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

//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); }