WAIT(2)                          System Calls                          WAIT(2)




NAME

       wait, waitpid, wait4, wait3 - wait for process termination


SYNOPSIS

       #include <sys/types.h>
       #include <sys/wait.h>

       pid_t wait (union wait *status);

       #include <sys/time.h>
       #include <sys/resource.h>

       pid_t waitpid (pid_t wpid, union wait *status, int options);

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

       pid_t wait4 (pid_t wpid, union wait *status, int options, struct rusage
       *rusage);


DESCRIPTION

       The wait function suspends execution of its calling process until  sta-
       tus  information is available for a terminated child process, or a sig-
       nal is received.  On return from a successful  wait  call,  the  status
       area  contains termination information about the process that exited as
       defined below.

       The wait4 call provides a more general interface for programs that need
       to  wait  for  certain  child processes, that need resource utilization
       statistics accumulated by child processes, or that require options.

       The wpid parameter 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 parameter is defined below.  The options parameter contains
       the bitwise OR of any of the following options.  The WNOHANG option  is
       used  to  indicate  that the call should not block if there are no pro-
       cesses that wish to report status.  If the  WUNTRACED  option  is  set,
       children  of  the  current  process  that are stopped due to a SIGTTIN,
       SIGTTOU, SIGTSTP, or SIGSTOP signal also have their status reported.

       If rusage is non-zero, a summary of the resources used  by  the  termi-
       nated  process  and  all  its children is returned (this information is
       currently not available for stopped processes).

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

       The  waitpid  call  is identical to wait4 with an rusage value of zero.
       The older wait3 call is the same as wait4 with a wpid value of -1.

       The following macros may be used to test the manner of exit of the pro-
       cess.  One of the first three macros will evaluate to a non-zero (true)
       value:

              WIFEXITED(status)
                     True if the process terminated  normally  by  a  call  to
                     _exit(2), exit(3), rexit(3), or a GS/OS quit call.

              WIFSIGNALED(status)
                     True  if  the process terminated due to receipt of a sig-
                     nal.

              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), exit(3),
                     rexit(3), or a GS/OS Quit call 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 cre-
                     ation of a core file containing an image of  the  process
                     when the signal was received.  This feature is not avail-
                     able under GNO.

              WSTOPSIG(status)
                     If WIFSTOPPED(status) is true, evaluates to the number of
                     the signal that caused the process to stop.


NOTES

       See  signal(2)  for a list of termination signals.  A status of 0 indi-
       cates normal termination.

       Some Unix uses of these functions expect status to be a pointer  to  an
       int  rather  than to a union wait.  It is safe to use a cast under such
       circumstances.

       If a parent process terminates without waiting for  all  of  its  child
       processes  to terminate, the remaining child processes are inherited by
       the Kernel Null Process (pid zero).

       If a signal is caught while any of the wait calls is pending, the  call
       may  be  interrupted  or  restarted  when  the  signal-catching routine
       returns, depending on  the  options  in  effect  for  the  signal;  see
       intro(2), System call restart.


BUGS

       Currently, only wait is implemented in the GNO kernel.  There is a min-
       imal version of waitpid in the libraries, but that version  of  waitpid
       ignores  its  options  parameter  (making  it impossible to make a non-
       blocking wait).  It may also give unexpected results when there is more
       than one child task being waited upon.

       wait4 and wait3 are not implemented.


RETURN VALUES

       If  wait returns due to a stopped or terminated child process, the pro-
       cess 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 wait4, wait3 or waitpid returns due to a stopped 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  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.


ERRORS

       Wait will fail and return immediately if:

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

              EFAULT The status  or  rusage  arguments  point  to  an  illegal
                     address.   (May  not  be  detected before exit of a child
                     process.)

              EINTR  The call was interrupted by a caught signal, or the  sig-
                     nal did not have the SA_RESTART flag set.


STANDARDS

       The  wait  and  waitpid functions are defined by POSIX; wait4 and wait3
       are not specified by POSIX.  The WCOREDUMP macro  and  the  ability  to
       restart a pending wait call are extensions to the POSIX interface.


SEE ALSO

       _exit(2), execve(2), signal(2), exit(3) rexit(3)


HISTORY

       A wait3 function call appeared in Version 6 AT&T UNIX.



GNO                             19 January 1997                        WAIT(2)

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