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.”

Go figure.

Monday, January 30, 2012

99 ways to program a hex, Part 22: C89, const correctness, assertive, system calls, full buffering

So yesterday I presented a non-portable version that was quite a bit faster than the portable version, but I'm not quite done yet. That version just buffered a line at a time—today's version buffers nearly 8k worth of data (it's not exact, but it's close enough) between calls to write().

/*************************************************************************
*
* Copyright 2012 by Sean Conner.  All Rights Reserved.
*
* This program is free software; you can redistribute it and/or
* modify it under the terms of the GNU General Public License
* as published by the Free Software Foundation; either version 2
* of the License, or (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
* GNU General Public License for more details.
*
* 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.
*
* Comments, questions and criticisms can be sent to: sean@conman.org
*
*************************************************************************/

/* Style: C89, const correctness, assertive, system calls, full buffering */

#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <assert.h>

#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>

#define LINESIZE	16

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

extern const char *sys_errlist[];
extern int         sys_nerr;

static void	do_dump		(const int,const int);
static size_t	dump_line	(char **const,unsigned char *,size_t,const unsigned long);
static void	hexout		(char *const,unsigned long,size_t,const int);
static void	myperror	(const char *const);
static size_t	myread		(const int,char *,size_t);
static void	mywrite		(const int,const char *const,const size_t);

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

int main(const int argc,const char *const argv[])
{
  if (argc == 1)
    do_dump(STDIN_FILENO,STDOUT_FILENO);
  else
  {
    int i;
    
    for (i = 1 ; i < argc ; i++)
    {
      int fhin;
      
      fhin = open(argv[i],O_RDONLY);
      if (fhin == -1)
      {
        myperror(argv[i]);
        continue;
      }
      
      mywrite(STDOUT_FILENO,"-----",5);
      mywrite(STDOUT_FILENO,argv[i],strlen(argv[i]));
      mywrite(STDOUT_FILENO,"-----\n",6);
      
      do_dump(fhin,STDOUT_FILENO);
      if (close(fhin) < 0)
        myperror(argv[i]);
    }
  }
  
  return EXIT_SUCCESS;
}
      
/************************************************************************/     

static void do_dump(const int fhin,const int fhout)
{
  unsigned char  buffer[4096];
  char           outbuffer[75 * 109];
  char          *pout;
  unsigned long  off;
  size_t         bytes;
  size_t         count;
  
  assert(fhin  >= 0);
  assert(fhout >= 0);

  memset(outbuffer,' ',sizeof(outbuffer));
  off      = 0;
  count    = 0;
  pout     = outbuffer;
  
  while((bytes = myread(fhin,(char *)buffer,sizeof(buffer))) > 0)
  {
    unsigned char *p = buffer;
    
    for (p = buffer ; bytes > 0 ; )
    {
      size_t amount;
      
      amount    = dump_line(&pout,p,bytes,off);
      p        += amount;
      bytes    -= amount;
      off      += amount;
      count++;
      
      if (count == 109)
      {
        mywrite(fhout,outbuffer,(size_t)(pout - outbuffer));
        memset(outbuffer,' ',sizeof(outbuffer));
        count    = 0;
        pout     = outbuffer;
      }      
    }
  }
  
  if ((size_t)(pout - outbuffer) > 0)
    mywrite(fhout,outbuffer,(size_t)(pout - outbuffer));
}

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

static size_t dump_line(
	char                **const pline,
	unsigned char              *p,
	size_t                      bytes,
	const unsigned long         off
)
{
  char   *line;
  char   *dh;
  char   *da;
  size_t  count;
  
  assert(pline  != NULL);
  assert(*pline != NULL);
  assert(p      != NULL);
  assert(bytes  >  0);
  
  line = *pline;
  
  hexout(line,off,8,':');
  if (bytes > LINESIZE)
    bytes = LINESIZE;
  
  p  += bytes;
  dh  = &line[10 + bytes * 3];
  da  = &line[58 + bytes];
  
  for (count = 0 ; count < bytes ; count++)
  {
    p  --;
    da --;
    dh -= 3;
    
    if ((*p >= ' ') && (*p <= '~'))
      *da = *p;
    else
      *da = '.';
    
    hexout(dh,(unsigned long)*p,2,' ');
  }
  
  line[58 + count] = '\n';
  *pline = &line[59 + count];
  return count;
}

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

static void hexout(char *const dest,unsigned long value,size_t size,const int padding)
{
  assert(dest != NULL);
  assert(size >  0);
  assert((padding >= ' ') && (padding <= '~'));
  
  dest[size] = padding;
  while(size--)
  {
    dest[size] = (char)((value & 0x0F) + '0');
    if (dest[size] > '9') dest[size] += 7;
    value >>= 4;
  }
}

