Manual Pages  — WAIT

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 wait6(), wait4(), wait3(), or waitpid() returns due to a stopped, continued, 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, continued 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.

If waitid() returns because one or more processes have a state change to report, 0 is returned. If an error is detected, a value of -1 is returned and errno is set to indicate the error. If WNOHANG is specified and there are no stopped, continued or exited children, 0 is returned. The si_signo and si_pid fields of infop must be checked against zero to determine if a process reported status.