Using shared memory in C - c

Question

I would like to create a server-client program in which the two processes pass information between each other using shared memory
information to be passed:
typedef struct shared_mem{
int board[BOARD_SIZE * BOARD_SIZE];
int goal;
}shared_mem;
shared_mem *msg;
server:
int main(int argc, char **argv) {
int shmid;
key_t key=ftok("2048_client", 42);
if(key == -1) {
printf("ftok failed");
return -1;
}
shared_mem *shm;
msg=(shared_mem *)malloc(sizeof(shared_mem));
/*
* Create the segment
*/
if ((shmid = shmget(key, sizeof(msg), IPC_CREAT)) < 0) {
perror("shmget");
exit(1);
}
/*
* Now we attach the segment to our data space.
*/
if ((shm = shmat(shmid, NULL, 0)) == (char *) -1) {
perror("shmat");
exit(1);
}
msg=shm;
(*msg).goal=64;
}
client:
int main(int argc, char **argv) {
int shmid;
key_t key=ftok("2048_client", 42);
if(key == -1) {
printf("ftok failed");
return -1;
}
shared_mem *shm;
msg=(shared_mem *)malloc(sizeof(shared_mem));
/*
* Create the segment.
*/
if ((shmid = shmget(key, sizeof(msg), 0)) < 0) {
perror("shmget");
exit(1);
}
/*
* Now we attach the segment to our data space.
*/
if ((shm = shmat(shmid, NULL, 0)) == (char *) -1) {
perror("shmat");
exit(1);
}
msg=shm;
printf("dscsadc: %d",msg->goal);
}
I am new to shared memory and i would like to understand why it doesn't work and how it is supposed to work. I am getting "shmat: Permission denied"

Answer 1

The problem is that you create the shared memory segment with 0000 permissions, so no-one can read or write it.
Change the shmget() call from:
if ((shmid = shmget(key, sizeof(msg), IPC_CREAT)) < 0) {
to:
if ((shmid = shmget(key, sizeof(msg), IPC_CREAT|0600)) < 0) {
Only the user running the program can access the shared memory that is created.
Note that POSIX shmget()
says:
The low-order nine bits of shm_perm.mode are set to the low-order nine bits of shmflg.

Answer 2

If you're not limited to C only, look at the boost library. It enables you to create shared memory segments for interprocess communication.
using boost::interprocess;
shared_memory_object shm_obj
(create_only, //only create
"shared_memory", //name
read_write //read-write mode
);
http://www.boost.org/doc/libs/1_54_0/doc/html/interprocess/sharedmemorybetweenprocesses.html
Other then that, you can always use pipes, or if you're thinking about windows - COM.