POSIX Semaphores. Operations on semaphores (taken from the Linux man page)

Transcription

1 POSIX Semaphores A variable of type sem_t Example Declaration of a semaphore sem_t sem; Operations on semaphores (taken from the Linux man page) int sem_init(sem_t *sem, int pshared, unsigned int value); int sem_wait(sem_t * sem); int sem_trywait(sem_t * sem); int sem_post(sem_t * sem); int sem_getvalue(sem_t * sem, int * sval); int sem_destroy(sem_t * sem); sem_init initializes the semaphore object pointed to by sem. The count associated with the semaphore is set initially to value. The pshared argument indicates whether the semaphore is local to the current process (pshared is zero) or is to be shared between several processes, e.g. a parent and its children processes (pshared is not zero). Linux currently does not support process-shared semaphores, thus sem_init always returns with error ENOSYS if pshared is not zero. sem_wait suspends the calling thread until the semaphore pointed to by sem has non-zero count. It then atomically decreases the semaphore count. sem_trywait is a non-blocking variant of sem_wait. If the semaphore pointed to by sem has non-zero count, then the count is atomically decreased and sem_trywait immediately returns 0. If the semaphore count is zero, sem_trywait immediately returns with error EAGAIN. sem_post atomically increases the count of the semaphore pointed to by sem. This function never blocks and can safely be used in asynchronous signal handlers. 1

2 sem_getvalue stores in the location pointed to by sval the current count of the semaphore sem. sem_destroy destroys a semaphore object, freeing the resources it might hold. No threads should be waiting on the semaphore at the time sem_destroy is called. In the Linux implementation, no resources are associated with semaphore objects, thus sem_destroy actually does nothing except checking that no thread is waiting on the semaphore. The sem_wait and sem_getvalue functions always return 0. All other semaphore functions return 0 on success and -1 on error, in addition to writing an error code in errno. A program that illustrates POSIX semaphores and System V semaphores on Linux. #include <stdio.h> #include <stdlib.h> #include <sys/types.h> #include <sys/stat.h> #include <string.h> #include <errno.h> #include <fcntl.h> #include <pthread.h> #include <unistd.h> #define POSIX_SEMAPHORE /* #define SYSV_SEMAPHORE */ sem_t my_sem; #define SEMINITVALUE 1 #include <sys/ipc.h> #include <sys/sem.h> int my_sem, semop_ret; #define PERMS S_IRUSR S_IWUSR #define SEM_SET_SIZE 1 struct sembuf semwait = 0, -1, 0 ; struct sembuf semsignal = 0, 1, 0 ; 2

3 /* function prototypes */ void *critical_section(void *); #define MAXTHREADS 100 /* * Main program * */ int main(int argc, char *argv[]) int n, i, error; pthread_t threadid[maxthreads]; /* check the number of arguments */ if ((argc!= 2) ((n = atoi(argv[1])) <= 0)) fprintf(stderr, "Usage: %s number_of_threads\n", argv[0]); /* initialise the semaphore */ if (sem_init(&my_sem, 0, SEMINITVALUE) < 0) perror("could not initialise semaphore"); if ((my_sem = semget(ipc_private, SEM_SET_SIZE, PERMS)) < 0) perror("could not initialise semaphore"); /* initialise the semaphore to one */ if (semop(my_sem, &semsignal, 1) == -1) fprintf(stderr, "Semaphore increment dailed %s\n", 3

4 /* create the threads */ for (i = 0; i < n; ++i) if (error=pthread_create(&threadid[i], NULL, critical_section, (void *)i)) fprintf(stderr, "Cannot create thread %d: %s\n", i, strerror(error)); /* wait for the threads to complete */ for (i = 0; i < n; ++i) if (error=pthread_join(threadid[i], NULL)) fprintf(stderr, "No thread %d to join: %s\n", i, strerror(error)); exit(0); void *critical_section(void *arg) int myindex, i, n; /* extract the index of the thread */ myindex = (int)arg; /* entry action */ if (sem_wait(&my_sem) == 1) perror("invalid semaphore"); while ((semop_ret=semop(my_sem, &semwait, 1)==-1) && (errno == EINTR)); if (semop_ret == -1) fprintf(stderr, "Semaphore decrement failed: %s\n", /* critical section */ for (i=1; i < 5; i++) 4

5 fprintf(stdout, "%d : %d\n", myindex, i); /* sleep for one second */ if ((n=sleep(1))!= 0) fprintf(stderr, "interrupted, no of secs left %d\n", n); /* exit action */ if (sem_post(&my_sem) == -1) perror("invalid semaphore"); while ((semop_ret=semop(my_sem, &semsignal, 1)==-1) && (errno == EINTR)); if (semop_ret == -1) fprintf(stderr, "Semaphore increment failed: %s\n", /* remainder */ pthread_exit(null); 5