comparison function returns incorrect data

i have modified the examples/c/ex_heap.c to read from one db and insert/delete in another.  For some reason one of the keys that the comparison function receives is either truncated or doesn't match any records in the db.  This is my comparison function

int compare_cmid(dbp, a, b, lob)

        DB *dbp;

        const DBT *a, *b;

        size_t *lob;

{

       //char* temp_a = (char*)a->data;

       //char* temp_b = (char*)b->data;

       //printf("temp_a = %s\n", temp_a);

       //printf("temp_b = %s\n", temp_b);

        char key_a[25];

        char key_b[25];

        memset(key_a,'\0',sizeof(key_a));

        memset(key_b,'\0',sizeof(key_b));

       int ascii_a[256];

       int ascii_b[256];

       int i;

       memset(ascii_a, 0, sizeof(ascii_a));

       memset(ascii_b, 0, sizeof(ascii_b));

       strcpy(key_a, a->data);

       strcpy(key_b, b->data);

       printf("key_a = %s\n", key_a);

       printf("key_b = %s\n", key_b);

       int l_a;

       l_a =  strlen(key_a);

       int l_b;

       l_b = strlen(key_b);

       int value;

       for (i=0; i<l_a; i++) {

         value = key_a[i];

         ascii_a[value]++;

       }

       for(i=0; i<l_b; i++) {

         value = key_b[i];

         ascii_b[value]++;

        }

       for(i=0; i<256; i++) {

         if(ascii_a[i] > ascii_b[i]) {

             return 1;

         } else if (ascii_a[i] < ascii_b[i]) {

             return -1;

         }

       }

       return 0;

}

b is often messed up.  I'm setting the comparison function before opening the db and it gets called fine but just not with the expected keys.  Please assist.