Shared Memory Segmentation Error

**doom** · 12-11-2005

This is a little urgent. I have to do use shared memory segments. I wrote this programme. I have tried everything, as you can see (void *, int * printing and so on), but havent been able to debug the error.
I will be grateful if some one can point out the bug.

Code:

#include<stdio.h>
#include<sys/ipc.h>
#define KEY 600
#define SIZE 20

int main()
{
	int sid;
	int i;
	void *a;
	int *shm;
	if(fork()>0)
	{
		printf("%d\n",getpid());
		sleep(1);
		if(sid=shmget(KEY,SIZE,0)<0)
		{
			printf("Client Error\n");
			exit(1);
		}
		a=shmat(sid,0,0);
		shm=(int*)a;
		shm++;
		printf("Client Attach\n");		
		i=*shm;
		printf("Read");
		printf("%d",i);
		printf("Client Release");
		shmdt(a)
		//shmctl(sid,IPC_RMID,(struct shmid_ds*)0);
	}
	else
	{
		if((sid=shmget(KEY,SIZE,IPC_CREAT|0666))<0)
		{
			printf("Error");
			exit(1);
		}
		printf("Got\n");
		a=shmat(sid,0,0);
		shm=(int*)a;
		printf("Attached\n");
		shm++;
		*shm=10;
		printf("Value put, waiting for %d\n",getppid());
		while(wait((int *)0)!=getppid());
		printf("got back");
		//shmdt(shm);		
		printf("Released\n");
		shmctl(sid,IPC_RMID,(struct shmid_ds*)0);
	}
	return 0;	
}

Please post any thing you think may be causing this error.
It prints till 'Value put, waiting for ...' in the child and then
Client Attach
this is followed by a Segmentation Fault message. It simply says 'Segmentation Fault'.

**doom** · 12-11-2005

Code:

              shm++;
                printf("Client Attach\n");              
                i=*shm; //Error in this line
                printf("Read");
                printf("%d",i);

If i remove this line, the program works fine.

**Salem** · 12-11-2005

> I will be grateful if some one can point out the bug.
Ah, just the one then huh?

Well lets see
1. if(fork()>0)
You have no control over which process next runs, and since you only go about creating the shared memory in one of them after the fork(), it seems likely to me that one could try and get it before the other has created it.

In the manual page for shmat(), there is
"fork() After a fork() the child inherits the attached shared memory segments."
So creating then attaching (just the once) before the fork() should be all you need to do.

2. shmat()/shmdt()
Your use seems to suggest that the act like a semaphore, one process has it at once.
This is not the case, so all your updates to the shared memory can happen in any order.

3. a=shmat(sid,0,0); //shmdt(shm);
The manual clearly states that the thing you detach is the thing you attached.
You did it right in the other place in the code.

**doom** · 12-11-2005

Thanks a lot. Problem solved. I attached the segment before forking. When i was later printing the memory address, it was giving different values. So didnt knw what to do

. But now works fine.

After forking, the parent process sleeps for 1 seconds so, the child should have run. Then Child waits for parent to terminate and then removes the segment. Thats why i didnt use semaphores. Already was having trouble, so simply decided to wait and sleep.
Thanks for the third point too. I overlooked earlier, sorry.

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