Description

   If the process was invoked by DCL, the status is interpreted by
   DCL, and a message is displayed.

   If the process was a child process created using vfork or an exec
   function, then the child process exits and control returns to the
   parent. The two functions are identical; the _exit function is
   retained for reasons of compatibility with VAX C.

   The exit and _exit functions make use of the $EXIT system
   service. If your process is being invoked by the RUN command
   using any of the hibernation and scheduled wakeup qualifiers, the
   process might not correctly return to hibernation state when an
   exit or _exit call is made.

   The C compiler command-line qualifier /[NO]MAIN=POSIX_EXIT can be
   used to direct the compiler to call __posix_exit instead of exit
   when returning from main. The default is /NOMAIN.

   Beginning with OpenVMS Version 8.3, C RTL contains a fix for
   the problem in which a call to _exit after a failed execl really
   exits but must not.

   In the OpenVMS implementation of vfork, a child process is not
   actually started as it is started on most UNIX systems. However,
   the C RTL creates some internal data structures intended to mimic
   child-process functionality (called the "child context").

   A bug occurred whereby after a vfork while in the child context,
   a call to an exec function justifiably fails, then calls _exit.
   On UNIX systems, after the failed exec call, the child process
   continues to execute. A subsequent call to _exit terminates
   the child. In the OpenVMS implementation, after the failed exec
   call, the child context terminates. A subsequent call to _exit
   terminates the parent. The C RTL fix is enabled by a feature
   logical switch, DECC$EXIT_AFTER_ FAILED_EXEC. Enabling this
   feature logical allows the child context to continue execution.

   With DECC$EXIT_AFTER_FAILED_EXEC disabled or not defined, the
   current behavior remains the default.

                                  NOTE

      EXIT_SUCCESS and EXIT_FAILURE are portable across any
      ANSI C compiler to indicate success or failure. On OpenVMS
      systems, they are mapped to OpenVMS condition codes with
      the severity set to success or failure, respectively. Values
      in the range of 3 to 255 can be used by a child process to
      communicate a small amount of data to the parent. The parent
      retrieves this data using the wait, wait3, wait4, or waitpid
      functions.