How to Transparently Intercept Procedure Calls in Windows

SUMMARY

A technique for aliasing procedure names in Windows allows procedure calls to be transparently intercepted. You can use this technique to monitor activities such as checking arguments for validity.

To intercept procedure calls transparently, first build a Windows executable code library that resembles the following:

          HANDLE MyGlobalAlloc( flags, size )
        WORD  flags;
        DWORD size;
        {
            /* Perform any type of monitoring necessary here.

               if ( !size )
                   return 0;

            */ 
            return RealGlobalAlloc( flags, size );
        }

        HANDLE FAR PASCAL main( argc, argv )
        WORD argc;
        LPSTR argv;
        {
            return 1;
        }
				

Next write the definitions file for the library above as follows:

   BOO.DEF:
   LIBRARY BOO
   DESCRIPTION "Procedure call interception library."
   DATA SINGLE MOVEABLE
   CODE MOVEABLE DISCARDABLE
   EXPORTS
          GlobalAlloc=MyGlobalAlloc
   IMPORTS
           RealGlobalAlloc=KERNEL.15
				

You can obtain the ordinal numbers for the KERNEL routines (or for any other routine) by using the LIB.EXE program to list the contents of SLIBW or MLIBW.

Build a dynamic link library (DLL) by running IMPLIB BOO.DEF as follows:

   IMPLIB BOO.DEF BOO.LIB
				

Build the library executable by using the following table:

   link boo,,,mlibw mlibc,boo.def;
				

Next dynamically link to BOO.EXE by linking to BOO.LIB as follows:

   link car,,,boo mlibw mlibc,car.def;
				

Any calls to GlobalAlloc() are routed to MyGlobalAlloc() first because of the name aliasing. This technique is very powerful and can be used to implement any type of monitoring function. In the case of GlobalAlloc(), the monitoring is transparent in the sense that it is not necessary to recompile the application to remove error checking; simply link to MLIBW.LIB instead of to BOO.LIB MLIBW.LIB.