Question

I’m reading in an NES ROM file, where the first four bytes are “\x4e\x45\x53\x1a”, or NES\x1a. In my actual code, the given file can be arbitrary, so I want to check to make sure this header is here. However, I’m running into some trouble, which the following code demonstrates:

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

int main()
{
    FILE *fp;
    fp = fopen("mario.nes", "rb");

    char nes[4];
    char real_nes[4] = "NES\x1a";
    fread(nes, 4, 1, fp);
    printf("A: %x\n", nes[3]);
    printf("B: %x\n", real_nes[3]);
    printf("C: %s\n", nes);
    printf("D: %s\n", real_nes);
    if (strcmp(nes, real_nes) != 0) {
        printf("not a match\n");
    }
    fclose(fp);
    return 0;
}

which returns:

A: 1a
B: 1a
C: NES?
D: NES
not a match

where the question mark is \x1a.

I’m new to C, so it’s possible I’m missing something subtle (or obvious) about why the two strings don’t match, and why the question mark doesn’t show when printing line D, to signify that \x1a is there at the end of the string, which line B seems to indicate it should be.

Answer 1

The major problem in your code is:

char real_nes[4] = "NES\x1a";

This not a string, since it does not end with the nul-terminator char (‘\0’).
This is the same problem for ‘nes’.

Just declare them like:

char real_nes[] = "NES\x1a"; /* this is a string, ended by '\0' */
char nes[sizeof real_nes];

To be sure that there is enouth place for the ‘\0’.

Now you can use the %s specifier, or strcmp(). Anyway, I recommand the use of strncmp() instead, like in:

if(0 != strncmp(real_nes, nes, sizeof real_nes)) { /* some stuff */ }

HTH.