Description

   The vfork function provided by VSI C for OpenVMS
   Systems differs from the fork function provided by other C
   implementations. The vfork and fork Functions shows the two major
   differences.

   Table REF-12 The vfork and fork Functions

   The vfork Function    The fork Function

   Used with the exec    Can be used without an exec function for
   functions.            asynchronous processing.
   Creates an            Creates an exact duplicate of the parent
   independent child     process that branches at the point where
   process that shares   vfork is called, as if the parent and the
   some of               child are the same process at different
   the parent's          stages of execution.
   characteristics.

   The vfork function provides the setup necessary for a subsequent
   call to an exec function. Although no process is created by
   vfork, it performs the following steps:

   o  It saves the return address (the address of the vfork call)
      to be used later as the return address for the call to an exec
      function.

   o  It saves the current context.

   o  It returns the integer 0 the first time it is called (before
      the call to an exec function is made). After the corresponding
      exec function call is made, the exec function returns control
      to the parent process, at the point of the vfork call, and
      it returns the process ID of the child as the return value.
      Unless the exec function fails, control appears to return
      twice from vfork even though one call was made to vfork and
      one call was made to the exec function.

   The behavior of the vfork function is similar to the behavior
   of the setjmp function. Both vfork and setjmp establish a return
   address for later use, both return the integer 0 when they are
   first called to set up this address, and both pass back the
   second return value as though it were returned by them rather
   than by their corresponding exec or longjmp function calls.

   However, unlike setjmp, with vfork, all local automatic
   variables, even those with volatile-qualified type, can have
   indeterminate values if they are modified between the call to
   vfork and the corresponding call to an exec routine.