生产者与消费者模型Linux下C语言的实现

 文章出自：http://page.renren.com/600235506/note/492983524

学习了信号量以及共享内存后，我们就可以实现进程的同步与互斥了。说到这里，最经典的例子莫过于生产者和消费者模型。下面就和大家一起分析，如何一步步实现这个经典模型。

下面程序，实现的是多个生产者和多个消费者对N个缓冲区（N个货架）进行访问的例子。现在先想想我们以前的伪代码是怎么写的？是不是这样：

//生产者：

while(1){p(semid,1);sleep(3);p(semid,0);//producer is producing a productgoods=rand()%10;shmaddr[indexaddr[0]]=goods;printf("producer:%d produces a product[%d]:%d\n",getpid(),indexaddr[0],goods);indexaddr[0]=(indexaddr[0]+1)%10;v(semid,0);sleep(3);v(semid,2);}

//消费者：

while(1){p(semid,2);sleep(1);p(semid,0);//consumer is consuming a productgoods=shmaddr[indexaddr[1]];printf("consumer:%d consumes a product[%d]:%d\n",getpid(),indexaddr[1],goods);indexaddr[1]=(indexaddr[1]+1)%num;v(semid,0);sleep(1);v(semid,1);}

 

可能上面的代码你有些眼熟，又有些困惑，因为它和课本上的代码不完全一样，其实上面的代码就是伪代码的linuxC语言具体实现。我们从上面的代码中慢慢寻找伪代码的踪迹：p(semid,0)和v(semid,0)的作用是让进程互斥访问临界区。临界区中包含的数据indexaddr[0]，indexaddr[1]，以及shmaddr数组分别对应伪代码中的in，out，buffer。p(semid,1)和v(semid,2)以及p(semid,2)和v(semid,1)实现的是同步作用。

并且，在生产者中，生产者生产了一个货物（goods=rand()%10;），然后将这个货物放上货架(shmaddr[indexaddr[0]]=goods;)。在消费者中，消费和从货架上取下货物（goods=shmaddr[indexaddr[1]];）。

好了，现在再看一边上面的代码，我想你的思路就清晰了。

了解了核心代码，并不能算就完成了生产者和消费者模型，因为生产者和消费者核心代码前还得做一些些准备工作，具体要准备些什么，我们具体来分析。

首先申请一块共享内存，这块共享内存用于存放生产者所生产的货物。同时我们可以看到这块共享内存大小为10字节。这里需要注意，每个生产着或消费者运行后，都要去试着分配这样的一块共享内存。如果在当前进程运行前已经有某个进程已经创建了这块共享内存，那么这个进程就不再创建（此时createshm会返回-1并且错误代码为EEXIST），只是打开这块共享内存。创建后，再将这块共享内存添加到当前进程的地址空间。

num=10;//create a shared memory as goods bufferif((shmid_goods=createshm(".",'s',num))==-1){if(errno==EEXIST){if((shmid_goods=openshm(".",'s'))==-1){exit(1);}}else{perror("create shared memory failed\n");        exit(1);}}//attach the shared memory to the current processif((shmaddr=shmat(shmid_goods,(char*)0,0))==(char*)-1){perror("attach shared memory error\n");exit(1);}

 

接下来还要再申请一块共享内存，用于存放两个整形变量in和out（其实就是申请一个含有2个整形变量的数组而已）。他们记录的是生产和消费货物时“货架”的索引。与上面情况相同，如果已经有其他进程创建了此块共享内存，那么当前进程只是打开它而已。

注意这里对两个整形变量的初始化时的值均为0。

//create a shared memory as indexif((shmid_index=createshm(".",'z',2))==-1){if(errno==EEXIST){if((shmid_index=openshm(".",'z'))==-1){exit(1);}}else{perror("create shared memory failed\n");        exit(1);}}else{is_noexist=1;}//attach the shared memory to the current processif((indexaddr=shmat(shmid_index,(int*)0,0))==(int*)-1){perror("attach shared memory error\n");exit(1);}if(is_noexist){indexaddr[0]=0;indexaddr[1]=0;}

 

