[BrittneTabor]

C Shared Memory and 2D Arrays
So here is my code for the client:

#include 
#include 
#include 
#include 
#include 

#define SHMSZ 27

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

   /* Application Step-through
      1. Create shared memory
      2. Init array
   */
   
   char   c;
   int      shmid;
   key_t   key;
   int   *shm, *s;
   int i, row, col;

   int      data_store[5][2];   
   /* Created Shared Memory */
   
   key = 0xFFFFFFFF;   /* Identifier Key for shared memory segment */
   
   if((shmid = shmget(key, SHMSZ, IPC_CREAT | 0666)) < 0)   /* shmget has failed */
   {
      perror("shmget");
      _Exit(1);
   }
   printf("[+] Shared Mem Segment Created. shmid: %d\n", shmid);
   
   /* Attatch segment to data space */
   if((shm = shmat(shmid,NULL,0)) == (char *)-1)
   {
      perror("shmat");
      _Exit(1);
   }
   printf("[+] Segment Attatched\n");

   int pid;
   
   pid = fork();
   if(pid < 0)
   {
      perror("fork");
      _Exit(1);
   }
   else if(pid == 0) /* Child Process */
   {
      while(1)
      {
         sleep(1);
      }
   }
   else         /* Parent Process */
   {
      printf("[+] Process ID: %d\n", pid);
                                       
      /* data_store ID, PID, PRI */            /*               0=PID 1=PRI
                                       data_store[0][0] */
      /* Write PID and PRI to data_store array */
      data_store[0][0] = pid;
      data_store[0][1] = 1;
      
      data_store[1][0] = 12345;
      data_store[1][1] = 2;
      
      /* DEBUG: Check Array is full */
      for(i=0; i <= 1; i++)
      {
         printf("%d\n", data_store[i][0]);
      }
      
      /*Write data_store to memory seg*/
      s = shm;

      for(row = 0; row <= 1; row++)
         for(col=0; col <=1; col++)
            s[row*2 + col] = 1;
            
      *s = (int)NULL;
      
   
   }

   return 0;
}

I hope this *should* write my array to the shared memory location.

How do I go about reading it from the Server? This is my current code;

#include 
#include 
#include 
#include 

#define SHMSZ 27

/* This bianry will simulate de-spooling */


int main(int argc, char* argv[])
{
   char c;
   int shmid;
   key_t key;
   int *shm, *s;

   /* Key for shared memory segment */
   key = 0xFFFFFFFF;


   if((shmid = shmget(key, SHMSZ, 0666)) < 0)
   {
      perror("shmget");
      _Exit(1);
   }
   printf("[+] Segment Found - shmid = %d\n",shmid);
   
   if((shm = shmat(shmid,NULL,0)) == (char *)-1)
   {
      perror("shmat");
      _Exit(1);
   }
   printf("[+] Segment Attatched\n");
   
   for(s=shm; *s != (int)NULL; s++)
      printf("%d",*s[0][0]);
      
   return 0;
}

[Abhey]

I am assuming that your question is how to read from the shared memory concurrently, without worrying about the data being corrupt by a writing process. First of all, you need some sort of locking mechanism. In the case of multiple processes trying to access a piece of data simultaneously, I do so through a monitor.

http://en.wikipedia.org/wiki/Monitor_(synchronization)

The monitor will basically ensure that you can have many simultaneous readers, but writers have exclusive access to a specific datum. I hope this answers the question.