97. Convert Hexadecimal String to Integer Without strtol or sscanf functions

#include <stdio.h>
#include <stdint.h>

// Convert a single hex char to its integer value
uint8_t hex_char_to_val(char c) {
    if (c >= '0' && c <= '9') return c - '0';
    else if (c >= 'A' && c <= 'F') return c - 'A' + 10;
    else if (c >= 'a' && c <= 'f') return c - 'a' + 10;
    else return 0; // Invalid character, could return error
}

// Convert hex string to unsigned integer
uint16_t hex_to_uint(const char *str) {
    uint16_t result = 0;
    for (int i = 0; str[i] != '\0'; i++) {
        result <<= 4; // shift by 4 bits (hex digit)
        result |= hex_char_to_val(str[i]);
    }
    return result;
}

int main() {
    char hex[10];
    scanf("%s", hex);

    printf("%u", hex_to_uint(hex));
    return 0;
}

Why This is Important in Firmware?

• Used in CLI tools, bootloaders, debug utilities
• Avoids linking large libraries (stdlib, sscanf)
• Needed in parsing hex configuration values or memory dump input

Logic Summary

• For each character in hex string:
  
  • Convert using manual mapping
  • Left shift result by 4 bits
  • OR in the new value