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

static void myperror(const char *const s)
{
  int err = errno;
  
  assert(s != NULL);
  
  mywrite(STDERR_FILENO,s,strlen(s));
  mywrite(STDERR_FILENO,": ",2);
  
  if (err > sys_nerr)
    mywrite(STDERR_FILENO,"(unknown)",9);
  else
    mywrite(STDERR_FILENO,sys_errlist[err],strlen(sys_errlist[err]));
  mywrite(STDERR_FILENO,"\n",1);
}

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

static size_t myread(const int fh,char *buf,size_t size)
{
  size_t amount = 0;
  
  assert(fh   >= 0);
  assert(buf  != NULL);
  assert(size >  0);
  
  while(size > 0)
  {
    ssize_t bytes;
    
    bytes = read(fh,buf,size);
    if (bytes < 0)
    {
      myperror("read()");
      exit(EXIT_FAILURE);
    }
    if (bytes == 0)
      break;
    
    amount += bytes;
    size   -= bytes;
    buf    += bytes;
  }
  
  return amount;
}

/*********************************************************************/  
  
static void mywrite(const int fh,const char *const msg,const size_t size)
{
  assert(fh   >= 0);
  assert(msg  != NULL);
  assert(size >  0);
  
  if (write(fh,msg,size) < (ssize_t)size)
  {
    if (fh != STDERR_FILENO)
      myperror("output");
      
    exit(EXIT_FAILURE);
  }
}

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

And that makes all the difference. The portable vesrsion:

[spc]lucy:~/projects/99/src>time ./12 ~/bin/firefox/libxul.so >/dev/null

real    0m4.985s
user    0m4.969s
sys     0m0.015s

Our first stab at a non-portable, but possibly faster version:

[spc]lucy:~/projects/99/src>time ./20 ~/bin/firefox/libxul.so >/dev/null

real    0m2.936s
user    0m1.511s
sys     0m1.425s

The “it's quite a bit faster” version:

[spc]lucy:~/projects/99/src>time ./21 ~/bin/firefox/libxul.so >/dev/null

real    0m0.957s
user    0m0.645s
sys     0m0.313s

And finally, the punchline—today's version:

[spc]lucy:~/projects/99/src>time ./22 ~/bin/firefox/libxul.so >/dev/null

real    0m0.460s
user    0m0.448s
sys     0m0.012s

And yes, that's the real output—1/10 the time of the portable version with a similar amount of time in the kernel.

Frankly, I was a bit surprised at these results—not that the non-portable version was faster (that's almost a given) but the magnitude of the results. I didn't think the standard C library had that much overhead. I was expecting easily a percentage increase in speed, but even twice would have been unexpected, but ten times faster?

Wow.

Increasing the size of the buffer past what I have probably won't help all that much, and in fact, when I doubled the buffer size:

[spc]lucy:~/projects/99/src>time ./22 ~/bin/firefox/libxul.so >/dev/null

real    0m0.592s
user    0m0.582s
sys     0m0.010s

The timing difference could be due to cache effects, maybe?

So I think we've maxed out the speed at which this program will run. As a test, I profiled the code to see if there was anything I migh have missed:

Each sample counts as 0.01 seconds.
  %   cumulative   self              self     total           
 time   seconds   seconds    calls  ms/call  ms/call  name    
100.21      0.41     0.41        1   410.86   410.86  do_dump
  0.00      0.41     0.00     9197     0.00     0.00  mywrite

I checked the code GCC produced (all code was compiled with -O3, a very high level of optimization) and well, I'm not sure I could have done much better, and probably would have done worse—GCC inlined everything into do_dump() (with the exception of main() and mywrite()), something I would not have done in assembly (and have any hope of code reuse for another project). So I think we're done with making this code fast.

That's not to say I won't do an assembly version of this program, but it probably won't be for the x86 line.

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