C Hexadecimal String To Byte Array Conversion

hex_str_2_byte_array.c

/* Libraries */

// Standard libc Library
#include <stdio.h>
#include <stdint.h>
#include <stdbool.h>

/*****************************************************************************/

/* Defines */

// Hexadecimal string max size
#define HEX_STR_SIZE 23

// Hexadecimal byte array size
#define BYTE_ARRAY_SIZE 11

/*****************************************************************************/

/* Function Prototypes */

/**
 * @brief Convert an hexadecimal string to uint8_t bytes array.
 * @param hex_str Hexadecimal string to convert.
 * @param hex_str_length Length of hexadecimal string.
 * @param byte_array Converted byte array.
 * @param byte_array_size Maximum size of byte array.
 * @return Conversion result success/fail (true/false).
 */
bool hex_str_2_byte_array(const char* hex_str, const uint16_t hex_str_length,
        uint8_t* byte_array, const uint16_t byte_array_size);

/**
 * @brief Convert an hexadecimal character to unsigned integer of 8 bits.
 * @param c Character to convert.
 * @param n Converted byte number.
 * @return Conversion result success/fail (true/false).
 */
bool char_2_hex(const char c, uint8_t* n);

/*****************************************************************************/

/* Main Function */

int main()
{
    char hex_str[HEX_STR_SIZE] = "0123456789abcdefABCDEF";
    uint8_t byte_array[BYTE_ARRAY_SIZE] = { 0x00 };
    bool fail = false;

    fail = !hex_str_2_byte_array(hex_str, HEX_STR_SIZE-1,
            byte_array, BYTE_ARRAY_SIZE);
    if (fail)
    {
        printf("\nConversion Fail\n");
        return 1;
    }
    printf("\nConversion Success\n");
    printf("hex_str:    %s\n", hex_str);
    printf("byte_array: ");
    for (uint8_t i = 0; i < BYTE_ARRAY_SIZE; i++)
        printf("%02X", byte_array[i]);
    printf("\n\n");

    return 0;
}

/*****************************************************************************/

/* Auxiliary Functions */

/**
 * @details
 * To make the conversion, it go through each hexadecimal string character and
 * use the char_2_hex() function on each of it to get the corresponding high
 * and low nibbles that are merged to compose the hexadecimal byte value.
 * The hexadecimal string provided must be an even length string and has 2 or
 * more characters, and the output byte array must has enough size to store
 * the full conversion of the string (byte_array_size >= (hex_str_length / 2)).
 */
bool hex_str_2_byte_array(const char* hex_str, const uint16_t hex_str_length,
        uint8_t* byte_array, const uint16_t byte_array_size)
{
    uint8_t n_left_nibble = 0;
    uint8_t n_right_nibble = 0;

    // Check for valid str, has 2 or more chars and is not an odd string
    if (hex_str == NULL)
        return false;
    if (hex_str_length < 2)
        return false;
    if ((hex_str_length % 2) != 0)
        return false;

    // Check if out byte array has enough size to store the conversion
    // Note that: (hex_str_length >> 1) == (hex_str_length / 2)
    if (byte_array_size < (hex_str_length >> 1))
        return false;

    // Convert each character of the string
    uint16_t ii = 0;
    for (uint16_t i = 0; i < hex_str_length; i = i + 2)
    {
        if (!char_2_hex(hex_str[i], &n_left_nibble))
            return false;
        if (!char_2_hex(hex_str[i+1], &n_right_nibble))
            return false;

        byte_array[ii] = (n_left_nibble << 4) | (n_right_nibble & 0x0f);
        ii = ii + 1;
    }

    return true;
}

/**
 * @details
 * To convert an hexadecimal character into an uint8_t data type, it checks
 * for expected valid characters (0-9, a-f or A-F) and substract the 
 * corresponding ASCII code offset to point into 0-15 integer value.
 * Check the ASCII Encode Table:
 * https://upload.wikimedia.org/wikipedia/commons/1/1b/ASCII-Table-wide.svg
 */
bool char_2_hex(const char c, uint8_t* n)
{
    if ((c >= '0') && (c <= '9'))
        *n = (uint8_t)(c - '0');
    else if ((c >= 'a') && (c <= 'f'))
        *n = (uint8_t)(c - 'a') + 10;
    else if ((c >= 'A') && (c <= 'F'))
        *n = (uint8_t)(c - 'A') + 10;
    else
        return false;

    return true;
}