Maximum buffer size for read() Post: 302601538

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Top Forums Programming Maximum buffer size for read() Post 302601538 by gabam on Friday 24th of February 2012 12:45:36 AM

02-24-2012

Registered User

Maximum buffer size for read()

Hi friends,
Hope everybody is fine. First have a look at my code, then we will talk about it.

Code:

 
$ cat copy.c
#include <stdio.h>
#define PERMS 0644 /*  RW for owner, R for group, others */
#define BUFSIZE 1
char *progname;
int main(int argc,char * argv[])
{
        int f1, f2, n;
        char buf[BUFSIZE];
        progname = argv[0];
        if(argc != 3)
        printf("Usage %s from to\n", progname);
        f1 = open(argv[1], 0);
        if(f1 == -1)
        {
        printf("Can't open %s\n", argv[1]);
        }
        f2 = creat(argv[2], 777);
        if(f2 == -1)
        {
        printf("Can't create %s\n", argv[2]);
        }
        while((n = read(f1, buf, BUFSIZE)) > 0)
        {
                printf("%c\n", buf[0]);
                if(write(f2, buf, n) != n)
                {
                printf("Write error\n");
                }
        }
        exit(0);
}

As you can see I've defined a macro bufsize 1. The value 1 is used for my buffer. My question is, what is the unit of 1, bytes, kilobytes, megabytes? Because, the file that I am reading is very lage, containing thousands of lines, how come it copies that large file into another file. Initially, I had set bufsize as 512, then i set it to 1, still the program works fine, copying the large file into another file.
Could you please help me here?

Looking forward to your wonderful replies.

Thanks in advance!

gabam

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LEARN ABOUT V7

ppmtosixel

ppmtosixel(1)						      General Commands Manual						     ppmtosixel(1)

NAME

       ppmtosixel - convert a portable pixmap into DEC sixel format

SYNOPSIS

       ppmtosixel [-raw] [-margin] [ppmfile]

DESCRIPTION

       Reads  a  portable  pixmap  as input.  Produces sixel commands (SIX) as output.	The output is formatted for color printing, e.g. for a DEC
       LJ250 color inkjet printer.

       If RGB values from the PPM file do not have maxval=100, the RGB values are rescaled.  A printer control header and a color assignment table
       begin the SIX file.  Image data is written in a compressed format by default.  A printer control footer ends the image file.

OPTIONS

       -raw   If  specified,  each  pixel  will  be explicitly described in the image file.  If -raw is not specified, output will default to com-
	      pressed format in which identical adjacent pixels are replaced by "repeat pixel" commands.  A raw file is often an order	of  magni-
	      tude larger than a compressed file and prints much slower.

       -margin
	      If  -margin  is not specified, the image will be start at the left margin (of the window, paper, or whatever).  If -margin is speci-
	      fied, a 1.5 inch left margin will offset the image.

PRINTING

       Generally, sixel files must reach the printer unfiltered.  Use the lpr -x option or cat filename > /dev/tty0?.

BUGS

       Upon rescaling, truncation of the least significant bits of RGB values may result in poor color conversion.  If the original PPM maxval was
       greater	than  100,  rescaling  also reduces the image depth.  While the actual RGB values from the ppm file are more or less retained, the
       color palette of the LJ250 may not match the colors on your screen.  This seems to be a printer limitation.

SEE ALSO

       ppm(5)

AUTHOR

       Copyright (C) 1991 by Rick Vinci.

								   26 April 1991						     ppmtosixel(1)

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