shared memory

[saidolsol]

hi all,

if i want to use pointers inside shared memory, how do i have to get and attach it. For example...

struct shm {
int *numbers;
int i1;
int i2;
int i3;
} *pshm;

Say i have N numbers to put into the array, then...

IDshm = shmget(KEY, (4*N)+4+4+4, IPC_CREAT | 0666)

then attach...

pshm = (struct shm*)shmat(IDshm, 0, 0)

Then i have a pointer to the shared memory segment.
To get the pointer to the array inside the structure...

pshm->numbers = (int *)pshm[4]; ( 4 ----> size of the pointer )

Is this ok? My application throws a segmentation fault and i think it could be the problem. The program is not so simple as above so this could not be the wrong thing.

I think that my explanation is not quite well ( as my english ), but i hope someone will help me...

thanx.
gracias.
gràcies.
a10.

 

[Cagliostro]

Are you using Windows? John Fill

ivfmd@mail.md

 

[saidolsol]

no.

 

[Zyrenthian]

I dont fully understand what you are trying to do but here is what i see

IDshm = shmget(KEY, (4*N)+4+4+4, IPC_CREAT | 0666)

this line here... it looks like you are getting a chunk of shared memory? And it looks like you are trying to use the (4*N)+4+4+4 as the size of the shared memory to create. This is where i think the problem is coming from. If by

"Say i have N numbers to put into the array, then..."

You mean you have N numbers you want to store in shared memory as well as i1,i2 & i3 I would suggest declaring your struct as follows

struct shmem{
int i1;
int i2;
int i3;

int* numbers;
};


I am assuming that either i1, i2 or i3 will tell you how many numbers are stored in the array "numbers", if not, having a value to tell you that will be helpful. For this example lets say i1 has the count of numbers;

now... when you create your shared memory... you know you want N numbers

pshm->i1 = N;

now, the easiest way i can see doing this is trough a loop. When the block of shared memory is created, i1,i2 & i3 are at the beginning, and at the end is all this allocated shared memory of size 4*N which is equivalent to sizeof(int)*N which i would suggest using instead. Now you can treat numbers as an array of N integers but as I am not 100% sure the [] operator on the array will work (first i would test it and see if I got a seg fault) i would also try it as follows

for(int i = 0;i<n;i++)
*(numbers+i)=next_int_value;

and see where that gets you.

Let me know if this was any help or if i mis-interpreted what you were asking.

Matt

 

[saidolsol]

hi Matt,
you have not mis-interpreted my question. Later i will test with that new struct, and with the array references you suggested.

bye.
Adiós.
Adéu.
a10.