Print binary representation of file

Question

I need a function which will print out the binary representation of a read file like the xxd program in unix, but I want to make my own. Hexidecimal works just fine with %x but there is no built in format for binary. Anyone know how to do this?

Answer 1

I usually do not believe in answering these sorts of questions with full code implementations, however I was handed this bit of code many years ago and I feel obligated to pass it on. I have removed all the comments except for the usage, so you can try to figure out how it works yourself.

Code _{base 16}

#include <stdio.h>
#include <ctype.h>

// Takes a pointer to an arbitrary chunk of data and prints the first-len bytes.
void dump (void* data, unsigned int len)
{
  printf ("Size:  %d\n", len);

  if (len > 0) {
    unsigned width = 16;
    char *str = (char *)data;
    unsigned int j, i = 0;

    while (i < len) {
      printf (" ");

      for (j = 0; j < width; j++) {
        if (i + j < len)
          printf ("%02x ", (unsigned char) str [j]);
        else
          printf ("   ");

        if ((j + 1) % (width / 2) == 0)
          printf (" -  ");
      }

      for (j = 0; j < width; j++) {
        if (i + j < len)
          printf ("%c", isprint (str [j]) ? str [j] : '.');
        else
          printf (" ");
       }

       str += width;
       i += j;

      printf ("\n");
    }
  }
}

Output _{base 16} (Excerpt from first 512 bytes* of a flash video)

Size:  512
 00 00 00 20 66 74 79 70  -  69 73 6f 6d 00 00 02 00  -  ... ftypisom....
 69 73 6f 6d 69 73 6f 32  -  61 76 63 31 6d 70 34 31  -  isomiso2avc1mp41
 00 06 e8 e6 6d 6f 6f 76  -  00 00 00 6c 6d 76 68 64  -  ....moov...lmvhd
 00 00 00 00 7c 25 b0 80  -  7c 25 b0 80 00 00 03 e8  -  ....|%..|%......
 00 0c d6 2a 00 01 00 00  -  01 00 00 00 00 00 00 00  -  ...*............
 00 00 00 00 00 01 00 00  -  00 00 00 00 00 00 00 00  -  ................
 00 00 00 00 00 01 00 00  -  00 00 00 00 00 00 00 00  -  ................
 00 00 00 00 40 00 00 00  -  00 00 00 00 00 00 00 00  -  ....@...........
 00 00 00 00 00 00 00 00  -  00 00 00 00 00 00 00 00  -  ................
 00 01 00 02 00 01 9f 38  -  74 72 61 6b 00 00 00 5c  -  .......8trak...\

I assume you already know how to tell the size of a file and read a file in binary mode, so I will leave that out of the discussion. Depending on your terminal width you may need to adjust the variable: width -- the code is currently designed for 80 character terminals.

I am also assuming that when you mentioned xxd in conjunction with "binary" you meant non-text as opposed to base 2. If you want base 2, set width to 6 and replace printf ("%02x ", (unsigned char) str [j]); with this:

{
  for (int k = 7; k >= 0; k--)
    printf ("%d", ((unsigned char)str [j] >> k) & 1);
  printf (" ");
}

The required change is pretty simple, you just need to individually shift all 8 bits of your octet and mask off all but the least-significant bit. Remember to do this in an order that seems counter-intuitive at first, since we print left-to-right.

Output _{base 2} (Excerpt from first 512 bytes* of a flash video)

Size:  512
 00000000 00000000 00000000  -  00100000 01100110 01110100  -  ... ft
 01111001 01110000 01101001  -  01110011 01101111 01101101  -  ypisom
 00000000 00000000 00000010  -  00000000 01101001 01110011  -  ....is
 01101111 01101101 01101001  -  01110011 01101111 00110010  -  omiso2
 01100001 01110110 01100011  -  00110001 01101101 01110000  -  avc1mp
 00110100 00110001 00000000  -  00000110 11101000 11100110  -  41....
 01101101 01101111 01101111  -  01110110 00000000 00000000  -  moov..
 00000000 01101100 01101101  -  01110110 01101000 01100100  -  .lmvhd
 00000000 00000000 00000000  -  00000000 01111100 00100101  -  ....|%
 10110000 10000000 01111100  -  00100101 10110000 10000000  -  ..|%..
 00000000 00000000 00000011  -  11101000 00000000 00001100  -  ......

_{*For the sake of simplicity, let us pretend that a byte is always 8-bits.}

Answer 2

Depending on the language, assuming you have bitwise operations, which lets you act on each bit of a variable, you can do the following. Read the file into a buffer, or a line, if encoding is needed, force it to extended ASCII (8 bit/ 1 byte character) now, when you get the buffer, you loop from 7 to 0 and using the and bitwise and a shift to check each bit value, let me give an example in C:

// gcc -Wall -Wextra -std=c99 xxd.c
#include <stdio.h>
#include <string.h>

int main() {
  // Whatever buffer size you chose.
  char buffer[32];
  //Feel free to replace stdin to a File Pointer, or any other stream
  // Reading into a char, means reading each byte at once
  while (!feof(stdin)) {
    // Read at most buffer bytes. Since its ASCII 1 byte = 1 char.
    fgets(buffer, sizeof(buffer), stdin);
    // Iterate though each character in the string/buffer.
    const size_t len = strlen(buffer);
    for (size_t j = 0; j < len; j++) {
      // Print the most significant bit first.
      for (int i = 7; i >=0; i--) {
        // Check if the i-th bit is set
        printf(buffer[j] & (1 << i) ? "1" : "0");
      }
    }
  }

  return 0;
}