Linux 系统应用编程——多线程经典问题（生产者-消费者）

         “生产者——消费者”问题是Linux多线程编程中的经典问题，主要是利用信号量处理线程间的同步和互斥问题。

         “生产者——消费者”问题描述如下：

          有一个有限缓冲区（这里用有名管道实现 FIFO 式缓冲区）和两个线程：生产者和消费者，它们分别不停地把产品放入缓冲区中拿走产品。一个生产者在缓冲区满的时候必须等待，一个消费者在缓冲区空的时候也不IXUS等待。另外，因为缓冲区是临界资源，所以生产者和消费者之间必须互斥进行。它们之间的关系如下：

这里要求使用有名管道来模拟有限缓冲区，并用信号量来解决“生产者——消费者”问题中的同步和互斥问题。

1、信号量分析

        这里使用3个信号量，其中两个信号量 avail 和 full 分别用于解决生产者和消费者线程之间的互斥问题。其中avail 表示缓冲区的空单元数，初始值为N；full 表示缓冲区非空单元数，初始值为 0 ； mutex 是互斥信号量 ，初始值为 1(当然也可以用互斥锁来实现互斥操作)。

2、画出流程图

3、编写代码

        本实验的代码中缓冲区拥有3个单元，每个单元为5个字节。为了尽量体现每个信号量的意义，在程序中生产过程和消费过程是随机（采取0~5s 的随机事件间隔）进行的，而且生产者的速度比消费者的速度平均快两倍左右。生产者一次生产一个单元的产品（放入hello字符串），消费者一次消费一个单元的产品。

#include <stdio.h>#include <stdlib.h>#include <string.h>#include <unistd.h>#include <pthread.h>#include <sys/types.h>#include <time.h>#include <fcntl.h>#include <semaphore.h>#include <sys/ipc.h>#include <errno.h>#define MYFIFO "myfifo"#define BUFFER_SIZE 3#define UNIT_SIZE 5#define RUN_TIME 30#define DELAY_TIME_LEVELS 5.0void *producer(void *arg);void *customer(void *arg);int fd;time_t end_time;sem_t mutex,full,avail;int main(){int ret;pthread_t thrd_prd_id,thrd_cst_id;srand(time(NULL));end_time = time(NULL) + RUN_TIME;        /*创建有名管道*/if((mkfifo(MYFIFO,0644) < 0) && (errno != EEXIST)){perror("mkfifo error!");exit(-1);}        /*打开管道*/fd = open(MYFIFO,O_RDWR);if(fd == -1){perror("open fifo error");exit(-1);}        /*初始化互斥信号量为1*/ret = sem_init(&mutex,0,1);        /*初始化avail信号量为 N */ret += sem_init(&avail,0,BUFFER_SIZE);/*初始化full信号量为0*/ret += sem_init(&full,0,0);if(ret != 0){perror("sem_init error");exit(-1);}        /*创建两个线程*/ret = pthread_create(&thrd_prd_id,NULL,producer,NULL);if(ret != 0){perror("producer pthread_create error");exit(-1);}ret = pthread_create(&thrd_cst_id,NULL,customer,NULL);if(ret != 0){perror("customer pthread_create error");exit(-1);}pthread_join(thrd_prd_id,NULL);pthread_join(thrd_cst_id,NULL);close(fd);unlink(MYFIFO);return 0;}void *producer(void *arg) //生产者线程{int real_write;int delay_time;while(time(NULL) < end_time){delay_time = (int)(rand() * DELAY_TIME_LEVELS/RAND_MAX/2.0) + 1;sleep(delay_time);                /*P操作信号量avail和mutex*/sem_wait(&avail);sem_wait(&mutex);printf("\nproducer have delayed %d seconds\n",delay_time);                /*生产者写入数据*/if((real_write = write(fd,"hello",UNIT_SIZE)) == -1){if(errno == EAGAIN){printf("The buffer is full,please wait for reading!\n");}}else{printf("producer writes %d bytes to the FIFO\n",real_write);printf("Now,the buffer left %d spaces!\n",avail);}                /*V操作信号量full 和 mutex*/sem_post(&full);sem_post(&mutex);}pthread_exit(NULL);}void *customer(void *arg) //消费者线程{unsigned char read_buffer[UNIT_SIZE];int real_read;int delay_time;while(time(NULL) < end_time){delay_time = (int)(rand() * DELAY_TIME_LEVELS/RAND_MAX/2.0) + 1;sleep(delay_time);sem_wait(&full); //P操作信号量full和mutexsem_wait(&mutex);memset(read_buffer,0,UNIT_SIZE);printf("\nCustomer have delayed %d seconds\n",delay_time);if((real_read = read(fd,read_buffer,UNIT_SIZE)) == -1){if(errno == EAGAIN){printf("The buffer is empty,please wait for writing!\n");}}else{printf("customer reads %d bytes from the FIFO\n",real_read);}sem_post(&avail); //V操作信号量 avail 和 mutexsem_post(&mutex);}pthread_exit(NULL);}

