arrayscmacrostype-conversionsizeof

Size of Operator and trying it to print array


Consider this C program

#include <stdio.h>
#define TOTAL_ELEMENTS (sizeof(arr))/sizeof(arr[0])
int arr[] = {23, 34, 12, 17, 204, 99, 16};
int main() {
 int d;
 printf("%zu\n", TOTAL_ELEMENTS);
 for (d = -1; d <= (TOTAL_ELEMENTS - 2); d++) {
  printf("%d ", arr[d + 1]);
 }
 return 0;
}

compilers are giving output as 7 according to my dry run and reasoning output should be
7
23 34 12 17 204 99 16

Now in order to see what is going wrong I applied some Variation consider this code

#include <stdio.h>
#define TOTAL_ELEMENTS (sizeof(arr))/sizeof(arr[0])
int arr[] = {23, 34, 12, 17, 204, 99, 16};
int main() {
 int d;
 printf("%zu\n", TOTAL_ELEMENTS);
 for (d = -1; d <=(int) ((TOTAL_ELEMENTS) - 2); d++) {
  printf("%d ", arr[d + 1]);
 }
 return 0;
}

Notice that the only change I done is introducing type casting with int, and now it's giving expected results.

Now as much as I know when we compare in C program it can do conversion to bigger data-type right, so suppose TOTAL_ELEMENT even has bigger type like long then d should be converted to long and program should run fine but that is not happening. What points I am missing here?


Solution

  • In the comparison d <= (TOTAL_ELEMENTS - 2) the rank of d is lower than the rank of (TOTAL_ELEMENTS - 2) as int has lower precision than size_t on your platform per 6.3.1.1:

    The rank of a signed integer type shall be greater than the rank of any signed integer type with less precision.

    The compiler will cast int d = -1 to size_t which yield a very large number (SIZE_MAX from stdint.h) so the loop condition is false and the body of the loop is never executed.

    1. Prefer the standard for-loop idiom. Then you can use size_t d which type matches the type of your upper bound.
    2. It's a really good idea to use () the right hand side of the macro and each macro argument to avoid surprises.
    3. Avoid global variables.
    4. Remove unused variables.
    5. Minimize scope of variables; in this case by declaring d in the for-loop.
    6. Use the correct format string %zu for size_t.
    7. (optional) You only need {} around the loop body if thers is more than 1 statement, and return 0 is default so I left both out.
    #include <stdio.h>
    
    #define TOTAL_ELEMENTS (sizeof (arr) / sizeof *(arr))
    
    int main() {
        int arr[] = {23, 34, 12, 17, 204, 99, 16};
        printf("%zu\n", TOTAL_ELEMENTS);
        for (size_t d = 0; d < TOTAL_ELEMENTS; d++)
            printf("%d ", arr[d]);
    }
    

    and you get the expected output:

    7
    23 34 12 17 204 99 16