Loading Data in shared memory (C++) Post: 302467563

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Top Forums Programming Loading Data in shared memory (C++) Post 302467563 by Corona688 on Friday 29th of October 2010 10:20:22 PM

10-29-2010

Registered User

Instead of using shmat, how about mmap? This solves the storage problem -- you no longer need to explicitly write to disk, just alter the memory and the OS will follow suit. It also makes it possible to use data larger than your system's memory without excessive swapping, since it only reads in the parts you're presently using. It also makes it easier for your readers to find the file -- it's just an ordinary file, not some weird special memory object thingamabob.

It does make the file fixed length though -- or at least, cumbersome to change the length of.

Code:

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

...

mystruct str, *shared;
int fd, n, len;

fd=open("filename", O_RDWR);
memset(&str, 0, sizeof(str));

// Create a blank file 1024 records long
for(n=0; n<1024; n++)
{
        write(fd, &str, shared);
}

// Mapped area must be a multiple of the page size
len=sizeof(str) * 1024;
if(len % getpagesize())
{
        len -= (len % getpagesize());
        len += getpagesize();
}

// Map the file into memory.
mystruct *mem=mmap(NULL, len, PROT_READ|PROT_WRITE,
        MAP_SHARED, fd, 0);
// We can now close the fd.
close(fd);

if(mem == MAP_FAILED)
{
        fprintf(stderr, "Couldn't map memory\n");
}
else
{
        fprintf(stderr, "mapped into %p\n", mem);
        // As the contents of the memory change, so will the contents on disk.
        // This I/O may just be cached in memory for the OS to write when
        // convenient, but the data shared between processes will remain
        // current and consistent.
        strcpy(mem[0].fName, "Foo");
        strcpy(mem[0].lName, "Bar");
        munmap(mem, len);
}

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LEARN ABOUT PHP

strlen

STRLEN(3)								 1								 STRLEN(3)

strlen - Get string length

SYNOPSIS

       int strlen (string  $string)

DESCRIPTION

	Returns the length of the given $string.

PARAMETERS

	      o $string
		- The string being measured for length.

RETURN VALUES

	The length of the $string on success, and 0 if the $string is empty.

CHANGELOG

       +--------+---------------------------------------------------+
       |Version |						    |
       |	|						    |
       |	|		     Description		    |
       |	|						    |
       +--------+---------------------------------------------------+
       | 5.3.0	|						    |
       |	|						    |
       |	|  Prior  versions  treated  arrays  as  the string |
       |	| Array, thus returning a string length  of  5	and |
       |	| emitting an E_NOTICE level error.		    |
       |	|						    |
       +--------+---------------------------------------------------+
EXAMPLES

       Example #1

	      A strlen(3) example

	      <?php
	      $str = 'abcdef';
	      echo strlen($str); // 6

	      $str = ' ab cd ';
	      echo strlen($str); // 7
	      ?>

NOTES

       Note

	      strlen(3) returns the number of bytes rather than the number of characters in a string.

       Note

	      strlen(3) returns NULL when executed on arrays, and an E_WARNING level error is emitted.

SEE ALSO

       count(3), grapheme_strlen(3), iconv_strlen(3), mb_strlen(3).

PHP Documentation Group 														 STRLEN(3)

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LEARN ABOUT PHP

strlen