Problems with shared memory and lists

BeNdErR · #1 (**permalink**) 06-28-2009

Hi, I need to put in shared memory a list made with object of this structure:

Code:

typedef struct Obj{
char objname[20];
struct Obj *nextObj;
}Object

I've filled my list with (for example) 10 elements, but when i try to put it in shared memory to be read by another process i get segmentation fault after first read record..
I've read in another thread (here -> dynamic shared memory )that malloc gives sone problems with shared memory

Quote:

If you called malloc to create linked_list_header->ptr, then the address returned by malloc is private to the calling (the process that calls malloc) process. Same with NODE->buffer. You will have to allocate these things in shared "by hand" without calling malloc.

but i don't understand what i have to do now and how to solve my problem.

i tried to share object in this way:

Code:

Obj *head;     //pointer to the head of my FILLED list
int shmid;
Obj *sh;
...
shmid = shmget(SHMKEY,(sizeof(Obj)*10),0666|IPC_CREAT|IPC_EXCL);
...
sh = (Obj *)shmat(shmid, 0, 0666);
...
*sh = *head;

probably I miss something or maybe I'm totally wrong... how can I allocate Obj in shared "by hand" without calling malloc?

Thanks for your support, best regards.

mgessner · #2 (**permalink**) 06-29-2009

Here's the problem: the pointers that malloc() returns are local to the address space of the process that called malloc(). Those values won't mean anything in the context of another process. Hence putting those numbers in shared memory and using them from another process will invoke undefined behaviour. If you're lucky, your program will then crash. If you're not lucky, you might cause a supernova somewhere (a bit of C humour).

So what you need to do is to allocate a separate chunk of shared memory, and write a "malloc" and "free" that use that pool of memory. In other words, you have to write a small memory allocator/deallocator that uses this shared memory pool, and then passes back offsets from. Since all of your objects are the same, this will be pretty trivial.

Notice I said "offsets" and not "addresses". I don't recall anywhere in anything I've ever read that guarantees that the shared memory will be assigned the same address in different processes. Assuming so, and using those values in your lists will likely lead you back to your original problem. So you will have to handle that problem in an intelligent way.

You can write this to be general (will handle any size allocation) or specific (only handles things of your struct Obj type). Either way is fine, but I think the exercise of writing the general will teach you more.

This is a fairly easy task, but there are some things to watch out for. I will give you two:

a) Only ONE process can initialize the shared memory pool. Start that process FIRST, initialize the pool, and THEN start the others. Otherwise, you'll potentially have multiple processes trashing your pool.

b) What happens if a process that is allocating something from this shared pool gets interrupted during the middle of an allocation, and another process also tries to allocate something from this shared pool. Warning: if you do this wrong, Bad Things will happen, and your mother will laugh at you. Note that accessing the list can ALSO lead to the same kinds of problems, because in the general case you don't necessarily know if another cooperating process has manipulated the list.

One other thing: in your post, you had:

Code:

*sh = *head;

This is wrong: a) you don't want to change the contents of sh, and b) you want head to point to sh, not the other way around.

HTH

BeNdErR · #3 (**permalink**) 06-29-2009

thanks for your reply

I solved my problem in another way.. but I'll try your solution too

pratice makes perfect!

but there are some other question/responses I'd like to know:

1) I laughed out loud on your humor, seriously xD I'm one of those who runs away if someone divide by zero on a calculator (I hate black holes);

2) I solved my problem in this way: (7 is the number in object of my list)

Code:

Obj *pp, *head;
...
..
if((shmid = shmget(SHMKEY, 7, 0777|IPC_CREAT|IPC_EXCL)) == -1){
			perror("shmget");
			exit(1);
		}
		
		if((sh = (Obj *)shmat(shmid, 0, 0777)) ==(Obj *) -1){
			perror("shmat");
			exit(1);
		}
		int a;
		pp = head;
		for(a=0; a<7; a++){
			*(sh+a) = *pp;
			pp = pp->next;
		}
...

it seems to work fine. On the other "side" i read data this way:

Code:

Obj *p, *List;
...
p=List;
for(a=0; a<7; a++){
		*p=*(sh+a);
		p=p->next;
	}
...

what do you think about that?

3) I'm a little messed up with shared memory. I mean, what I suppose about how it works is - (hope you'll understand my english-for-dummies-language-level) - like a "box" where I can put things. But if i try to put inside a pointer I'll get maaany problems. So the list I've putted in (with my method) are no longer behaving as a list, but more likely as a """""string""""" (watch for ""

).
example:

Code:

if((shmid = shmget(SHMKEY, 7, 0777|IPC_CREAT|IPC_EXCL)) == -1){
			perror("shmget");
			exit(1);

now i've a box made of 7 slots of x-dimension. once I attach memory
(...shmat..) X becomes of a true value, in my case Obj. then I have 7 slots of Obj-size each.
when i fill my box up, i fill slots individually moving through shared memory with sh++, and not with pointer to the next obj.

is my opinion right? or completely wrong?!

thanks again

mgessner · #4 (**permalink**) 06-30-2009

This is a problem:

Code:

Obj *pp, *head;
...
pp = head;

/* other stuff involving pp, which is wrong because pp was set to an unknown value. */

head was never initialized. You must always initialize a pointer before you use it. It must either be NULL (in which case you don't dereference it) or you must point it to something.

I think what you need to do is draw a picture (a box, perhaps) and divide it up into pieces (as many as you have objects) and then draw the arrows that represent the pointers.

Your code below doesn't work because it's correct; it works because you are unlucky.

BeNdErR · #5 (**permalink**) 06-30-2009

wait, I forgot to say that *head is the head of my entire list.. So I've to initialize each pointer at least to NULL value?

mgessner · #6 (**permalink**) 06-30-2009

You set pp = head, but you never set head to anything in your example.

BeNdErR · #7 (**permalink**) 06-30-2009

yes, i did not because of the length of the part where i set the value of head

thougth was better just say that head was the pointer to the head of my entire list instead of posting the code entirely..

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