Convert a hexadecimal string to an integer efficiently in C?

Question

In C, what is the most efficient way to convert a string of hex digits into a binary unsigned int or unsigned long?

For example, if I have

Answer 1

@Eric

I was actually hoping to see a C wizard post something really cool, sort of like what I did but less verbose, while still doing it "manually".

Well, I'm no C guru, but here's what I came up with:

unsigned int parseHex(const char * str)
{
    unsigned int val = 0;
    char c;

    while(c = *str++)
    {
        val <<= 4;

        if (c >= '0' && c <= '9')
        {
            val += c & 0x0F;
            continue;
        }

        c &= 0xDF;
        if (c >= 'A' && c <= 'F')
        {
            val += (c & 0x07) + 9;
            continue;
        }

        errno = EINVAL;
        return 0;
    }

    return val;
}

I originally had more bitmasking going on instead of comparisons, but I seriously doubt bitmasking is any faster than comparison on modern hardware.