[c] Hex to ascii string conversion

i have an hex string and want it to be converted to ascii string in C. How can i accomplish this??

you need to take 2 (hex) chars at the same time... then calculate the int value and after that make the char conversion like...

char d = (char)intValue;

do this for every 2chars in the hex string

this works if the string chars are only 0-9A-F:

#include <stdio.h>
#include <string.h>

int hex_to_int(char c){
        int first = c / 16 - 3;
        int second = c % 16;
        int result = first*10 + second;
        if(result > 9) result--;
        return result;
}

int hex_to_ascii(char c, char d){
        int high = hex_to_int(c) * 16;
        int low = hex_to_int(d);
        return high+low;
}

int main(){
        const char* st = "48656C6C6F3B";
        int length = strlen(st);
        int i;
        char buf = 0;
        for(i = 0; i < length; i++){
                if(i % 2 != 0){
                        printf("%c", hex_to_ascii(buf, st[i]));
                }else{
                        buf = st[i];
                }
        }
}

Few characters like alphabets i-o couldn't be converted into respective ASCII chars . like in string '6631653064316f30723161' corresponds to fedora . but it gives fedra

Just modify hex_to_int() function a little and it will work for all characters. modified function is

int hex_to_int(char c)
{
    if (c >= 97)
        c = c - 32;
    int first = c / 16 - 3;
    int second = c % 16;
    int result = first * 10 + second;
    if (result > 9) result--;
    return result;
}

Now try it will work for all characters.

strtol() is your friend here. The third parameter is the numerical base that you are converting.

Example:

#include <stdio.h>      /* printf */
#include <stdlib.h>     /* strtol */

int main(int argc, char **argv)
{
    long int num  = 0;
    long int num2 =0;
    char * str. = "f00d";
    char * str2 = "0xf00d";

    num = strtol( str, 0, 16);  //converts hexadecimal string to long.
    num2 = strtol( str2, 0, 0); //conversion depends on the string passed in, 0x... Is hex, 0... Is octal and everything else is decimal.

    printf( "%ld\n", num);
    printf( "%ld\n", num);
}

If I understand correctly, you want to know how to convert bytes encoded as a hex string to its form as an ASCII text, like "537461636B" would be converted to "Stack", in such case then the following code should solve your problem.

Have not run any benchmarks but I assume it is not the peak of efficiency.

static char ByteToAscii(const char *input) {
  char singleChar, out;
  memcpy(&singleChar, input, 2);
  sprintf(&out, "%c", (int)strtol(&singleChar, NULL, 16));
  return out;
}

int HexStringToAscii(const char *input, unsigned int length,
                            char **output) {
  int mIndex, sIndex = 0;
  char buffer[length];
  for (mIndex = 0; mIndex < length; mIndex++) {
    sIndex = mIndex * 2;
    char b = ByteToAscii(&input[sIndex]);
    memcpy(&buffer[mIndex], &b, 1);
  }
  *output = strdup(buffer);
  return 0;
}