The Boston Diaries

The ongoing saga of a programmer who doesn't live in Boston, nor does he even like Boston, but yet named his weblog/journal “The Boston Diaries.”

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Tuesday, January 17, 2012

99 ways to program a hex, Part 9: C89, const correctness, assertive

This is a minor variation on part 7—the use of assert():

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* Copyright 2012 by Sean Conner.  All Rights Reserved.
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/* Style: C89, const correctness, assertive */

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

#define LINESIZE	  16

static void 	do_dump		(FILE *const,FILE *const);

/****************************************************************/

int main(const int argc,char *const argv[])
{
  assert(argc    >= 1);
  assert(argv    != NULL);
  assert(argv[0] != NULL);
  
  if (argc == 1)
    do_dump(stdin,stdout);
  else
  {
    int i;
    
    for (i = 1 ; i < argc ; i++)
    {
      FILE *fp;
      
      fp = fopen(argv[i],"rb");
      if (fp == NULL)
      {
        perror(argv[i]);
        continue;
      }

      printf("-----%s-----\n",argv[i]);
      do_dump(fp,stdout);
      fclose(fp);
    }
  }

  return EXIT_SUCCESS;
}

/******************************************************************/

static void do_dump(FILE *const fpin,FILE *const fpout)
{
  unsigned char  buffer[BUFSIZ];
  unsigned char *pbyte;
  size_t         offset;
  size_t         bread;
  size_t         j;
  char           ascii[LINESIZE + 1];
  
  assert(fpin  != NULL);
  assert(fpout != NULL);
  
  offset = 0;

  while((bread = fread(buffer,1,BUFSIZ,fpin)) > 0)
  {
    pbyte = buffer;
    while (bread > 0)
    {
      fprintf(fpout,"%08lX: ",(unsigned long)offset);
      j = 0;
      do
      {
        fprintf(fpout,"%02X ",*pbyte);
        if (isprint(*pbyte))
          ascii [j] = *pbyte;
        else
          ascii [j] = '.';
        pbyte  ++;
        offset ++;
        j      ++;
        bread  --;
      } while ((j < LINESIZE) && (bread > 0));
      ascii [j] = '\0';
      if (j < LINESIZE)
      {
	size_t i;

	for (i = j ; i < LINESIZE ; i++) fprintf(fpout,"   ");
      }
      fprintf(fpout,"%s\n",ascii);      
    }
    
    if (fflush(fpout) == EOF)
    {
      perror("output");
      exit(EXIT_FAILURE);
    }
  }
}

/***************************************************************/

Writing Solid Code is one of only two programming books that really change how I write code (the other being Thinking Forth but that's for another episode post), begining with the liberal use of assert() to, well, not validate input parameters, but to enforce that they're valid.

Prior to this book, I wrote defensive code, so prior to reading the book, I would have coded do_dump() as:

static void do_dump(FILE *const fpin,FILE *const fpout)
{
  /* vars vars vars */

  if ((fpin == NULL) || (fpout == NULL))
    return;

  /* rest of code */
}

Not very much code (and in this code, useless as well), but in a larger codebase, it does add up. And it hides problems with the code. The first project I liberally used assert() I really went crazy with it. The codebase implemented “window regions” on a text screen, and every routine used assert() to not only check that I didn't slip in a NULL pointer, but that every field of all the structures I defined had reasonable values.

And doing so saved me a lot of debugging time in the corner cases, like, what exactly does it mean to have a “window” that's only one character wide? Or even a window that's one character wide by one line high? The assert()s would trip up on all sorts of corner cases like this, and given that I was programming the code under MS-DOS, an errant pointer could not only crash the program, but the entire machine (at best—at worst, it could corrupt memory that wouldn't be detected until some other program ran).

I still use assert()s to this day.

Now, I'll grant you the following bit of code:

int main(const int argc,char *const argv[])
{
  assert(argc    >= 1);
  assert(argv    != NULL);
  assert(argv[0] != NULL);

is going a bit too far, only because this is guaranteed to be true by the C standard, and if it's not, I have more pressing issues to worry about.

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