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.
