Problem with writing a hexadecimal string

Question

Here is my code

/*
gcc -c -Wall -g main.c
gcc -g -lm -o main main.o
*/

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

void stringToHex(const char* string, char* hex) {
 int i = 0;

 for(i = 0; i < strlen(string)/2; i++) {
  printf("s%x", string[2*i]); //for debugging
  sprintf(&hex[i], "%x", string[2*i]);
  printf("h%x\n", hex[i]); //for debugging
 }
}

void writeHex(char* hex, int length, FILE* file, long position) {
 fseek(file, position, SEEK_SET);
 fwrite(hex, sizeof(char), length, file);
}

int main(int argc, char** argv) {
 FILE* pic = fopen("hi.bmp", "w+b");
 const char* string = "f2";
 char hex[strlen(string)/2];

 stringToHex(string, hex);
 writeHex(hex, strlen(string)/2, pic, 0);




 fclose(pic);
 return 0;
}

I want it to save the hexadecimal number 0xf2 to a file (later I will have to write bigger/longer numbers though). The program prints out -

s66h36

And when I use hexedit to view the file I see the number '36' in it.

Why is my code not working? Thanks!

Answer 1

It's not (at all!) apparent from your code what you really want to accomplish. You're starting with a string containing hexadecimal digits, and then (apparently) trying to convert that to hexadecimal...

Normally, you'd do something like this:

int x = 0xf2;

printf("%0x", x);

Answer 2

When it processes "f", it converted it to the character f, which is ascii 102, which is hex 66. That's why you got the "s66" part of your answer.

%x prints out an integer in its hexadecimal representation.

I think you want sscanf( string, "%x", &hexInt )

That'll read in string as a hexadecimal string and save its value in the int hexInt.

Answer 3

Using muddybruin's answer I built this-

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

void stringToHex(char* string, int* hex) {
    int i = 0;
    char tmpByte[3] = {'\0', '\0', '\0'};

    for(i = 0; i < strlen(string)/2; i++) {
        memcpy(tmpByte, &string[2*i], 2);
        sscanf(tmpByte, "%x", &hex[i]);
    }
}

void writeHex(int* hex, int length, FILE* file, long position) {
    int i = 0;

    fseek(file, position, SEEK_SET);
    for(i = 0; i < length; i++) {
        fwrite(&hex[i], sizeof(char), 1, file);
        fseek(file, -3/4*sizeof(int), SEEK_CUR);
    }
}

int main(int argc, char** argv) {
    FILE* pic = fopen("hi.bmp", "w+b");
    char* string = "424D460000000000000036000000280000000200000002000000010018000000000010000000130B0000130B000000000000000000000000AAAFFFFF0000FF000000FF000000";
    int* hex = calloc(1000, sizeof(int));

    stringToHex(string, hex);
    writeHex(hex, strlen(string)/2, pic, 0);

    fclose(pic);
    return 0;
}

Not the best code in the world, but it works :).