Question

I’ve read a file into an array of characters using fread. Now I want to search that array for two consecutive hex values, namely FF followed by D9 (its a jpeg marker signifying end of file). Here is the code I use to do that:

char* searchBuffer(char* b) {
    char* p1 = b;
    char* p2 = ++b;
    int count = 0;

    while (*p1 != (unsigned char)0xFF && *p2 != (unsigned char)0xD9) {
        p1++;
        p2++;
        count++;
    }

    count = count;
    return p1;
}

Now I know this code works if I search for hex values that don’t include 0xFF (eg 4E followed by 46), but every time I try searching for 0xFF it fails. When I don’t cast the hex values to unsigned char the program doesn’t enter the while loop, when I do the program goes through all the chars in the array and doesn’t stop until I get an out of bounds error. I’m stumped, please help.

Ignore count, its just a variable that helps me debug.

Thanks in advance.

Answer 1

You are falling foul of integer promotions. Both operands for != (and similar) are promoted to int. And if at least one of them is unsigned, then both of them are treated as unsigned (actually that isn’t 100% accurate, but for this particular situation, it should suffice). So this:

*p1 != (unsigned char)0xFF

is equivalent to:

(unsigned int)*p1 != (unsigned int)(unsigned char)0xFF

On your platform, char is evidently signed, in which case it can never take on the value of (unsigned int)0xFF.

So try casting *p1 as follows:

(unsigned char)*p1 != 0xFF

Alternatively, you could have the function take unsigned char arguments instead of char, and avoid all the casting.

[Note that on top of all of this, your loop logic is incorrect, as pointed out in various comments.]