CODEWITHSUNDEEP
cshared-memory
Milmar
Milmar

Reputation: 17

using Shmget() in c with structure

am using C on OSx and using shmget() to get memory to be used between two different programs i have a structure of 2 int values does this mean if i want a size of 27 instead it will have to be 54 and how would this work with the pointers used for the structure or any help would be greatly appreciated

Upvotes: 0

Views: 1116

Answers (2)

Reeno Joseph
Reeno Joseph

Reputation: 21

Assume you have a struct like this:

typedef struct {
    int aCount;
    int bCount;
    int cCount;
} data_t;

This below function will create shared memory for the specified size.

int shared_memory_create(size_t memory_size)
{
    int shm_key = shmget(IPC_PRIVATE, memory_size, IPC_CREAT | IPC_EXCL | 0666);
    if(shm_key < 0) {
        perror("Failed to Create Shared Memory Key");
        exit(0);
    }

    return shm_key;
}

If you want to create shared memory to accommodate 27 elements, call the function shared_memory_create as shown below:

shm_key = shared_memory_create(27*sizeof(data_t));

Upvotes: 0

Randy Howard
Randy Howard

Reputation: 2156

No, you need to use sizeof on the struct. You can't guarantee how the implementation will pad the values inside the struct, so you don't assume the size of them. Also, if the size changes later due to adding member(s) to the struct, the memory allocation will still work correctly.

Then use that value from sizeof to multiply by the number of instances of the struct you intend to use in the shared memory area.

Upvotes: 1

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