wait, waitpid - wait for process to terminate

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

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

     Wait causes its caller to delay until a signal is received or one of  its
     child  processes  terminates.  If any child has died since the last wait,
     return is immediate, returning the process id and exit status of  one  of
     the  terminated  children.  If there are no children, return is immediate
     with the value -1 returned.

     On return from a successful wait call, status is nonzero,  and  the  high
     byte  of status contains the low byte of the argument to exit supplied by
     the child process; the low byte of status contains the termination status
     of the process.  A more precise definition of the status word is given in
     <sys/wait.h>.  Wait can  be  called  with  a  null  pointer  argument  to
     indicate that no status need be returned.

     Waitpid provides an alternate interface for programs that must not  block
     when  collecting  the status of child processes, or that wish to wait for
     one particular child.  The pid parameter is the process ID of  the  child
     to wait for, -1 for any child.  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 wish to report status (WNOHANG), and/or that
     children of the current process  that  are  stopped  due  to  a  SIGTTIN,
     SIGTTOU,  SIGTSTP,  or  SIGSTOP  signal  should  also  have  their status
     reported (WUNTRACED).  (Job control is not  implemented  for  Minix,  but
     these symbols and signals are.)

     When the WNOHANG option is specified and  no  processes  wish  to  report
     status,  waitpid  either returns 0 under some implementations, or -1 with
     errno set to EAGAIN under others.   (Under  Minix  it  returns  0.)   The
     WNOHANG and WUNTRACED options may be combined by or'ing the two values.

     The call wait(&status) is equivalent to waitpid(-1, &status, 0).

     See sigaction(2) for a list of termination statuses (signals);  0  status
     indicates  normal termination.  A special status (0177) is returned for a
     stopped process that  has  not  terminated  and  can  be  restarted;  see
     ptrace(2).   If  the  0200  bit  of the termination status is set, a core
     image of the process was produced by the system.

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

     <sys/wait.h> defines a number of macros that operate on a status word:

          True if normal exit.

          Exit  status  if  the  process  returned  by  a  normal  exit,  zero

          Signal number if the process died by a signal, zero otherwise.

          True if the process died by a signal.

          True if the process is stopped.  (Never true under Minix.)

          Signal number of the signal that stopped the process.

     If wait 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.

     Waitpid returns -1 if there are no children not previously waited for  or
     if the process that it wants to wait for doesn't exist.

     Waitpid returns 0 if WNOHANG is specified and there  are  no  stopped  or
     exited  children.  (Under other implementations it may return -1 instead.
     Portable code should test for both possibilities.)

     Wait will fail and return immediately if one or more of the following are

     [ECHILD]       The calling process has  no  existing  unwaited-for  child

     [EFAULT]       The status argument points to an illegal address.

     [EAGAIN]       Waitpid is called with the WNOHANG option and no child has
                    exited yet.  (Not under Minix, it'll return 0 in this case
                    and leave errno alone.)

     execve(2), exit(2), sigaction(2).