Operating Systems 173323 (3 cu)

Exercise 6

1. Shared memory is the fastest way for the communication between the processes: processes have a common area in the memory and they use that to communicate. The following program uses shared memory. Add comment lines before each command line to explain the program code.

   #include <sys/types.h>
   #include <sys/ipc.h>
   #include <sys/shm.h>
   
   int key=2709;
   
   char *messu[] = { 
           "Hello",
           "Countdown",
           "5", "4","3","1","sorry, 2","1","GO\007",
           "------------------------------"
           };
   
   int n=10;
   
   char *shmat();
   
   main()
   {
     int id,i;
     char *seg;
   
     id = shmget(key,1024,IPC_CREAT | 0777);
     if (id == -1) {perror("shmget"); exit(1);}
   
     seg = shmat(id,(char *)0,0);
   
     for(i=0;;) {
           sprintf(seg,"%s",messu[i]);
           i++; i = i % n;
           sleep(1);
     }
   }
   

2. The following program also uses shared memory. Add comment lines before each command line to explain the program code.

   #include <sys/types.h>
   #include <sys/ipc.h>
   #include <sys/shm.h>
   
   char *shmat();
   
   int key=2709;
   
   main()
   {
     int id,i;
     char *seg;
   
     id = shmget(key,1024, SHM_RDONLY);
     if (id == -1) {perror("shmget"); exit(1);}
   
     seg = shmat(id,(char *)0,0);
   
     while(1){
       printf("%s\n",seg);
       sleep(1);
       }
   }
   

3. Save the programs in task 1 and task 2 in cs-computer and compile them. Start the execution of the program in task 1 in one session and in another session start the execution of the program in task 2. What happens and why ? Use ctrl-c to quit the programs.