Using shared memory in 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"

The only things that seem obviously wrong here are that you have to supply a value for key (ftok may help with that), and you may also be required to round sizeof(msg) up to a multiple of sysconf(_SC_PAGESIZE). Beyond that: what is this doing that you did not expect it to do? — zwol
– zwol, Commented May 26, 2014 at 18:35
@Zack I tried using ftok but i get the error: shmat: Permission denied — Syn
– Syn, Commented May 26, 2014 at 19:15
In the server, the line msg=shm; leaks the carefully allocated memory; remove the malloc() line. Similarly in the client. The line (*msg).goal=64; would conventionally be written msg->goal = 64; — the -> operator was invented for a reason. — Jonathan Leffler
– Jonathan Leffler, Commented May 26, 2014 at 19:22
@JonathanLeffler I tried doing that but I get an error saying every function is defined more than once:2048.c:120:40: warning: comparison of distinct pointer types lacks a cast [enabled by default] /tmp/ccNMeNPm.o: In function main': 2048.c:(.text+0x45): multiple definition of main' /tmp/ccLsff3I.o:2048_client.c:(.text+0x0): first defined here 2048: In function move_board': (.text+0xaff): multiple definition of move_board' /tmp/ccNMeNPm.o:2048.c:(.text+0xa1b): first defined here And so on for all the other functions. — Syn
– Syn, Commented May 26, 2014 at 19:28

Jonathan Leffler · Accepted Answer · 2014-05-26 19:57:52Z

2

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.

answered May 26, 2014 at 19:57

Jonathan Leffler

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Comments

Andro · Accepted Answer · 2014-05-26 19:19:27Z

0

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.

edited May 26, 2014 at 19:19

answered May 26, 2014 at 19:12

Andro

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2 Comments

Syn Over a year ago

I am limited to C only and not allowed to use Pipes and I am really trying to understand shared memory.

Andro Over a year ago

sorry I couldn't be more help then

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