How to change HEX String to Byte Array

Hi All, I want to change HEX String to Byte Array, but I can't get the expected result.

Code:

//Hex String
char* HEXStr="A4505D0B0F6AEDAA";

unsigned char* tmpByte=malloc(9);
for(int i=0; i<16; i++)
{
  sscanf(HEXStr, "02X", &tmpByte[i]);
  HEXStr+2*sizeof(char);
}

//Change it to Hex
char* backHex=malloc(17);
for(int i =0; i<8; i++)
{
  sprintf(backHex+2*i, "02X", tmpByte[i]); 
}

HEXStr ="A4505D0B0F6AEDAA";
Result of backHEX="FF505D0B0F6AFFFF";

Someone was having trouble before with

Code:

//Hex String
char* HEXStr="A4505D0B0F6AEDAA";

The changed it to

Code:

//Hex String
char HEXStr[]="A4505D0B0F6AEDAA";

It gave the string a place in memory

I think that you are not showing all your code (right?) - I'm saying that because you are not freeing any memory you are "mallocing".

Code:

unsigned char* tmpByte=malloc(9);
for(int i=0; i<16; i++)
{
  sscanf(HEXStr, "02X", &tmpByte[i]); // <--------- For i up to 16?!

I think that you should get rid of the mallocs

I don't understand how you got any result at all. Your format specifiers are completely wrong (hint: they start with a percent sign). The line: HEXStr+2*sizeof(char); does nothing at all.

Code:

#include <stdio.h>

int main() {
    int i, n;
    char *HEXStr = "A4505D0B0F6AEDAA";
    unsigned char tmpByte[9];
    char backHex[17];

    for(i = 0; i < 8; i++) {
        sscanf(HEXStr+2*i, "%2X", &n);
        tmpByte[i] = (char)n;
    }

    for(i = 0; i < 8; i++)
        sprintf(backHex+2*i, "%02X", tmpByte[i]);
    backHex[16] = '\0';

    printf("%s\n", backHex);
    return 0;
}

This is not the exact code that you compiled, is it? Can you copy and paste the actual code that gives the results you posted?

I think for the most part you are just doing unnecessary calculations and using unnecessary temporary space.

One of the reasons hexadecimal is so nice is because 1 hexa digit is equal to a nibble or 4 bits, so if I were supposed to convert your string into a byte string it would have to be like:

Code:

A4	 50 	 5D 	  0B 	   0F 	    6A	     ED	      AA
10100100 0101000 01011101 00001011 00001111 01101010 11101101 10101010

Each column is one byte of string. BTW this is for illustration. Just don't mind the numbers too much, I tried to convert to binary and I think it's right, but I might have transposed some digits or something.

The good news is that sscanf can indeed do this work, but you need to handle it two characters at a time, because it is the same as storing the result one byte at a time.

Additionally, if you actually look up the %X conversion for sscanf (check your syntax!) you will find that the result is expected to be a unsigned int. Just use a temporary variable with sscanf, and cast over to unsigned char or char, whichever is right, when you put it into the byte string. I am fairly sure this was completely ignored in your attempt and contributed to things being very wrong.

Code:

#include <stdio.h>
 
int main() {
    int i, n;
    char *HEXStr = "A4505D0B0F6AEDAA";
    unsigned char tmpByte[9];
    char backHex[17];
 
    for(i = 0; i < 16; i++) {
        sscanf(HEXStr+2*i, "%2X", &n);
        tmpByte[i] = (char)n;
    }
 
    for(i = 0; i < 8; i++)
        sprintf(backHex+2*i, "%02X", tmpByte[i]);
    backHex[16] = '\0';
 
    printf("%s\n", backHex);
    return 0;
}

Whoops