int main( void ) {
/* create an a shared memory segment */
seg_id = shmget ( IPC_PRIVATE, sizeof(int), IPC_CREAT | 0666 );
if ( seg_id == -1 )
printf("error\n");
/* attach to segment and get a pointer to it */
mem_ptr = shmat ( seg_id, NULL, 0 );
/* create a semaphore pair, set both to 0 */
semid = sem_init( 2, 0);
/* trap for CTRL-C - use sigaction in your code */
signal(SIGINT, ctrlc_handler);
/* create one Reader and one Writer thread. The Reader thread performs a
loop of read operations, reading only when the Writer has finished.
demonstrates the strong-reader preference protocol; e.g. the writer must
wait until the Reader finishes */
pthread_t W1, W2, R1, R2, R3;
if ( pthread_create( &W1, NULL, Writer, (void *)"1" ) != 0)
printf("Error creating thread\n");
if ( pthread_create( &W2, NULL, Writer, (void *)"2" ) != 0)
printf("Error creating thread\n");
if ( pthread_create( &R1, NULL, Reader, (void *)"1" ) != 0)
printf("Error creating thread\n");
if ( pthread_create( &R2, NULL, Reader, (void *)"2" ) != 0)
printf("Error creating thread\n");
if ( pthread_create( &R3, NULL, Reader, (void *)"3" ) != 0)
printf("Error creating thread\n");
/* rejoin from threads */
if ( pthread_join(W1, NULL) != 0 )
printf("Error rejoining from thread\n");
if ( pthread_join(W2, NULL) != 0 )
printf("Error rejoining from thread\n");
if ( pthread_join(R1, NULL) != 0 )
printf("Error rejoining from thread\n");
if ( pthread_join(R2, NULL) != 0 )
printf("Error rejoining from thread\n");
if ( pthread_join(R3, NULL) != 0 )
printf("Error rejoining from thread\n");
/* parent cleans up */
sleep(5);
removeid( semid); // remove semaphore set
shmctl( seg_id, IPC_RMID, NULL); // remove shared memory
} // end main
int main (void) {
/* set stdout to autoflush - this call is not portable but works on linux */
setvbuf(stdout, NULL, _IONBF, 0);
/* create an a shared memory segment - IPC_PRIVATE creates unique set_id */
shmid = shmget(IPC_PRIVATE, sizeof(int), IPC_CREAT | 0666);
if (shmid == -1)
perror("shmget:");
/* attach to segment and get a pointer to it */
shared = shmat(shmid, NULL, 0 );
/* initialize mem segment */
*shared=0;
/* create a semaphore pair, set both to 0 */
semid = init_my_sems(2, 0);
/* trap for CTRL-C */
struct sigaction sa;
sa.sa_handler = ctrlc_handler;
sigfillset(&sa.sa_mask); /* block signals while in the handler */
sigaction(SIGINT, &sa, NULL);
/* create one Reader and one Writer thread. The Reader thread performs a
* loop of read operations, reading only when the Writer has finished.
* demonstrates the strong-reader preference protocol; e.g. the writer must
* wait until the Reader finishes
*/
pthread_t W1, W2, W3, R1, R2, R3, R4;
if (pthread_create( &W1, NULL, Writer, (void *)"1" ) != 0)
perror("Error creating thread\n");
if (pthread_create( &W2, NULL, Writer, (void *)"2" ) != 0)
perror("Error creating thread\n");
if (pthread_create( &W3, NULL, Writer, (void *)"3" ) != 0)
perror("Error creating thread\n");
if (pthread_create( &R1, NULL, Reader, (void *)"1" ) != 0)
perror("Error creating thread\n");
if (pthread_create( &R2, NULL, Reader, (void *)"2" ) != 0)
perror("Error creating thread\n");
if (pthread_create( &R3, NULL, Reader, (void *)"3" ) != 0)
perror("Error creating thread\n");
if (pthread_create( &R4, NULL, Reader, (void *)"4" ) != 0)
perror("Error creating thread\n");
/* rejoin from threads */
if (pthread_join(W1, NULL) != 0 )
perror("Error rejoining from thread\n");
if (pthread_join(W2, NULL) != 0 )
perror("Error rejoining from thread\n");
if (pthread_join(W3, NULL) != 0 )
perror("Error rejoining from thread\n");
if (pthread_join(R1, NULL) != 0 )
perror("Error rejoining from thread\n");
if (pthread_join(R2, NULL) != 0 )
perror("Error rejoining from thread\n");
if (pthread_join(R3, NULL) != 0 )
perror("Error rejoining from thread\n");
if (pthread_join(R4, NULL) != 0 )
perror("Error rejoining from thread\n");
/* parent cleans up */
removeid( semid); // remove semaphore set
shmctl( shmid, IPC_RMID, NULL); // remove shared memory
} // end main
