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  pro-
     cess  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 follow-
     ing 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.

     If status is not NULL, wait or waitpid store status informa-
     tion 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 argument 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.

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

     ECHILD  if the process specified in pid does not exist or is
             not  a child of the calling process.  (This can hap-
             pen for one's own child if the action for SIGCHLD is
             set to SIG_IGN.)

     EINVAL  if the options argument was invalid.

     ERESTARTSYS
             if WNOHANG was not set and an unblocked signal or  a
             SIGCHLD  was  caught.  This error is returned by the
             system call.  The library interface is  not  allowed
             to return ERESTARTSYS, but will return EINTR.

     The Single Unix Specification describes a flag  SA_NOCLDWAIT
     (not  present  under Linux) such that if either this flag is
     set, or the action for SIGCHLD is set to SIG_IGN (which,  by
     the  way,  is not allowed by POSIX), 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.

     SVr4, POSIX.1

     signal(2), wait4(2), signal(7)