Why does the 'for' loop condition fail? [duplicate]

Answer 1

When I try to print TOTAL_ELEMENTS - 2 like this:

printf("total %d\n", TOTAL_ELEMENTS - 2);

I got an warning (using gcc 4.8) saying:

test.c:8:2: warning: format ‘%d’ expects argument of type ‘int’, but argument 2 has type ‘long unsigned int’ [-Wformat=]
  printf("total %d\n", TOTAL_ELEMENTS - 2);
  ^

The warning means that TOTAL_ELEMENTS - 2 is long unsigned. Now when you compare a signed int with unsigned int, that signed int is treated as unsigned. So in d <= (TOTAL_ELEMENTS-2), d becomes a very high valued positive number (assuming 2's complement number system is used).

You can cast the result to int to fix the issue.

d <= (int)(TOTAL_ELEMENTS-2)

Or if you are using the macro in many places then you can change that like this:

#define TOTAL_ELEMENTS (int)(sizeof(array) / sizeof(array[0]))