wait() delays its caller until a signal is received or one of its child processes terminates or stops due to tracing. If any child process has died or stopped due to tracing and this has not been reported using wait(), return is immediate, returning the process ID and exit status of one of those children. If that child process has died, it is discarded. If there are no children, return is immediate with the value -1 returned. If there are only running or stopped but reported children, the calling process is blocked.

If status is not a NULL pointer, then on return from a successful wait() call the status of the child process whose process ID is the return value of wait() is stored in the wait() union pointed to by status. The w_status member of that union is an int; it indicates the cause of termination and other information about the terminated process in the following manner:

• If the low-order 8 bits of w_status are equal to 0177, the child process has stopped; the 8 bits higher up from the low-order 8 bits of w_status contain the number of the signal that caused the process to stop. See ptrace(2) and sigvec(3UCB).
• If the low-order 8 bits of w_status are non-zero and are not equal to 0177, the child process terminated due to a signal; the low-order 7 bits of w_status contain the number of the signal that terminated the process. In addition, if the low-order seventh bit of w_status (that is, bit 0200) is set, a ``core image'' of the process was produced; see sigvec(3UCB).
• Otherwise, the child process terminated due to an exit() call; the 8 bits higher up from the low-order 8 bits of w_status contain the low-order 8 bits of the argument that the child process passed to exit(); see exit(2).

waitpid() behaves identically to wait() if pid has a value of -1 and options has a value of zero. Otherwise, the behavior of waitpid() is modified by the values of pid and options as follows:

pid specifies a set of child processes for which status is requested. waitpid() only returns the status of a child process from this set.

• If pid is equal to -1, status is requested for any child process. In this respect, waitpid() is then equivalent to wait().
• If pid is greater than zero, it specifies the process ID of a single child process for which status is requested.
• If pid is equal to zero, status is requested for any child process whose process group ID is equal to that of the calling process.
• If pid is less than -1, status is requested for any child process whose process group ID is equal to the absolute value of pid.

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

WNOHANG

waitpid() does not suspend execution of the calling process if status is not immediately available for one of the child processes specified by pid.

WUNTRACED

The status of any child processes specified by pid that are stopped, and whose status has not yet been reported since they stopped, are also reported to the requesting process.

wait3() is an alternate interface that allows both non-blocking status collection and the collection of the status of children stopped by any means. The status parameter is defined as above. The options parameter is used to indicate the call should not block if there are no processes that have status to report (WNOHANG), and/or that children of the current process that are stopped due to a SIGTTIN, SIGTTOU, SIGTSTP, or SIGSTOP signal are eligible to have their status reported as well (WUNTRACED). A terminated child is discarded after it reports status, and a stopped process will not report its status more than once. If rusage is not a NULL pointer, a summary of the resources used by the terminated process and all its children is returned. Only the user time used and the system time used are currently available. They are returned in rusage.ru_utime and rusage.ru_stime, respectively.

When the WNOHANG option is specified and no processes have status to report, wait3() returns 0. The WNOHANG and WUNTRACED options may be combined by ORing the two values.

wait4() is another alternate interface. With a pid argument of 0, it is equivalent to wait3(). If pid has a nonzero value, then wait4() returns status only for the indicated process ID, but not for any other child processes.

WIFSTOPPED, WIFSIGNALED, WIFEXITED, are macros that take an argument status, of type int, as returned by wait(), or wait3(), or wait4(). WIFSTOPPED evaluates to true (1) when the process for which the wait() call was made is stopped, or to false (0) otherwise. WIFSIGNALED evaluates to true when the process was terminated with a signal. WIFEXITED evaluates to true when the process exited by using an exit(2) call.

If wait()or waitpid() 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 wait() or waitpid() return due to the delivery of a signal to the calling process, a value of -1 is returned and errno is set to EINTR. If waitpid() function 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. Otherwise, a value of -1 is returned, and errno is set to indicate the error.

wait3() and wait4() returns 0 if WNOHANG is specified and there are no stopped or exited children, and returns the process ID of the child process if it returns due to a stopped or terminated child process. Otherwise, they returns a value of -1 and sets errno to indicate the error.

wait(), wait3()or wait4() will fail and return immediately if one or more of the following are true:

ECHILD

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

EFAULT

The status or rusage arguments point to an illegal address.

waitpid() may set errno to:

ECHILD

The process or process group specified by pid does not exist or is not a child of the calling process.

EINTR

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

EINVAL

The value of options is not valid.

wait(), and wait3(), and wait4() will terminate prematurely, return -1, and set errno to EINTR upon the arrival of a signal whose SV_INTERRUPT bit in its flags field is set (see sigvec(3UCB) and siginterrupt(3UCB)). signal(3UCB), sets this bit for any signal it catches.

exit(2), ptrace(2), wait(2), waitpid(2), getrusage(3C), siginterrupt(3UCB), signal(3UCB), sigvec(3UCB), signal(3C)

Use of these interfaces should be restricted to only applications written on BSD platforms. Use of these interfaces with any of the system libraries or in multi-thread applications is unsupported.

If a parent process terminates without waiting on its children, the initialization process (process ID = 1) inherits the children.

wait(), and wait3(), and wait4() are automatically restarted when a process receives a signal while awaiting termination of a child process, unless the SV_INTERRUPT bit is set in the flags for that signal.

Calls to wait() with an argument of 0 should be cast to type `int *', as in:

wait((int *)0)

Previous SunOS releases used union wait*statusp and union wait status in place of int *statusp and int status. The union contained a member w_status that could be treated in the same way as status.

Other members of the wait union could be used to extract this information more conveniently:

• If the w_stopval member had the value WSTOPPED, the child process had stopped; the value of the w_stopsig member was the signal that stopped the process.
• If the w_termsig member was non-zero, the child process terminated due to a signal; the value of the w_termsig member was the number of the signal that terminated the process. If the w_coredump member was non-zero, a core dump was produced.
• Otherwise, the child process terminated due to a call to exit(). The value of the w_retcode member was the low-order 8 bits of the argument that the child process passed to exit().

union wait is obsolete in light of the new specifications provided by IEEE Std 1003.1-1988 and endorsed by SVID89 and XPG3. SunOS Release 4.1 supports unionwait for backward compatibility, but it will disappear in a future release.