Thread: How to change HEX String to Byte Array

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;
}

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.