Description

   The waitpid function suspends the calling process until the
   request is completed. It is redefined so that only the calling
   thread is suspended.

   If the process_id argument is -1 and the options argument is
   0, the waitpid function behaves the same as the wait function.
   If these arguments have other values, the waitpid function is
   changed as specified by those values.

   The process_id argument allows the calling process to gather
   status from a specific set of child processes, according to the
   following rules:

   If the
   process_
   id is          Then status is requested

   Equal to -1    For any child process. In this respect, the
                  waitpid function is equivalent to the wait
                  function.
   Greater than   For a single child process and specifies the
   0              process ID.

   The waitpid function only returns the status of a child process
   from this set.

   The options argument to the waitpid function modifies the
   behavior of the function. You can combine the flags for the
   options argument by specifying their bitwise-inclusive OR. The
   flags are:

   WCONTINUED     Specifies that the following is reported to the
                  calling process: the status of any continued child
                  process specified by the process_id argument whose
                  status is unreported since it continued.
   WNOWAIT        Specifies that the process whose status is
                  returned in status_location is kept in a waitable
                  state. You can wait for the process again with the
                  same results.
   WNOHANG        Prevents the calling process from being suspended.
                  If there are child processes that stopped or
                  terminated, one is chosen and waitpid returns its
                  PID, as when you do not specify the WNOHANG flag.
                  If there are no terminated processes (that is,
                  if waitpid suspends the calling process without
                  the WNOHANG flag), 0 (zero) is returned. Because
                  you can never wait for process 0, there is no
                  confusion arising from this return.
   WUNTRACED      Specifies that the call return additional
                  information when the child processes of the
                  current process stop because the child process
                  received a SIGTTIN, SIGTTOU, SIGSTOP, or SIGTSTOP
                  signal.

   If the waitpid function returns because the status of a child
   process is available, the process ID of the child process is
   returned. Information is stored in the location pointed to by
   status_location, if this pointer is not null. The value stored
   in the location pointed to by status_location is 0 only if the
   status is returned from a terminated child process that did one
   of the following:

   o  Returned 0 from the main function.

   o  Passed 0 as the status argument to the _exit or exit function.

   Regardless of the value of status_location, you can define this
   information using the macros defined in the <wait.h> header file,
   which evaluate to integral expressions. In the following function
   descriptions, status_value is equal to the integer value pointed
   to by status_location:

   WIFEXITED(status_  Evaluates to a nonzero value if status was
   value)             returned for a child process that terminated
                      normally.
   WEXITSTATUS(status_If the value of WIFEXITED(status_value) is
   value)             nonzero, this macro evaluates to the low-order
                      8 bits of the status argument that the child
                      process passed to the _exit or exit function,
                      or to the value the child process returned
                      from the main function.
   WIFSIGNALED(status_Evaluates to a nonzero value if status
   value)             returned for a child process that terminated
                      due to the receipt of a signal not
                      intercepted.
   WTERMSIG(status_   If the value of WIFSIGNALED(status_value) is
   value)             nonzero, this macro evaluates to the number of
                      the signal that caused the termination of the
                      child process.
   WIFSTOPPED(status_ Evaluates to a nonzero value if status was
   value)             returned for a child process that is currently
                      stopped.
   WSTOPSIG(status_   If the value of WIFSTOPPED(status_value) is
   value)             nonzero, this macro evaluates to the number
                      of the signal that caused the child process to
                      stop.
   WIFCONTINUED(status_valuates to a nonzero value if status
   value)             returned for a child process that continued.

   If the information stored at the location pointed to by status_
   location is stored there by a call to waitpid that specified
   the WUNTRACED flag, one of the following macros evaluates to a
   nonzero value:

   o  WIFEXITED(*status_value)

   o  WIFSIGNALED(*status_value)

   o  WIFSTOPPED(*status_value)

   o  WIFCONTINUED(*status_value)

   If the information stored in the buffer pointed to by status_
   location resulted from a call to waitpid without the WUNTRACED
   flag specified, one of the following macros evaluates to a
   nonzero value:

   o  WIFEXITED(*status_value)

   o  WIFSIGNALED(*status_value)

   If a parent process terminates without waiting for all of its
   child processes to terminate, the remaining child processes is
   assigned a parent process ID equal to the process ID of the init
   process.

   See also exit, _exit, and wait.