the same data as for a SIGCHLD signal delivered when the process changed state.

The set of child processes to be queried is specified by the arguments idtype and id. The separate idtype and id arguments support many other types of identifiers in addition to process IDs and process group IDs.

• If idtype is P_PID, waitid() and wait6() wait for the child process with a process ID equal to (pid_t)id.
• If idtype is P_PGID, waitid() and wait6() wait for the child process with a process group ID equal to (pid_t)id.
• If idtype is P_ALL, waitid() and wait6() wait for any child process and the id is ignored.
• If idtype is P_PID or P_PGID and the id is zero, waitid() and wait6() wait for any child process in the same process group as the caller.

Non-standard identifier types supported by this implementation of waitid() and wait6() are:

P_UID

Wait for processes whose effective user ID is equal to (uid_t) id.

P_GID

Wait for processes whose effective group ID is equal to (gid_t) id.

P_SID

Wait for processes whose session ID is equal to id. If the child process started its own session, its session ID will be the same as its process ID. Otherwise the session ID of a child process will match the caller's session ID.

P_JAILID

Waits for processes within a jail whose jail identifier is equal to id.

For the waitpid() and wait4() functions, the single wpid argument 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 argument is defined below.

The options argument contains the bitwise OR of any of the following options.

WCONTINUED

Report the status of selected processes that have continued from a job control stop by receiving a SIGCONT signal.

WNOHANG

Do not block when there are no processes wishing to report status.

WUNTRACED

Report the status of selected processes which are stopped due to a SIGTTIN, SIGTTOU, SIGTSTP, or SIGSTOP signal.

WSTOPPED

An alias for WUNTRACED.

WTRAPPED

Report the status of selected processes which are being traced via ptrace(2) and have trapped or reached a breakpoint. This flag is implicitly set for the functions wait(), waitpid(), wait3(), and wait4().
For the waitid() and wait6() functions, the flag has to be explicitly included in options if status reports from trapped processes are expected.

WEXITED

Report the status of selected processes which have terminated. This flag is implicitly set for the functions wait(), waitpid(), wait3(), and wait4().
For the waitid() and wait6() functions, the flag has to be explicitly included in options if status reports from terminated processes are expected.

WNOWAIT

Keep the process whose status is returned in a waitable state. The process may be waited for again after this call completes.

For the waitid() and wait6() functions, at least one of the options WEXITED, WUNTRACED, WSTOPPED, WTRAPPED, or WCONTINUED must be specified. Otherwise there will be no events for the call to report. To avoid hanging indefinitely in such a case these functions return -1 with errno set to EINVAL.

If rusage is non-NULL, a summary of the resources used by the terminated process and all its children is returned.

If wrusage is non-NULL, separate summaries are returned for the resources used by the terminated process and the resources used by all its children.

If infop is non-NULL, a siginfo_t structure is returned with the si_signo field set to SIGCHLD and the si_pid field set to the process ID of the process reporting status. For the exited process, the si_status field of the siginfo_t structure contains the full 32 bit exit status passed to _exit(2); the status argument of other calls only returns 8 lowest bits of the exit status.

When the WNOHANG option is specified and no processes wish to report status, waitid() sets the si_signo and si_pid fields in infop to zero. Checking these fields is the only way to know if a status change was reported.

When the WNOHANG option is specified and no processes wish to report status, wait4() and wait6() return a process id of 0.

The wait() call is the same as wait4() with a wpid value of -1, with an options value of zero, and a rusage value of NULL. The waitpid() function is identical to wait4() with an rusage value of NULL. The older wait3() call is the same as wait4() with a wpid value of -1. The wait4() function is identical to wait6() with the flags WEXITED and WTRAPPED set in options and infop set to NULL.

The following macros may be used to test the current status of the process. Exactly one of the following four macros will evaluate to a non-zero (true) value:

WIFCONTINUED(status)

True if the process has not terminated, and has continued after a job control stop. This macro can be true only if the wait call specified the WCONTINUED option.

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.