Designing a Queue to be a shared memory

Question

I\'m attempting to design/implement a (circular) queue (in C) as a shared memory so that it can be shared between multiple threads/processes.

The queue structure is as

Answer 1

When I messed with Unix IPC, I followed Beej's guide to Unix IPC. It even has some jokes! You can go directly to the shared memory section. It has snippets explaining each step, and a full example at the end.

Answer 2

Here is a simple example that creates shared memory the size of a structure, writes some data to it and prints it out. Run one instance and it will create the shared memory and put some "data" in it, and then wait for a key press. Run a second instance in a different command prompt, and the second instance will print the contents of the memory.

typedef struct
   {
   char a[24];
   int i;
   int j;
   } somestruct;


void fillshm(int shmid) {
   somestruct *p;

   if ( (p = shmat (shmid, NULL, 0)) < 0 )
      {
      perror("shmat");
      exit(1);
      }

   printf("writing to shared memory\n");
   strcpy(p->a, "my shared memory");
   p->i = 123;
   p->j = 456;
}


void printshm(int shmid)
{
   somestruct *p;
   if ( (p = shmat (shmid, NULL, 0)) < 0 )
      {
      perror("shmat");
      exit(1);
      }

   printf( "%s, %d, %d\n", p->a, p->i, p->j );
}

int main( int argc, char *argv[] ) {

   int shmid;

   // see if the memory exists and print it if so
   if ( (shmid = shmget (1234, 0, 0)) >= 0 )
      printshm( shmid );
   else
      {
      // didn't exist, so create it
      if ( (shmid = shmget (1234, sizeof( somestruct ), IPC_CREAT | 0600)) < 0 )
         {
         perror("shmget");
         exit(1);
         }

      printf( "shmid = %d\n", shmid );

      fillshm(shmid);
      printf( "Run another instance of this app to read the memory... (press a key): " );
      getchar();

      // delete it
      if ( shmctl (shmid, IPC_RMID, NULL) < 0 )
         {
         perror("semctl");
         exit(1);
         }
      }

   return 0;
}