manpage

If anyone is actually intending on installing dehexer, here is the manpage:

.\" Dehexer
.TH dehexer 1 "20 April 2008" "1.0" "dehexer"
.SH NAME
.B dehexer 
is a simple program that converts a file of ascii characters into
the equivalent binary file. Any character not in the range 0-9 or
letters not A-F or a-f are simply ignored.
.SH EXAMPLES
dehexer < 
.I ASCII_FILE 
> 
.I BINARY_FILE
.SH DESCRIPTION
dehexer is used to convert a human-readable (i.e. description of
the bytes) into an actual binary file. Thus we get "0a" is
transformed into the byte 00001010 on disk (of course, it would
actually be the bits themselves).
.SH OPTIONS
None, just do normal I/O redirection.
.SH FILES
.P 
.I /usr/share/man/man1/dehexer.1.gz
.SH SEE ALSO
.BR hexer(1)
.SH BUGS
No known bugs at this time. 
.SH AUTHOR
.nf
Chris Wilson (christopher.j.wilson@gmail.com)
This program is dedicated to the public domain.
.fi
.SH HISTORY
2008 - Written as a compliment to Hal Canary's hexer program.

Gzip the above text and store it in

/usr/share/man/man1/

(at least on my system).

dehexer

I was looking through Hal's blog when I came across his hexer program. I thought that I should write the complimentary program, that is, given an ascii file of hex characters (e.g. "01CAFEBABE") it will write actual bytes to stdout.

chris@papaya:c$ cc helloworld.c
chris@papaya:c$ ./a.out 
Hello World!
chris@papaya:c$ cat a.out | ./hexer | ./dehexer > helloworld
chris@papaya:c$ chmod a+x helloworld
chris@papaya:c$ ./helloworld 
Hello World!

It seems to work (I changed the name to 'helloworld' because doing

cat a.out | ./hexer | ./dehexer > a.out

seems to clobber the file in a bad way.

Without further ado:

/* dehexer - Convert an ascii file of hex characters into the
   corresponding binary file. Any non-hex characters are silently
   skipped (newlines, tabs, etc.)
   Copyright 2008 Christopher Wilson, based in part on hexer by Hal
   Canary (also DTPD)
   Dedicated to the Public Domain */
/* cc -o dehexer dehexer.c */
#include 
#include 
int main (int argc, char *argv[])
{
  char x;
  int char_out = 0;
  int low = 0; // LSBs, 1 for true
  while (fread(&x, sizeof(x), 1, stdin) == 1) {
    if (x > 47 && x < 58) { 
      // digit is 0-9
      x = x - 48;
      handle_byte(x, &char_out, low);
    } else if (x > 64 && x < 71) {
      // digit is A-F
      x = x - 55;
      handle_byte(x, &char_out, low);
    } else if (x > 96 && x < 103) {
      // digit a-f
      x = x - 87;
      handle_byte(x, &char_out, low);
    } else {
      // skip anything that isn't 0-9A-Z, or a-z
      continue;
    }
    low = (low + 1) % 2; // flip the high/low bit marker
  }
  return(0);
}

/* handle_byte - if low is true then it prints the full byte. If low
     is false, set char_out to 16 times x.
*/
int handle_byte(int x, char *char_out, int low)
{
  if(low) {
    *char_out += x;
    putc(*char_out, stdout);
  } else {
    *char_out = x * 16;
  }
  return 0;
}
/* EOF */