(BSD System Compatibility)

wait(3bsd)

wait: wait3, WIFSTOPPED, WIFSIGNALED, WIFEXITED -- (BSD) wait for process to terminate or stop

Synopsis

   /usr/ucb/cc [flag . . . ] file . . .
   
   #include <sys/wait.h>
   #include <sys/time.h>
   #include <sys/resource.h>
   

   int wait3(union wait *statusp, int options, struct rusage *rusage);
   

   WIFSTOPPED(union wait status);
   

   WIFSIGNALED(union wait status);
   

   WIFEXITED(union wait status);

Description

wait delays its caller until a signal is received or one of its child processes terminates or stops due to tracing. If any child 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 had 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(3bsd).
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(3bsd).
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).

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

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

The other members of the wait union merely provide an alternate way of analyzing the status. The value stored in the w_status field is compatible with the values stored by other versions of the UNIX system, and an argument of type int * may be provided instead of an argument of type union wait * for compatibility with those versions.

wait3 is an alternate interface to wait(2) 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 an OR of the two values.

WIFSTOPPED, WIFSIGNALED, WIFEXITED, are macros that take an argument status, of type `union wait', as returned by wait3. 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.

Return values

wait3 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, wait3 returns a value of -1 and sets errno to indicate the error.

wait3 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.

wait3 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(3bsd) and siginterrupt(3bsd)). signal(3bsd), in the System V compatibility library, sets this bit for any signal it catches.

Since System V Release 4 does not implement this function directly as a system call, an illegal address (status or rusage) argument may result in a core dump as opposed to returning EFAULT.

References

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

Notices

wait (see wait(2)) is found in libc, not libucb. However, its description is provided here to offer a comparison to the wait3 functionality.

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

wait3 is 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.