Guard Page Exception Handler

ULONG GPHandler(PEXCEPTIONREPORTRECORD pX)
{
      ULONG ulAttribs;                       /* Memory attributes        */
      ULONG ulSize;                          /* Range in pages           */

      if (pX->ExceptionNum ==                /* If guard page exception  */
          XCPT_GUARD_PAGE_VIOLATION)
         {
            ulSize=1L;                       /* One page                 */
            DosQueryMem(                     /* Query memory attributes  */
               (PVOID)pX->ExceptionInfo[1],  /* Page base address        */
               &ulSize,                      /* Single page              */
               &ulAttribs);                  /* Memory attributes        */
            if (((ulAttrs & PAG_FREE) ||     /* If page is available     */
                (ulAttrs & PAG_COMMIT))==0)  /* but is not committed     */
               {
               DosSetMem(                    /* Commit page              */
                  (PVOID)pX->ExceptionInfo[1], /* Page base address      */
                  1L,                        /* Single page only         */
                  PAG_DEFAULT |              /* Default attributes       */
                  PAG_COMMIT);               /* Set commit flag          */
               return(XCPT_CONTINUE_EXECUTION); /* Done                  */
               }
         }
      if (pX->ExceptionNum ==                /* If access violation      */
          XCPT_ACCESS_VIOLATION)
         {
         if (pX->ExceptionInfo[1])           /* If page address not NULL */
            {
            ulSize=1L;                       /* One page                 */
            DosQueryMem(                     /* Query memory attributes  */
               (PVOID)pX->ExceptionInfo[1],  /* Page base address        */
               &ulSize,                      /* Single page              */
               &ulAttribs);                  /* Memory attributes        */
            if (((ulAttrs & PAG_FREE) ||     /* If page is available     */
                (ulAttrs & PAG_COMMIT))==0)  /* but is not committed     */
               {
               DosSetMem(                    /* Commit page              */
                  (PVOID)pX->ExceptionInfo[1], /* Page base address      */
                  1L,                        /* Single page only         */
                  PAG_DEFAULT |              /* Default attributes       */
                  PAG_COMMIT);               /* Set commit flag          */
               return(XCPT_CONTINUE_EXECUTION); /* Done                  */
               }
            }
         }
      return(XCPT_CONTINUE_SEARCH);          /* Chain to next handler if */
}                                            /* any other exception      */

This exception handler also handles the situation where an application writes directly to an uncommitted page rather than to the guard page, as is possible with non-sequential write operations.