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 _xvlock()
 Lock a VM segment
------------------------------------------------------------------------------
 C Prototype

     #include "vm.api"
     FARP _xvlock(
                   HANDLE hSegment
                 )

 Arguments

     hSegment is the VM segment handle returned by _xvalloc().

 Returns

     _xvlock() returns a far pointer to the base of the locked VM segment, or
     a NULL pointer if the VM segment handle is not valid (i.e., does not
     represent an allocated VM segment).

 Description

     _xvlock() guarantees that the VM segment is located in conventional
     memory and cannot be moved or swapped.  The pointer returned is valid
     until the segment is unlocked using _xvunlock().

     Note:  A VM segment may be locked more than once.  The VMM system
     maintains, for each segment, a lock count (_xvlockcount()) which is
     incremented each time you lock the segment and decremented each time you
     unlock the segment.  A segment is not physically unlocked until its lock
     count is zero.  You must eventually unlock all VM segments with
     _xvunlock().

     Warning!  Do not leave a VM segment locked unnecessarily.  Because
     all locked segments reside in conventional memory, the more of them you
     lock the greater is the chance of exhausting conventional memory.
     Therefore, always unlock a VM segment with _xvunlock() when you are not
     actively accessing it.  Typically, this will mean locking and unlocking
     the same VM segment several times within one function call.

     If you have to lock a VM segment for an extended period of time, use
     _xvwire().  This function places the VM segment in an area of the swap
     space that least inhibits the VMM system.

 Notes

     .  Error conditions:  If there is not enough VM swap space available
        for the segment to be loaded into conventional memory, the system
        raises an internal error and halts.

     .  Garbage collection:  The VMM system may take up to several seconds
        to lock a VM segment if locking it triggers garbage collection.  Use
        _xvwire() for routines that require fast access to a buffer.

 Examples

     .  This example allocates a segment with _xvalloc() and locks it
        using _xvlock().  After a string is copied into the locked segment,
        the segment is unlocked and freed (with _xvunlock() and _xvfree()):

        // CA-Clipper include files
        #include "extend.api"
        #include "vm.api"

        // Microsoft C include files
        #include "string.h"

        // Prototype
        Boolean VMExample(char * spSrc);


        Boolean VMExample(char * spSrc)
        {
           HANDLE hSegment;
           char * spString;
           Boolean bResult = FALSE;

           if (hSegment = _xvalloc(strlen(spSrc) + 1, 0))
              {
              spString = _xvlock(hSegment);
              if (spString != NULL)
                 {
                 strcpy(spString, spSrc);

                 .
                 . <statements>
                 .

                 bResult = TRUE;

                 _xvunlock(hSegment);
                 }
              _xvfree(hSegment);
              }

           return (bResult);
        }

 Files:  Library is CLIPPER.LIB, header file is Vm.api.