Two variables in one shared memory

Question

Is there a way to use one shared memory,

shmid = shmget (shmkey, 2*sizeof(int), 0644 | IPC_CREAT);

For two variables with different values?

int *a, *b;
a = (int *) shmat (shmid, NULL, 0);
b = (int *) shmat (shmid, NULL, 0); // use the same block of shared memory ??

Thank you very much!

Answer 1

Apparently (reading the manual) shmat gets you here a single block of memory, of size 2*sizeof(int).

If so, then you can just adjust the pointer:

int *a, *b;
a = shmat(shmid, NULL, 0);
b = a+1;

Also, casting here is wrong, for reasons listed here (while the question is about malloc, the same arguments apply)

Answer 2

I'm not familiar with the shmget and similar, but I imagine if the memory is contiguous just increment the pointer.

int *a, *b;
i = (int *) shmat (shmid, NULL, 0);
a = ((int *) shmat (shmid, NULL, 0)) + 1;

Better still, just write:

int *myMemory = shmat (shmid, NULL, 0);

myMemory[0] = 5;
myMemory[1] = 10;