C turning char arrays into int behaving weird

Question

I have a much longer char array that I am turning into integers, but I cannot figure out why it behaves weird in some spots.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main () 
{
    char x[60] = "08 02 22 97 38 15 00 40 00 75 04 05 07 78 52 12 50 77 91 08";
    printf("%lu\n\n", strlen(x));

    for ( int i = 0; i < strlen(x); i+=3 ) {
        char num[2];
        num[0] = (char)x[i];
        num[1] = (char)x[i+1];
        printf("%d, ", atoi(num));
    }

}

The Output:

8, 2, 22, 97, 38, 15, 0, 40, 0, 75, 4, 5, 7, 78, 52, 12, 500, 773, 916, 89, 

Everything is great until.....500, 773, 916, 89...what is happening?

Answer 1

As you can see atoi wants a C-String: a null terminated array of character.

So, this

    char num[2];
    num[0] = (char)x[i];
    num[1] = (char)x[i+1];

Have to be

    char num[3] = {0};
    num[0] = (char)x[i];
    num[1] = (char)x[i+1];        
    num[2] = '\0'; // this could be avoided in your specific case

Answer 2

The need for a proper string with its null character has been posted by many.

Just wanted to add another coding idea: compound literal. (char[]) { x[i], x[i + 1], '\0' } to implement that.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main(void) {
  char x[] = "08 02 22 97 38 15 00 40 00 75 04 05 07 78 52 12 50 77 91 08";
  size_t len = strlen(x);
  printf("%zu\n\n", len);

  for (size_t i = 0; i < len; i += 3) {
    printf("%d, ", atoi((char[] ) { x[i], x[i + 1], '\0' }));
  }
}

Output

59

8, 2, 22, 97, 38, 15, 0, 40, 0, 75, 4, 5, 7, 78, 52, 12, 50, 77, 91, 8, 

Some other fixes made too.