WAIT(2) 						     Linux Programmer's Manual							   WAIT(2)

NAME

       wait, waitpid - wait for process termination

SYNOPSIS

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

       pid_t wait(int *status);
       pid_t waitpid(pid_t pid, int *status, int options);

DESCRIPTION

       The  wait  function  suspends  execution of the current process until a child has exited, or until a signal is delivered whose action is to
       terminate the current process or to call a signal handling function.  If a child has already exited by the time of the  call  (a  so-called
       "zombie" process), the function returns immediately.  Any system resources used by the child are freed.

       The waitpid function suspends execution of the current process until a child as specified by the pid argument has exited, or until a signal
       is delivered whose action is to terminate the current process or to call a signal handling function.  If a child as requested  by  pid  has
       already	exited by the time of the call (a so-called "zombie" process), the function returns immediately.  Any system resources used by the
       child are freed.

       The value of pid can be one of:

       < -1   which means to wait for any child process whose process group ID is equal to the absolute value of pid.

       -1     which means to wait for any child process; this is the same behaviour which wait exhibits.

       0      which means to wait for any child process whose process group ID is equal to that of the calling process.

       > 0    which means to wait for the child whose process ID is equal to the value of pid.

       The value of options is an OR of zero or more of the following constants:

       WNOHANG
	      which means to return immediately if no child has exited.

       WUNTRACED
	      which means to also return for children which are stopped, and whose status has not been reported.

       (For Linux-only options, see below.)

       If status is not NULL, wait or waitpid store status information in the location pointed to by status.

       This status can be evaluated with the following macros (these macros take the stat buffer (an int) as an argument -- not a pointer  to  the
       buffer!):

       WIFEXITED(status)
	      is non-zero if the child exited normally.

       WEXITSTATUS(status)
	      evaluates to the least significant eight bits of the return code of the child which terminated, which may have been set as the argu-
	      ment to a call to exit() or as the argument for a return statement in the main program.  This macro can only be evaluated if  WIFEX-
	      ITED returned non-zero.

       WIFSIGNALED(status)
	      returns true if the child process exited because of a signal which was not caught.

       WTERMSIG(status)
	      returns  the  number  of	the  signal  that  caused  the child process to terminate. This macro can only be evaluated if WIFSIGNALED
	      returned non-zero.

       WIFSTOPPED(status)
	      returns true if the child process which caused the return is currently stopped; this is only possible if the  call  was  done  using
	      WUNTRACED.

       WSTOPSIG(status)
	      returns the number of the signal which caused the child to stop.	This macro can only be evaluated if WIFSTOPPED returned non-zero.

       Some  versions  of  Unix  (e.g. Linux, Solaris, but not AIX, SunOS) also define a macro WCOREDUMP(status) to test whether the child process
       dumped core. Only use this enclosed in #ifdef WCOREDUMP ... #endif.

RETURN VALUE

       The process ID of the child which exited, or zero if WNOHANG was used and no child was available, or -1 on error (in which  case  errno	is
       set to an appropriate value).

ERRORS

       ECHILD if  the  process	specified in pid does not exist or is not a child of the calling process.  (This can happen for one's own child if
	      the action for SIGCHLD is set to SIG_IGN. See also the LINUX NOTES section about threads.)

       EINVAL if the options argument was invalid.

       EINTR  if WNOHANG was not set and an unblocked signal or a SIGCHLD was caught.

NOTES

       The Single Unix Specification describes a flag SA_NOCLDWAIT (not supported under Linux) such that if either this flag is set, or the action
       for SIGCHLD is set to SIG_IGN then children that exit do not become zombies and a call to wait() or waitpid() will block until all children
       have exited, and then fail with errno set to ECHILD.

       The original POSIX standard left the behaviour of setting SIGCHLD to SIG_IGN unspecified.   Later  standards,  including  SUSv2	and  POSIX
       1003.1-2001  specify the behaviour just described as an XSI-compliance option.  Linux does not conform to the second of the two points just
       described: if a wait() or waitpid() call is made while SIGCHLD is being ignored, the call behaves just as though  SIGCHLD  were	not  being
       igored, that is, the call blocks until the next child terminates and then returns the PID and status of that child.

LINUX NOTES

       In  the	Linux  kernel, a kernel-scheduled thread is not a distinct construct from a process. Instead, a thread is simply a process that is
       created using the Linux-unique clone(2) system call; other routines such as the	portable  pthread_create(3)  call  are	implemented  using
       clone(2).   Before Linux 2.4, a thread was just a special case of a process, and as a consequence one thread could not wait on the children
       of another thread, even when the latter belongs to the same thread group.  However, POSIX prescribes such functionality,  and  since  Linux
       2.4 a thread can, and by default will, wait on children of other threads in the same thread group.

       The following Linux-specific options are for use with children created using clone(2).

       __WCLONE
	      Wait for "clone" children only.  If omitted then wait for "non-clone" children only.  (A "clone" child is one which delivers no sig-
	      nal, or a signal other than SIGCHLD to its parent upon termination.)  This option is ignored if __WALL is also specified.

       __WALL (Since Linux 2.4) Wait for all children, regardless of type ("clone" or "non-clone").

       __WNOTHREAD
	      (Since Linux 2.4) Do not wait for children of other threads in the same thread group. This was the default before Linux 2.4.

CONFORMING TO

       SVr4, POSIX.1

SEE ALSO

       clone(2), signal(2), wait4(2), pthread_create(3), signal(7)

Linux								    2000-07-24								   WAIT(2)