How would I add a common semaphore, to shared1.cpp and shared2.cpp to protect the shared memory so that when a process accesses the shared area, the other is excluded from doing so. You may use the POSIX semaphore, which is simpler or the UNIX semaphore. Test your modified programs to see if they work properly. //shared1.cpp /* After the headers the shared memory segment (the size of our shared memory structure) is created with a call to shmget, with the IPC_CREAT bit specified. It reads data from the shared memory. */ #include #include #include #include #include #include #include #define TEXT_SZ 2048 struct shared_use_st { int written_by_you; char some_text[TEXT_SZ]; }; int main() { int running = 1; void *shared_memory = (void *)0; struct shared_use_st *shared_stuff; int shmid; srand((unsigned int)getpid()); shmid = shmget((key_t)1234, sizeof(struct shared_use_st), 0666 | IPC_CREAT); if (shmid == -1) { fprintf(stderr, "shmget failed "); exit(EXIT_FAILURE); } /* We now make the shared memory accessible to the program. */ shared_memory = shmat(shmid, (void *)0, 0); if (shared_memory == (void *)-1) { fprintf(stderr, "shmat failed "); exit(EXIT_FAILURE); } printf("Memory attached at %X ", (long)shared_memory); /* The next portion of the program assigns the shared_memory segment to shared_stuff, which then prints out any text in written_by_you. The loop continues until end is found in written_by_you. The call to sleep forces the consumer to sit in its critical section, which makes the producer wait. */ shared_stuff = (struct shared_use_st *)shared_memory; shared_stuff->written_by_you = 0; while(running) { if (shared_stuff->written_by_you) { printf("You wrote: %s", shared_stuff->some_text); sleep( rand() % 4 ); /* make the other process wait for us ! */ shared_stuff->written_by_you = 0; if (strncmp(shared_stuff->some_text, "end", 3) == 0) { running = 0; } } } /* Lastly, the shared memory is detached and then deleted. */ if (shmdt(shared_memory) == -1) { fprintf(stderr, "shmdt failed "); exit(EXIT_FAILURE); } if (shmctl(shmid, IPC_RMID, 0) == -1) { fprintf(stderr, "shmctl(IPC_RMID) failed "); exit(EXIT_FAILURE); } exit(EXIT_SUCCESS); } 

/* shared2.cpp: Similar to shared1.cpp except that it writes data to the shared memory. */ #include #include #include #include #include #include #include #define TEXT_SZ 2048 struct shared_use_st { int written_by_you; char some_text[TEXT_SZ]; }; int main() { int running = 1; void *shared_memory = (void *)0; struct shared_use_st *shared_stuff; char buffer[BUFSIZ]; int shmid; shmid = shmget((key_t)1234, sizeof(struct shared_use_st), 0666 | IPC_CREAT); if (shmid == -1) { fprintf(stderr, "shmget failed "); exit(EXIT_FAILURE); } shared_memory = shmat(shmid, (void *)0, 0); if (shared_memory == (void *)-1) { fprintf(stderr, "shmat failed "); exit(EXIT_FAILURE); } printf("Memory attached at %X ", (long)shared_memory); shared_stuff = (struct shared_use_st *)shared_memory; while(running) { while(shared_stuff->written_by_you == 1) { sleep(1); printf("waiting for client... "); } printf("Enter some text: "); fgets(buffer, BUFSIZ, stdin); strncpy(shared_stuff->some_text, buffer, TEXT_SZ); shared_stuff->written_by_you = 1; if (strncmp(buffer, "end", 3) == 0) { running = 0; } } if (shmdt(shared_memory) == -1) { fprintf(stderr, "shmdt failed "); exit(EXIT_FAILURE); } exit(EXIT_SUCCESS); }