Reading hex from a file

Question

I am trying to read hex values from a text file. I then need to store their individual 4 bit binary equivalents so that i can access the individual bits for comparison. In the file the hex is in this format:

5104
de61
567f

And I would like to be able to have each line stored one at a time as binary in such a way that I can access a specific bit, maybe in an array? Like: {0,1,1,0,0,0,0,1...}

My vague attempts at understanding C has yielded this:

int main(int argc, char *argv[]){
    FILE *file;
    char instructions[8];
    file = fopen(argv[1], "r");
    while(fgets(instructions, sizeof(instructions), file) != NULL){
        unsigned char a[8];
        int i = 0;
        while (instructions[i] != '\n'){
            int b;
            sscanf(&instructions[i], "%2x", &b);
            a[i] = b;
            i += 2;
        }
    }
    fclose(file);
    return 0;
}

**Post what you tried**, and your speculation for why you thought it would work and why it seems to have failed. — WhozCraig, Oct 01 '14 at 22:16
Store them in bytes and use bitwise operators to check the bits — M.M, Oct 01 '14 at 22:17
"I've tried every way I can think of" That's good. Please show your closest attempt, because good attempts at entry-level problems are usually good enough to be fixed. — Sergey Kalinichenko, Oct 01 '14 at 22:18
You need two indexes: one for the source, one for the destination. — wildplasser, Oct 01 '14 at 22:52

score 2 · Accepted Answer · edited May 23 '17 at 11:52

Taken from here. With this code you can read the file as it is.

#include <stdio.h>

int main() {
    FILE *f;
    unsigned int num[80];
    int i=0;
    int rv;
    int num_values;

    f=fopen("test.txt","r");
    if (f==NULL){
        printf("file doesnt exist?!\n");
        return 1;
    }

    while (i < 80) {
        rv = fscanf(f, "%x", &num[i]);

        if (rv != 1)
            break;

        i++;
    }
    fclose(f);
    num_values = i;

    if (i >= 80)
    {
        printf("Warning: Stopped reading input due to input too long.\n");
    }
    else if (rv != EOF)
    {
        printf("Warning: Stopped reading input due to bad value.\n");
    }
    else
    {
        printf("Reached end of input.\n");
    }

    printf("Successfully read %d values:\n", num_values);
    for (i = 0; i < num_values; i++)
    {
        printf("\t%x\n", num[i]);
    }

    return 0;
}

Now, if you want to convert hex in binary, you could do it with many was. Check some links that provide methods.

Just for the reference, here is the code from the first link:

const size_t numdigits = strlen(input) / 2;

uint8_t * const output = malloc(numdigits);

for (size_t i = 0; i != numdigits; ++i)
{
  output[i] = 16 * toInt(input[2*i]) + toInt(intput[2*i+1]);
}

unsigned int toInt(char c)
{
  if (c >= '0' && c <= '9') return      c - '0';
  if (c >= 'A' && c <= 'F') return 10 + c - 'A';
  if (c >= 'a' && c <= 'f') return 10 + c - 'a';
  return -1;
}

Reading hex from a file

1 Answers1