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.
