1 files changed, 126 insertions, 0 deletions
diff --git a/reference/C/CONTRIB/SNIP/checkexe.c b/reference/C/CONTRIB/SNIP/checkexe.c
new file mode 100755
index 0000000..8ea43cb
--- /dev/null
+++ b/reference/C/CONTRIB/SNIP/checkexe.c
@@ -0,0 +1,126 @@
+/*
+** CHECKEXE.C - checksum protection for executable files
+**
+** by: Bob Jarvis
+*/
+
+#include <stdio.h>
+#include <string.h>
+#include <time.h>
+#include <stdlib.h>
+
+static struct {
+      unsigned char marker[16];
+      unsigned long checksum;
+} marker_struct = {"CHECKEXE MARKER",0L};
+
+void checkexe(char *fname)
+{
+      FILE *fptr;
+      unsigned int c;
+      int first_time = 0, i;
+      char buffer[14];
+      unsigned long chksum = 0L;
+      unsigned long l;
+      unsigned long marker_offset;
+      unsigned char *charptr;
+      time_t tm;
+
+      fptr = fopen(fname,"r+b");
+      if(fptr == NULL)
+      {
+            fprintf(stderr,"checkexe : unable to open input file '%s'\n",
+                  fname);
+            exit(99);
+      }
+
+      setvbuf(fptr, NULL, _IOFBF, 32767);    /* try to get a 32K buffer */
+
+   /* 
+    * If this is the first time the check has been run, scan the entire file
+    * to find the marker.  Otherwise proceed.
+    */
+
+      if(marker_struct.checksum == 0L)
+      {
+            first_time = 1;
+
+            c = fgetc(fptr);
+            while(!feof(fptr))
+            {
+                  if(c == (unsigned int)marker_struct.marker[0])
+                  {
+                        fread(buffer,sizeof(buffer),1,fptr);
+                        if(memcmp(buffer,&marker_struct.marker[1],
+                              sizeof(buffer)) == 0)
+                        {
+                              marker_offset = ftell(fptr) + 1L;
+                              break;
+                        }
+                        fseek(fptr,-13L,SEEK_CUR);
+                  }
+
+                  c = fgetc(fptr);
+            }
+
+            if(feof(fptr))
+            {
+                  fprintf(stderr,"checkexe : unable to locate marker\n");
+                  exit(99);
+            }
+
+      /* Change the marker field to random values */
+
+            tm = time(NULL);
+
+            srand((unsigned int)tm);
+
+            for(i = 0 ; i < sizeof(marker_struct.marker) ; ++i)
+                  marker_struct.marker[i] = (unsigned char) rand();
+
+            fseek(fptr,marker_offset - sizeof(marker_struct.marker),SEEK_SET);
+
+            fwrite(marker_struct.marker,sizeof(marker_struct.marker),1,fptr);
+      }
+
+   /* Calculate the checksum for the entire file */
+
+      rewind(fptr);
+
+      c = fgetc(fptr);
+      for(l = 0 ; !feof(fptr) ; ++l)
+      {
+            chksum += (unsigned long)c;
+            c = fgetc(fptr);
+      }
+
+      if(first_time)
+      {
+            marker_struct.checksum = chksum;
+            fseek(fptr,marker_offset,SEEK_SET);
+            fwrite(&marker_struct.checksum,sizeof(unsigned long),1,fptr);
+      }
+      else
+      {
+            charptr = (unsigned char*) &marker_struct.checksum;
+
+            for(i = 0 ; i < sizeof(marker_struct.checksum) ; ++i)
+                  chksum -= (unsigned long)(charptr[i]);
+
+            if(chksum != marker_struct.checksum)
+            {
+                  fprintf(stderr, "\acheckexe : %s has been altered, "
+                        "possibly by a virus\n", fname);
+                  exit(99);
+            }
+      }
+
+      fclose(fptr);
+      return;
+}
+
+main(int argc, char *argv[])
+{
+      checkexe(argv[0]);
+      return EXIT_SUCCESS;
+}