How to handle structs in Shared memory

Question

I have created a struct in C. Now I want to write some data in that struct and want other process to read it. Let call the writer be server and reader be client.

For writer code goes like:

typedef struct 
{
    pthread_mutex_t mutex;
    char * data;

} shm_data_struct, *shm_data_struct_t;
int shmid;
char * shm_address;
shm_data_struct_t shm_ptr;


int main(int argc, char const *argv[])
{
    shmid = shmget(KEY, sizeof(shm_data_struct), IPC_CREAT | 0666)
    shm_address = shmat(shmid, (void*)0, 0)
    shm_ptr = (shm_data_struct_t)shm_address;
    //Writing into struct
    shm_ptr->data = "String";
    while(shm_ptr->data != '*'){
        sleep(1);
    }
}

For the client Side:

typedef struct 
{
    pthread_mutex_t mutex;
    char * data;

} shm_data_struct, *shm_data_struct_t;

int main(int argc, char const *argv[])
{
    int shmid;
    key_t key;
    char *shm;
    shm_data_struct_t shm_ptr;
    key = 120;
    shmid = shmget(key, sizeof(shm_data_struct), 0666)
    shm = shmat(shmid, NULL, 0)

    /*
     * Now read what the server put in the memory.
     */
    shm_ptr = (shm_data_struct_t)shm;
    printf("%s\n", shm_ptr->data);
    shm_ptr->data = '*';

    exit(0);
}

This code is giving me segmentation fault in print statement in client's code. Can anybody help what I am doing wrong?

Which code exactly is giving segfault in which situation precisely? — Yunnosch
– Yunnosch, Commented Mar 11, 2018 at 7:51
@Yunnosch I have edited in the question. It's in print statement in client code. — Sand Box
– Sand Box, Commented Mar 11, 2018 at 7:53
Pointers in shared memory don't work, because the address that it points to in the server doesn't point to anything in the client. — user3386109
– user3386109, Commented Mar 11, 2018 at 8:10
See Is it a good idea to typedef pointers — the short answer is No except perhaps for function pointers. — Jonathan Leffler
– Jonathan Leffler, Commented Mar 11, 2018 at 8:37

user149341 · Accepted Answer · 2018-03-11 08:15:22Z

1

This is your problem:

    char * data;

Only the pointer (that is, an address) is being stored in shared memory. The data pointed to by the pointer is not shared, and the pointer itself will be invalid, or will point to unexpected data, in another process.

Generally speaking, SysV shared memory is a poor form of IPC, and should usually be avoided. As you've discovered, it cannot easily store even moderately complex data structures; it is also prone to race conditions, and the locking structures used to avoid that (i.e, SysV semaphores) will often create deadlocks if a process using them crashes.

answered Mar 11, 2018 at 8:15

user149341

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2 Comments

Peter - Reinstate Monica Over a year ago

One might add that the remedy is to include the data proper in the struct, i.e. change the definition of the struct member data to something like char data[MAX_STRING_SZ]; with some suitable size. The data must be copied into it with strcpy() or the like. (I realize that this is an elaboration of @user3386109's comment.)

Sand Box Over a year ago

Is there any way we can do this. Because I am suppose to use SYSV for this project.

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