接下来就是创建一个信号量集，这个信号量集中包含三个信号量。第一个信号量实现的互斥作用，即进程对临界区的互斥访问。剩下两个均实现的是同步作用，协调生产者和消费者的合理运行，即货架上没有空位时候生产者不再生产，货架上无商品时消费者不再消费。

注意下面对每个信号量的赋值情况。互斥信号量当然初值为1。而同步信号量两者之和不能大于num的值。

//create a semaphore set including 3 semaphoresif((semid=createsem(".",'t',3,0))==-1){if(errno==EEXIST){if((semid=opensem(".",'t'))==-1){exit(1);}}else{perror("semget error:");exit(1);}}else{union semun arg;        //seting value for mutex semaphore         arg.val=1;         if(semctl(semid,0,SETVAL,arg)==-1)         {perror("setting semaphore value failed\n");        return -1;            }          //set value for synchronous semaphore        arg.val=num;            //the num means that the producer can continue to produce num products          if(semctl(semid,1,SETVAL,arg)==-1)            {perror("setting semaphore value failed\n");         return -1;         }           //the last semaphore's value is default           //the default value '0' means that the consumer is not use any product now    }

基本上这样，就算完成了生产者和消费者的前期工作。我们可以看到，在核心代码中，我们只需要“装模作样”的将代码“各就各位”即可，当然这需要你理解生产者消费者这个基本模型。而在下面的准备代码中，则需要我们理解关于信号量和共享内存的一些基本函数。

最后再说说使用，建议先运行一个生产者和一个消费者，观察两者是如何协调工作的。然后再只运行一个生产者或一个消费者，看其是否会阻塞。了解了以上情况后，你就可以同时运行多个生产者和消费者了。

下面是源代码：

shm.h

#include <stdio.h>#include <string.h>#include <stdlib.h>#include <unistd.h>#include <sys/types.h>#include <sys/ipc.h>#include <sys/sem.h>#include <sys/shm.h>#include <errno.h>#define SHM_SIZE 1024union semun{int val;struct semid_ds *buf;unsigned short *array;};//create a semaphore setint createsem(const char *pathname,int proj_id,int num,int init_val){key_t key;int i,semid;union semun arg;if((key=ftok(pathname,proj_id))==-1){perror("ftok error:");return -1;}if((semid=semget(key,num,IPC_CREAT|IPC_EXCL|0666))==-1){return -1;}//initialize the value of semaphorearg.val=init_val;for(i=0;i<num;i++){if(semctl(semid,i,SETVAL,arg)==-1){perror("semctl error:");return -1;}}return (semid);}//open the semaphore setint opensem(const char*pathname,int proj_id){key_t key;int semid;if((key=ftok(pathname,proj_id))==-1){perror("ftok error:");return -1;}//just get the id of semaphore setif((semid=semget(key,0,IPC_CREAT|0666))==-1){perror("semget error:");return -1;}return (semid);}//P operationint p(int semid,int index){struct sembuf buf={0,-1,0};if(index<0){printf("error:the index is invalid\n");return -1;}buf.sem_num=index;if(semop(semid,&buf,1)==-1){perror("semop error:");return -1;}return 1;}//V opeationint v(int semid,int index){struct sembuf buf={0,+1,0};if(index<0){printf("error:the index is invalid\n");return -1;}buf.sem_num=index;if(semop(semid,&buf,1)==-1){perror("semop error:");return -1;}return 1;}//delete the semaphore setint deletesem(int semid){return (semctl(semid,0,IPC_RMID,0)==-1);}//waiting for the semaphore is equal to 1int waitsem(int semid,int index){while(semctl(semid,index,GETVAL,0)==0){sleep(1);printf("I am waiting for semval equals 1..\n");}return 1;}//create share memoryint createshm(char *pathname,int proj_id,size_t size){key_t key;int shmid;if((key=ftok(pathname,proj_id))==-1){perror("ftok error:");return -1;}if((shmid=shmget(key,size,IPC_CREAT|IPC_EXCL|0666))==-1){return -1;}return (shmid);}//open share memoryint openshm(char *pathname,int proj_id){key_t key;int shmid;if((key=ftok(pathname,proj_id))==-1){perror("ftok error:");return -1;}if((shmid=shmget(key,0,IPC_CREAT|0666))==-1){perror("shmget error:");return -1;}return (shmid);}

