Question

How can I access an array in a parent process from a child process? (fork)

Asked by: shadow_shooter

Hello,

I created two arrays in a parent process. What I want to do is accessing this array from a child process. I know I can do this by using a shared memory but I don't want to use it. Is there a way to overcome this problem?

If I make sure parent process doesn't end before child processes complete, is it sufficient to use the array that the parent process created? As far as I experienced, it is not. I still see garbage values in both arr1 and arr2.

Can anyone please help me?

#include <stdio.h>
#include "functions.h"
 
int main()
{
	int pid1, pid2;
	int arr1[248], arr2[248], grid[496] i, j;
	FILE *fp;
	fp = fopen("unsorted.txt", "r"); 
	if(fp == NULL)
	{
		printf("An error has occured during reading the file. Check if the file name or path is correct.\n");
 
	}
 
	pid1=fork();
	if(pid1==0)
	{
		pid2=fork();
		
		if(pid2==0)
		{
			//SECOND CHILD               
			heapSort(arr1, 248);  //PROBLEM IS HERE: arr1 is not filled though it was, by Parent process.
			printArray(arr1,248);
		}
		else
		{
			//FIRST CHILD
			wait(&pid2);
			heapsort(arr2,248);
			printArray(arr2,248);
		}
	}
	else
	{		
		while(fscanf(fp, "%d", &grid[count]) != EOF) 
		{
			count++;
		}
		for(i=0; i<248; i++)
		{
			arr1[i]=grid[i];
		}
 
		i=0;
		for(j=248; j<496; j++)
		{
			arr2[i]=grid[j];
			i++;
		}
		wait(&pid1);
		fclose(fp);
	}
}
                                  
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Asked On
2009-03-21 at 08:41:42ID24252085
Tags

C

,

Linux

,

Fedora

,

Ubuntu

,

Programming

,

Child

,

Parent

Topics

C Programming Language

,

Linux Programming

,

Linux Networking

Participating Experts
1
Points
310
Comments
5

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Answers

 

by: KdoPosted on 2009-03-21 at 08:48:22ID: 23947738

Hi Shadow,

You've got several very workable choices.  The first you've already discounted, that is shared memory.  (Check the mmap function, it's not hard to use.)

The second is to use threads instead of spawning a new task.  Threads share the entire memory region of the original task so all memory is shared.

On some systems you can use system calls to access memory in another task, but that's not portable and certainly more work than either method listed above.


Good Luck,
Kent

 

by: shadow_shooterPosted on 2009-03-21 at 09:35:57ID: 23947900

Kdo,

The reason why I don't want to use shared memory is I am a newbie and dont want to be stuck in that.

Can you give a link to a good tutorial explaining the shared memory? I'm working on Fedora now. I am not sure if the things I use change for different operating systems.

There are not many decent, easy, self explaining tutorials for linux development. I'm not surprised Linux is on the par.

 

by: shadow_shooterPosted on 2009-03-21 at 09:59:04ID: 23947967

I think storing the array in a shared memory is very complex. I just saw a tutorial which tries to store a single character into shared memory and it was complicated.

Could anyone please tell me how I can store arr1 and arr2 arrays in a shared memory then access them from a child memory?

 

by: KdoPosted on 2009-03-21 at 11:16:02ID: 23948217

Hi shadow,

Actually, shared memory is incredibly easy.  :)

  man mmap

Will get you the basics of mmap (memory mapping).  I'll find you a like with a good tutorial.


Kent

 

by: KdoPosted on 2009-03-21 at 11:26:10ID: 23948262


Here are three links.  One with a very length description of mmap() and a two-line example of how to use it.  :)

  http://www.opengroup.org/onlinepubs/000095399/functions/mmap.html


This second link is sample code that copies a file by mapping it to memory.

  http://www.cs.purdue.edu/homes/fahmy/cs503/mmap.txt


The third link is from our very own Experts-Exchange library.  :)  It appears to be an example of exactly what you're trying to do.

  http://www.experts-exchange.com/Programming/Languages/C/Q_10083462.html


Kent

20120131-EE-VQP-002

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