binary decimal conversion

Since I'm an avid C hobbyist (and I have the day off work) I thought I'd tackle the "bits and nibbles" problem with a simple conversion program. There are some things about it that I suspect are wrong -- ie, I can see that converting with atoi() rather than atol() gives me a different result with very large or negative numbers, and I'm not sure which gives the more appropriate result in all cases. I've looked at strtoul(), though I have to admit the man page made my eyes cross a bit -- is this what I need? I'm afraid that as much as I enjoy coding, math is not my strongest point

Suggestions, feedback, and guidance are welcome ...

Code:

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

/* cvtbin.c - a simple calculator for binary to decimal
 * and decimal to binary conversion */

#define UNIX 1

#if UNIX
# define CLEAR_SCREEN "clear"
#else
# define CLEAR_SCREEN "clr"
#endif

int c[32];  char s[40];


char *
itob (unsigned long int n)
{
  int i, j, k = 0;  char *p;

  for (i = 0; i < 32; i++)
    c[i] = (n >> i) & 0x1;

  for (i = 0, j = 31; j >= 0; i++, j--)
  {
    if (k == 4)
    {
      k = 0;
      s[i++] = ' ';
    }
    s[i] = c[j]+'0';  k++;
  }
  if ((p = strchr (s, '1')) == NULL)
    return s+35;
  k = p - s;
  k /= 5;  k *= 5;
  return s+k;
}


unsigned long int
btoi (char *sb)
{
  int i, j;  unsigned long int n;
  size_t k = strlen (sb)-1;

  for (i = 0, j = k; j >= 0; i++, j--)
  {
    if (sb[j] > '1')
      return 0;
    if (sb[j] != ' ')
      c[i] = sb[j]-'0';
  }
  i = k+1;  n = 0;
  while (i--)
  {
    n <<= 1;
    n += c[i];
  }
  return n;
}


void
clr_stdin (char *s)
{
  char ch, *p;
  if ((p = strchr (s, '\n')) == NULL)
    while ((ch = getchar()) != '\n' && ch != EOF)
      ;
  else *p = '\0';
}


void
to_binary (void)
{
  char in[10+1];  unsigned long int n;

  system (CLEAR_SCREEN);
  while (1)
  {
    printf ("Enter a number to convert to binary > ");
    if (fgets (in, 10+1, stdin))
    {
      if (*in == '\n' || (isdigit (*in) == 0 && *in != '-'))
        break;
      clr_stdin (in);
      n = atol (in);
      printf ("\ndecimal %lu is binary %s\n\n", n, itob (n));
    }
  }
}


void
to_decimal (void)
{
  char in[39+1];  unsigned long int t;

  system (CLEAR_SCREEN);
  while (1)
  {
    printf ("Enter a binary string to convert to decimal\n> ");
    if (fgets (in, 39+1, stdin))
    {
      if (*in == '\n' || isdigit (*in) == 0)
        break;
      clr_stdin (in);
      t = btoi (in);
      printf ("\nbinary %s is decimal %lu", in, t);
      if (t == 0)
        printf (" or not binary");
      printf ("\n\n");
    }
  }
}


int
main (void)
{
  char in[2+1];

  while (1)
  {
    system (CLEAR_SCREEN);
    printf ("Enter your selection\n"
            "    1) convert decimal to binary\n"
            "    2) convert binary to decimal\n"
            "or any other key to quit > ");

    if (fgets (in, 2+1, stdin))
    {
      clr_stdin (in);
      if (in[1])
        break;
      if (*in == '1')
        to_binary();
      else if (*in == '2')
        to_decimal();
      else break;
    }
  }
  printf ("\nGood bye\n\n");
  return 0;
}

This stuff is a no-go for me.

Code:

#if UNIX
# define CLEAR_SCREEN "clear"
#else
# define CLEAR_SCREEN "clr"
#endif

System calls in general are not too popular with folks.

Globals may be frowned upon too.

The strtoul approach need not be as brutal as it may look.

Code:

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

int main(void)
{
   char binary[] = "1101010110", *end;
   unsigned long value = strtoul(binary, &end, 2);
   if ( *end == '\0' )
   {
      printf("value = %lu\n", value);
   }
   return 0;
}

/* my output
value = 854
*/

This seems a bit cryptic: anytime "magic numbers" pop up.

Code:

  if ((p = strchr (s, '1')) == NULL)
    return s+35;
  k = p - s;
  k /= 5;  k *= 5;
  return s+k;

If you changed an array size, for example changing s from a size of 40 to say 120, would a maintainer (even yourself) have enough hints to track down other places to update?

The 32-bit-centricity can be avoided.

But it looks like you're having fun, so I'll stop picking nits.

Thanks for your comments. It's true I get sloppy with globals -- that's a known bad habit of mine. And the part you call "cryptic" I knew was a horrible kludge ... I sort of winced when I coded it.

That strtoul() example you give is nice; I should've looked at that earlier since it's definitely a more practical way to convert a binary string.

Anyway, yes, I'm having fun, but the nits are good, too