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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Monday, January 16, 2012

99 ways to program a hex, Part 8: C99, const and restrict correctness

Much like the difference between part 1 and part 4, there is very little difference between today's code and yesterday's code:

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*
* Copyright 2012 by Sean Conner.  All Rights Reserved.
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* GNU General Public License for more details.
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* You should have received a copy of the GNU General Public License
* along with this program; if not, write to the Free Software
* Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA  02111-1307, USA.
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/* Style: C99, const and restrict correctness */

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

#define LINESIZE	  16

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

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

int main(const int argc,char *const restrict argv[])
{
  if (argc == 1)
    do_dump(stdin,stdout);
  else
  {
    for (int i = 1 ; i < argc ; i++)
    {
      FILE *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 restrict fpin,FILE *const restrict fpout)
{
  unsigned char  buffer[BUFSIZ];
  size_t         offset;
  size_t         bread;
  
  offset = 0;

  while((bread = fread(buffer,1,BUFSIZ,fpin)) > 0)
  {
    unsigned char *pbyte = buffer;

    while (bread > 0)
    {
      char ascii[LINESIZE + 1];
      
      fprintf(fpout,"%08lX: ",(unsigned long)offset);
      size_t 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)
	for (size_t i = j ; i < LINESIZE ; i++) fprintf(fpout,"   ");

      fprintf(fpout,"%s\n",ascii);      
    }
    
    if (fflush(fpout) == EOF)
    {
      perror("output");
      exit(EXIT_FAILURE);
    }
  }
}

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

C99 adds restrict to the ways one can modify the access to a variable. The rational behind this is a bit esoteric—it tells the compiler that a pointer is the only pointer to a block of memory.

Yes, it does seem odd to have to add a keyword for that, but it does help with code optimization. For instance, the following (silly) function:

int foo(int *p1,int *p2)
{
  *p2 =  *p1 * 17;
  return *p1 * 17;
}

The problem here is that the compiler has to do the multiplication twice, as p2 could be pointing to the same location as p1, and thus, the contents pointed to by p1 could be modified. So the compiler is forced to write machine code like:

foo:		mov	ebx,[esp + 4]	; get p1
		mov	eax,[ebx]	; read *p1
		imul	eax,17		; multiply
		mov	edx,[esp + 8]	; get p2
		mov	[edx],eax	; save results in *p2
		mov	eax,[ebx]	; read *p1
		imul	eax,17		; multiply
		ret

Not exactly optimum, but the C compiler is constrained because of the semantics of pointers in C. Change the C code a bit:

int foo(int *restrict p1,int *restrict p2)
{
  *p2 =  *p1 * 17;
  return *p1 * 17;
}

And the compiler can now produce:

foo:		mov	ebx,[esp + 4]	; get p1
		mov	eax,[ebx]	; read *p1
		imul	eax,17		; multiply
		mov	edx,[esp + 8]	; save
		mov	[edx],eax	; return result
		ret

Okay, it's only a savings of two instructions (plus an an additional read) but when you're trying to multiply huge matrices, it can add up.

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