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.

Sunday, January 29, 2012

99 ways to program a hex, Part 21: C89, const correctness, assertive, system calls, per line buffering

Yesterday's version was faster than the portable version, but spent nearly 100 times longer in the kernel than the portable version, and that's because the portable version, using the standard C library, buffers the output way more than my non-portable system calling version did. Making a subroutine call into the kernel (a “system call”) takes way more time than just calling a regular subroutine call.

So, we need to avoid making a ton of system calls, and to do that, we need to buffer the output a bit more. This version, we buffer an entire line's worth of data before writing it out.

/*************************************************************************
*
* 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, per line 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	(const int,unsigned char *,size_t,const unsigned long);
static void	hexout		(char *,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 0;
}
      
/************************************************************************/     

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

  off = 0;
  
  while((bytes = myread(fhin,(char *)buffer,sizeof(buffer))) > 0)
  {
    unsigned char *p = buffer;
    
    for (p = buffer ; bytes > 0 ; )
    {
      size_t amount;
      
      amount = dump_line(fhout,p,bytes,off);
      p     += amount;
      bytes -= amount;
      off   += amount;
    }
  }
}

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

static size_t dump_line(
	const int            fhout,
	unsigned char       *p,
	size_t               bytes,
	const unsigned long  off
)
{
  char    line[75];
  char   *dh;
  char   *da;
  size_t  count;
  
  assert(fhout >= 0);
  assert(p     != NULL);
  assert(bytes >  0);
  
  memset(line,' ',sizeof(line));
  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';
  mywrite(fhout,line,59 + count);
  return count;
}

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

static void hexout(char *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);
  }
}

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

Okay, so how does this version fare?

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

real    0m2.941s
user    0m1.499s
sys     0m1.441s
[spc]lucy:~/projects/99/src>time ./21 ~/bin/firefox/libxul.so >/dev/null

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

Not bad—one third the time overall, and one fifth the amount of time spent in the kernel. And compared to the portable version, this only takes one fifth the total time, although it's still spending over twenty times as long in kernel space.

We can do better—it just takes more buffering and less system calls.

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