Description

   The wait4 function suspends the calling process until the request
   is completed.

   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 wait4 function only returns the status of a child process
   from this set.

   The options argument to the wait4 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:

   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 suspension of the calling process.
                  If there are child processes that stopped or
                  terminated, one is chosen and the waitpid function
                  returns its process ID, 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 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 wait4 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 status_location value, you can define this
   information using the macros defined in the <wait.h> header file,
   which evaluate to integral expressions. In the following macro
   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 was
   value)             returned for a child process that terminated
                      due to the receipt of a signal that was 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)             was returned for a child process that has
                      continued.

   If the information stored at the location pointed to by status_
   location was stored there by a call to wait4 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 location pointed to by status_
   location resulted from a call to wait4 without the WUNTRACED flag
   specified, one of the following macros evaluates to a nonzero
   value:

   o  WIFEXITED(*status_value)

   o  WIFSIGNALED(*status_value)

   The wait4 function is similar to the wait3 function. However,
   the wait4 function waits for a specific child as indicated by
   the process_id argument. The resource_usage argument points to a
   location that contains resource usage information for the child
   processes as defined in the <resource.h> header file.

   See also exit and _exit.