CODEWITHSUNDEEP
cunixposixshared-memory
VINOTH ENERGETIC
VINOTH ENERGETIC

Reputation: 1843

How to store the multiple structures instances in shared memory?

I have a struct definition as follows

struct s
{
int a;
};
struct s s1;
struct s s2;

I need to push both structure instances to shared memory.

char *data=(char *)shmat(m_sharedMemoryId,NULL,0);
memcpy(data,&s1,sizeof struct s);

I did for the 1st instance of structure.

How to do for the second instance? Is this possible in shared memory?

Plaftform : UNIX

Upvotes: 0

Views: 1178

Answers (1)

Serge Ballesta
Serge Ballesta

Reputation: 148965

The shmat calls only gets a pointer to a shared memory segment that should have previously be created with shmget. You have 2 ways to deal with that :

  • allocate a segment that will hold an array for all your structs and then copy your structs to this array :

    m_sharedMemoryId = shmget(key, sizeof(struct s) * nb, perm_flag);
struct s* data = shmat(m_sharedMemoryId,NULL,0);

    and then you copy a struct s where you want in the array, here in position i :

    memcpy(data + i,&s1,sizeof(struct s));

    this is mainly to use if you have many little structures

  • allocate a different segment per each struct

    m_sharedMemoryId = shmget(key, sizeof(struct s), perm_flag);
struct s* data = shmat(m_sharedMemoryId,NULL,0);
memcpy(data,&s1,sizeof(struct s));

    and you repeat the allocation for each struct. This is to use if you have few big structs that you want to be able to deallocate separately

Upvotes: 2

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