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

For AVR Microcontrollers I wrote the following function, including relevant comments to make it easy to understand:

/**
 * hex2int
 * take a hex string and convert it to a 32bit number (max 8 hex digits)
 */
uint32_t hex2int(char *hex) {
    uint32_t val = 0;
    while (*hex) {
        // get current character then increment
        char byte = *hex++; 
        // transform hex character to the 4bit equivalent number, using the ascii table indexes
        if (byte >= '0' && byte <= '9') byte = byte - '0';
        else if (byte >= 'a' && byte <='f') byte = byte - 'a' + 10;
        else if (byte >= 'A' && byte <='F') byte = byte - 'A' + 10;    
        // shift 4 to make space for new digit, and add the 4 bits of the new digit 
        val = (val << 4) | (byte & 0xF);
    }
    return val;
}

Example:

char *z ="82ABC1EF";
uint32_t x = hex2int(z);
printf("Number is [%X]\n", x);

Will output: