Reading a particular line from a .txt file Post: 302425811

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Top Forums Programming Reading a particular line from a .txt file Post 302425811 by achenle on Sunday 30th of May 2010 12:34:29 PM

05-30-2010

Registered User

That's only going to work if you can guarantee each line fits into your buffer.

If you can do that, great. If not, you pretty much have to count newline characters. This works, and should be fairly fast as you'd be relying on the OS to page in the data, which should be fast enough. If it's a really big file, and you know you're only going through it once, it'd be faster to use open() and read() with direct IO set so you bypass any page cache (if you're only looking once at each byte of a bunch of gigabytes of data, any caching is wasted cycles):

Code:

struct stat sb;
int fd = open( filename, O_RDONLY );
fstat( fd, &sb );
char *ptr = mmap( NULL, sb.st_size, PROT_READ, MAP_PRIVATE, fd, 0 );
uint64_t offset;
uint64_t line_count = 0;
uint64_t desired_line = 123455;

for ( offset = 0; offset < sb.st_size; offset++ )
{
    if ( '\n' == ptr[ offset ] )
    {
        line_count++;

        if ( line_count == desired_line )
        {
            break;
        }
    }
}

// if offset is less than file size, the desired line was found
if ( offset < sb.st_size )
{
    // line starts at offset + 1
    offset++;
        .
        .
        .
}

Note that has no error checking.

achenle

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LEARN ABOUT REDHAT

pyro

XScreenSaver(1) 					      General Commands Manual						   XScreenSaver(1)

NAME

       pyro - simulate fireworks

SYNOPSIS

       pyro  [-display	host:display.screen] [-foreground color] [-background color] [-window] [-root] [-mono] [-install] [-visual visual] [-count
       integer] [-frequency integer] [-scatter integer]

DESCRIPTION

       The pyro program simulates fireworks, in a way similar to a Macintosh program of the same name.

OPTIONS

       pyro accepts the following options:

       -window Draw on a newly-created window.	This is the default.

       -root   Draw on the root window.

       -mono   If on a color display, pretend we're on a monochrome display.

       -install
	       Install a private colormap for the window.

       -visual visual
	       Specify which visual to use.  Legal values are the name of a visual class, or the id number (decimal or hex) of a specific visual.

       -count integer
	       How many particles should be allowed on the screen at once.  Default 100.

       -frequency integer
	       How often new missiles should launch.  Default 30.

       -scatter integer
	       How many particles should appear when a missile explodes.  Default 20.  The actual number used is between N and N+(N/2).

ENVIRONMENT

       DISPLAY to get the default host and display number.

       XENVIRONMENT
	       to get the name of a resource file that overrides the global resources stored in the RESOURCE_MANAGER property.

SEE ALSO

       X(1), xscreensaver(1)

COPYRIGHT

       Copyright (C) 1992 by Jamie Zawinski.  Permission to use, copy, modify, distribute, and sell this software and its  documentation  for  any
       purpose	is  hereby  granted without fee, provided that the above copyright notice appear in all copies and that both that copyright notice
       and this permission notice appear in supporting documentation.  No representations are made about the suitability of this software for  any
       purpose.  It is provided "as is" without express or implied warranty.

AUTHOR

       Jamie Zawinski <jwz@jwz.org>, 13-aug-92.

X Version 11							     13-aug-92							   XScreenSaver(1)