执行结果如下：

fs@ubuntu:~/qiang/pthread$ ./cust_prod producer have delayed 2 secondsproducer writes 5 bytes to the FIFONow,the buffer left 2 spaces!Customer have delayed 2 secondscustomer reads 5 bytes from the FIFOproducer have delayed 2 secondsproducer writes 5 bytes to the FIFONow,the buffer left 2 spaces!Customer have delayed 2 secondscustomer reads 5 bytes from the FIFOproducer have delayed 2 secondsproducer writes 5 bytes to the FIFONow,the buffer left 2 spaces!Customer have delayed 2 secondscustomer reads 5 bytes from the FIFOproducer have delayed 1 secondsproducer writes 5 bytes to the FIFONow,the buffer left 2 spaces!Customer have delayed 2 secondscustomer reads 5 bytes from the FIFOproducer have delayed 1 secondsproducer writes 5 bytes to the FIFONow,the buffer left 2 spaces!Customer have delayed 3 secondscustomer reads 5 bytes from the FIFOproducer have delayed 3 secondsproducer writes 5 bytes to the FIFONow,the buffer left 2 spaces!producer have delayed 1 secondsproducer writes 5 bytes to the FIFONow,the buffer left 1 spaces!Customer have delayed 2 secondscustomer reads 5 bytes from the FIFOCustomer have delayed 1 secondscustomer reads 5 bytes from the FIFOproducer have delayed 3 secondsproducer writes 5 bytes to the FIFONow,the buffer left 2 spaces!Customer have delayed 1 secondscustomer reads 5 bytes from the FIFOproducer have delayed 2 secondsproducer writes 5 bytes to the FIFONow,the buffer left 2 spaces!Customer have delayed 2 secondscustomer reads 5 bytes from the FIFOproducer have delayed 1 secondsproducer writes 5 bytes to the FIFONow,the buffer left 2 spaces!Customer have delayed 2 secondscustomer reads 5 bytes from the FIFOproducer have delayed 1 secondsproducer writes 5 bytes to the FIFONow,the buffer left 2 spaces!producer have delayed 1 secondsproducer writes 5 bytes to the FIFONow,the buffer left 1 spaces!Customer have delayed 2 secondscustomer reads 5 bytes from the FIFOproducer have delayed 2 secondsproducer writes 5 bytes to the FIFONow,the buffer left 1 spaces!Customer have delayed 3 secondscustomer reads 5 bytes from the FIFOCustomer have delayed 1 secondscustomer reads 5 bytes from the FIFOproducer have delayed 3 secondsproducer writes 5 bytes to the FIFONow,the buffer left 2 spaces!producer have delayed 1 secondsproducer writes 5 bytes to the FIFONow,the buffer left 1 spaces!Customer have delayed 2 secondscustomer reads 5 bytes from the FIFOCustomer have delayed 1 secondscustomer reads 5 bytes from the FIFOproducer have delayed 3 secondsproducer writes 5 bytes to the FIFONow,the buffer left 2 spaces!Customer have delayed 2 secondscustomer reads 5 bytes from the FIFOproducer have delayed 2 secondsproducer writes 5 bytes to the FIFONow,the buffer left 2 spaces!fs@ubuntu:~/qiang/pthread$