linux下信号量semaphore的应用

生产者线程用于往链表里添加节点，数个工作线程从链表取出节点并处理。对于一般的mutex_lock,mutex_unlock方式，如果这一段时间没有工作，那么工作线程将会不停的调用lock,unlock操作。而这样的操作毫无疑义。

在这里系统给我们提供了另外一种同步机制，信号灯,Semaphore。

信号灯其实就是一个计数器，也是一个整数。每一次调用wait操作将会使semaphore值减一，而如果semaphore值已经为0，则wait操作将会阻塞。每一次调用post操作将会使semaphore值加一。将这些操作用到上面的问题中。工作线程每一次调用wait操作，如果此时链表中没有节点，则工作线程将会阻塞，直到链表中有节点。生产者线程在每次往链表中添加节点后调用post操作，信号灯值会加一。这样阻塞的工作线程就会停止阻塞，继续往下执行。

信号灯的类型为sem_t。在声明后必须调用sem_init()。需要传递两个参数，第一个参数就是你之前声明的sem_t变量，第二个必须为0。当你不再需要信号灯时，你必须调用sem_destroy()来释放资源。

等待信号灯的操作为sem_wait()。投递一个信号的操作为sem_wait()。和互斥量一样，等待信号灯也有一个非阻塞的操作，sem_trywait()。该操作在没有信号灯的时候返回EAGAIN。

下面是一个结合了互斥量和信号灯的例子：

#include <malloc.h>
#include <pthread.h>
#include <semaphore.h>
struct job {
/* Link field for linked list. */
struct job* next;
/* Other fields describing work to be done... */
};
/* A linked list of pending jobs. */
struct job* job_queue;
/* A mutex protecting job_queue. */
pthread_mutex_t job_queue_mutex = PTHREAD_MUTEX_INITIALIZER;
/* A semaphore counting the number of jobs in the queue. */
sem_t job_queue_count;
/* Perform one-time initialization of the job queue. */
void initialize_job_queue ()
{
/* The queue is initially empty. */
job_queue = NULL;
/* Initialize the semaphore which counts jobs in the queue. Its
initial value should be zero. */
sem_init (&job_queue_count, 0, 0);
}
/* Process queued jobs until the queue is empty. */
void* thread_function (void* arg)
{
while (1) {
struct job* next_job;
/* Wait on the job queue semaphore. If its value is positive,
indicating that the queue is not empty, decrement the count by
1. If the queue is empty, block until a new job is enqueued. */
sem_wait (&job_queue_count);
/* Lock the mutex on the job queue. */
pthread_mutex_lock (&job_queue_mutex);
/* Because of the semaphore, we know the queue is not empty. Get
the next available job. */
next_job = job_queue;
/* Remove this job from the list. */
job_queue = job_queue->next;
/* Unlock the mutex on the job queue because we’re done with the
queue for now. */
pthread_mutex_unlock (&job_queue_mutex);
/* Carry out the work. */
process_job (next_job);
/* Clean up. */
free (next_job);
}
return NULL;
}
/* Add a new job to the front of the job queue. */
void enqueue_job (/* Pass job-specific data here... */)
{
struct job* new_job;
/* Allocate a new job object. */
new_job = (struct job*) malloc (sizeof (struct job));
/* Set the other fields of the job struct here... */
/* Lock the mutex on the job queue before accessing it. */
pthread_mutex_lock (&job_queue_mutex);
/* Place the new job at the head of the queue. */
new_job->next = job_queue;
job_queue = new_job;
/* Post to the semaphore to indicate that another job is available. If
threads are blocked, waiting on the semaphore, one will become
unblocked so it can process the job. */
sem_post (&job_queue_count);
/* Unlock the job queue mutex. */
pthread_mutex_unlock (&job_queue_mutex);
}