IPC using file pointer

vikasbansal27 · 06-03-2012, 04:23 AM

Take the general scenario of a parent creating a child process. Parent process declares a file pointer *fp.
then it points it to a file say 'new1' by
fp = fopen("new1","w+"). Opening it for both reading and writing.
Now the parent process creates a child process. While parent process waits for the child, child uses
execlp("./child_new",0); calling a c file child_new.
In child_new we have same File pointer declared. Now can we use that file pointer to make changes in same file which we did in parent??

Can processes share a file pointer?
I read somewhere that we can but I tried and it is showing some segmentation fault.

******Parent************************

Code:

#include<stdio.h>
#include<fcntl.h>
main()
{
   FILE *fp;
fp = fopen("new1","w+");
if(fork()==0)
{
	printf("\nIn child process\n");
	execlp("./child_new",0);
	exit(1);
}
else  {
	wait();
	printf("In parent");
	exit(1);
      }
}

***********child****************

Code:

#include<stdio.h>
#include<fcntl.h>
main()
{
	FILE *fp;
	printf("In child file");
	fseek(fp,0L,0);
}

Sergei Steshenko · 06-03-2012, 01:03 PM

Quote:

Originally Posted by vikasbansal27

... it is showing some segmentation fault.
...

So, what did you do to debug the segmentation fault ? I.e. have you tried web search for, say, 'gdb debugging segmentation fault' ?
...
Opening file for both reading and writing at the same time is generally speaking a bad idea. In the end it's much easier to have two files:

from_parent_to_child - parent writes, child reads;
form_child_to_parent - child writes, parent read.

Think also about using shared memory for IPC.

vikasbansal27 · 06-03-2012, 01:55 PM

Hey thank you for telling me about gdb, that sure will help but in this program, I '//' both fseek and fp, then it was doing fine. So there is some problem with sharing file pointers. I searched on the internet, but there is not much about sharing file pointer technique.

Sergei Steshenko · 06-03-2012, 02:12 PM

Quote:

Originally Posted by vikasbansal27

...there is not much about sharing file pointer technique.

Start from something really simple and safe. E.g. create a text file with known contents, write your parent -> child code, write it in a manner that the parent simply reads the file line by line and outputs its contents to screen, then the child is created, the child rewinds the file and also outputs the text file contents to screen line by line.

vikasbansal27 · 06-03-2012, 03:14 PM

I made a very simple program..but I am getting an unusual output this program should show the 10 characters of file 'first' two times. But it is showing it three times.

Code:

#include<stdio.h>
#include<sys/types.h>
#include<unistd.h>
main()
{
FILE *fp;
char c;
int i,pid;
fp=fopen("first","r");
c=getc(fp);	
printf("\nIn Parent...\n");

for(i=0;i<10;i++)
{
	printf("%c",c);
	c=getc(fp);
}

pid=fork();
if(pid==0)
 {
	printf("\nIn Child...\n");
	fseek(fp,0,0);
	c=getc(fp);
	for(i=0;i<10;i++)
	{
		printf("%c",c);
		c=getc(fp);
	}
printf("\n");
 }
else {
	wait(NULL);
	exit(0);
}

}

first contains 'VikasBansal' as text.
output is

In Parent...
VikasBansa
In Child...
VikasBansa
VikasBansa

vikasbansal27 · 06-03-2012, 03:20 PM

It is executing the loop just above fork() TWICE. But if I place a printf statement between them, it runs normal.