Problem with hex literal in string comparison

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.

Answer 2

Some remarks and suggestions:

• open the files in binary mode - otherwise, funny things may happen on non-POSIX systems (fixed)

  fp = fopen("mario.nes", "rb");
  

• null-terminate your buffers if you want to print or compare them or use functions like strncmp() which accept the string's length as extra argument

  printf("C: %.4s\n", nes);
  printf("D: %.4s\n", real_nes);
  if (strncmp(nes, real_nes, 4) != 0) {
  

• '\x1a' is the non-graphic substitute character ^Z

• check the return values of io functions for errors

Answer 3

Well, one problem is your use of strcmp. This function expects a ZERO-TERMINATED string (neither nes nor real_nes are zero-terminated string in your code).

Another problem is fread. Use it like this:

fread(nes, 1, 4, fp); // first size_t param is size and second is member count

Change your code like this:

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

        char nes[5];
        char real_nes[5] = "NES\x1a";
        fread(nes, 1, 4, fp);
        nes[4] = '\0';
        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;
}

And see if it works.

Answer 4

Do not use string functions on not-zero-terminated byte arrays.

The problem is you have two 4 byte arrays which should contain the string "NES\x1a" (no space left for '\0' since it is already 4 bytes long), but the %s format and the strcmp need a '\0' termination at the end to know the strings end. That's why it doesn't work correctly.

1.: Do not use printf with %s format on this byte array. 2.: Use memcmp to compare the bytes.

Try this instead:

int i;

printf("Read bytes: 0x");
for(i = 0; i < sizeof(nes); i ++)
  printf("%02X", nes[i]);
printf("\n");

if (memcmp(nes, real_nes, sizeof(nes)) != 0) {
  printf("not a match\n");
}

Answer 5

A little too late maybe, but here's how I do it:

 // Read the 16 byte iNES header
    char header[16];
    fread( header, 16, 1, file );

    // Search for the "NES^Z" signature
    if( memcmp( header, "NES\x1A", 4 ) )
    {

As Xeno proposed, with memcmp you don't care about null terminators. After all, you are not really using strings, but more like char arrays, which is not the same due to the null terminators. As you don't really need to print the signature other than for debugging, you shouldn't care using string functions at all.