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.