How to use shared memory as an array of pointers?

Question

Hi,
Can we use the pointer returned by "void** ptr = (void * *)shmat();" for accessing the elements in array fashion?
I have a pointer to pointer to struct - an array of pointers & I want to access the elements by using array notation of ptr[index].
But, when I do so, my process crashes with SSEGV.
My code is somewhat as below:-
size_t size = nSize*sizeof(void *);
key_t key = 90000;
int id = shmget(key,size,0777 | IPC_CREATE);
void** ptr = (void**)shmat(id,(void*)0,0777 | SHM_SHARE_MMU);
memset(ptr,0,size);
void* tmpPtr = ptr[100]; // this line causes crash.I was trying to do this on the basis of following code, which uses malloc & works fine:-
uht->t = (void **)malloc(nSize*sizeof(void *));
pUHT_ITEM curr_item = (pUHT_ITEM)uht->t;
How can I achieve this?
Please help. Thanks.
Message was edited by:
grbSunUser

Answer 1

After debugging the program using dbx, I found that after certain specific index, I get an error of "dbx: cannot access address 0xfd032000", if I do "print suht->t[51200]".
What I did is that I created shm segment of size = 50000 * (sizeof(viod*))
This turns out to be 200084.
By doing step by step print of each index in this shm segment, I reached upto the index of 51199. When I did "print suht->t[51199]" in dbx, I got "suht->t[51199] = (nil)", means this was accessible location.
But, when I tried "print suht->t[51200]", I got an error of "cannot access...".
I think this is index from which, if process tries to access, will cause crash.
How are these numbers related? Should I not do "50000 * sizeof(void*)"?