[koraX]

I've found it

	FILE *fp,*fp2;

      fp = fopen ("client.dll", "rb+"); // opens CLIENT.DLL file readonly
      if (fp != NULL)
      {
         fp2 = fopen ("client.bak", "wb"); // opens backup file for writing
         if (fp2 != NULL)
         {
            int c, count = 0;

            // get the size of the CLIENT.DLL file
            while ((c = fgetc (fp)) != EOF)
               count++;

            //  if it differs from the CS 1.3 original DLL size
/*            if (count != 655360)
            {
               printf ("CLIENT.DLL is not the genuine Counter-Strike 1.3 DLL. Patch cancelled.\n");
               return -1; // don't allow it to be patched
            }
*/
            rewind (fp); // return back to beginning of file
            while ((c = fgetc (fp)) != EOF)
               fputc (c, fp2); // duplicate client.dll to client.bak

            fclose (fp2); // close backup file

            fseek (fp, 191620L, SEEK_SET); // seek at the start of patch offset
            fputc (0x2B, fp); // actually patch client.dll (at offset 191620)
            fclose (fp); // close patched file

            printf ("Counter-Strike's CLIENT.DLL has been successfully patched.\n");
         }
      }
      else
         printf ("CLIENT.DLL not found in ../cstrike/cl_dlls. Patch cancelled.\n");

      return 0; // end of processing

but it does not work in CS 1.5. It crashes. Of course size of client.dll is different than 1.3. If only somebody would know the position of 'patch offset' in cs 1.5 ...