 

producer.c

#include "shm.h"int main(){int num;int shmid_goods,shmid_index,semid;char* shmaddr=NULL;int *indexaddr=NULL;int is_noexist=0;num=10;//create a shared memory as goods bufferif((shmid_goods=createshm(".",'s',num))==-1){if(errno==EEXIST){if((shmid_goods=openshm(".",'s'))==-1){exit(1);}}else{perror("create shared memory failed\n");        exit(1);}}//attach the shared memory to the current processif((shmaddr=shmat(shmid_goods,(char*)0,0))==(char*)-1){perror("attach shared memory error\n");exit(1);}//create a shared memory as indexif((shmid_index=createshm(".",'z',2))==-1){if(errno==EEXIST){if((shmid_index=openshm(".",'z'))==-1){exit(1);}}else{perror("create shared memory failed\n");        exit(1);}}else{is_noexist=1;}//attach the shared memory to the current processif((indexaddr=shmat(shmid_index,(int*)0,0))==(int*)-1){perror("attach shared memory error\n");exit(1);}if(is_noexist){indexaddr[0]=0;indexaddr[1]=0;}//create a semaphore set including 3 semaphoresif((semid=createsem(".",'t',3,0))==-1){if(errno==EEXIST){if((semid=opensem(".",'t'))==-1){exit(1);}}else{perror("semget error:");exit(1);}}else{union semun arg;        //seting value for mutex semaphore         arg.val=1;         if(semctl(semid,0,SETVAL,arg)==-1)         {perror("setting semaphore value failed\n");        return -1;            }          //set value for synchronous semaphore        arg.val=num;            //the num means that the producer can continue to produce num products          if(semctl(semid,1,SETVAL,arg)==-1)            {perror("setting semaphore value failed\n");         return -1;         }           //the last semaphore's value is default           //the default value '0' means that the consumer is not use any product now      }int goods=0;while(1){p(semid,1);sleep(3);p(semid,0);//producer is producing a productgoods=rand()%10;shmaddr[indexaddr[0]]=goods;printf("producer:%d produces a product[%d]:%d\n",getpid(),indexaddr[0],goods);indexaddr[0]=(indexaddr[0]+1)%10;v(semid,0);sleep(3);v(semid,2);}}

 

consumer.c

#include "shm.h"int main(int argc,char **argv){int num;int shmid_goods,shmid_index,semid;char* shmaddr=NULL;int* indexaddr=NULL;int is_noexist=0;num=10;//create a shared memory as goods bufferif((shmid_goods=createshm(".",'s',num))==-1){if(errno==EEXIST){if((shmid_goods=openshm(".",'s'))==-1){exit(1);}}else{perror("create shared memory failed\n");        exit(1);}}//attach the shared memory to the current processif((shmaddr=shmat(shmid_goods,(char*)0,0))==(char*)-1){perror("attach shared memory error\n");exit(1);}//create a shared memory as indexif((shmid_index=createshm(".",'z',2))==-1){if(errno==EEXIST){if((shmid_index=openshm(".",'z'))==-1){exit(1);}}else{perror("create shared memory failed\n");        exit(1);}}else{is_noexist=1;}//attach the shared memory to the current processif((indexaddr=shmat(shmid_index,(int*)0,0))==(int*)-1){perror("attach shared memory error\n");exit(1);}if(is_noexist){indexaddr[0]=0;indexaddr[1]=0;}//create a semaphore set including 3 semaphoresif((semid=createsem(".",'t',3,0))==-1){if(errno==EEXIST){if((semid=opensem(".",'t'))==-1){exit(1);}}else{perror("semget error:");exit(1);}}else{union semun arg;        //seting value for mutex semaphore         arg.val=1;         if(semctl(semid,0,SETVAL,arg)==-1)         {perror("setting semaphore value failed\n");        return -1;            }          //set value for synchronous semaphore        arg.val=num;            //the num means that the producer can continue to produce num products          if(semctl(semid,1,SETVAL,arg)==-1)            {perror("setting semaphore value failed\n");         return -1;         }           //the last semaphore's value is default           //the default value '0' means that the consumer is not use any product now    }int goods=0;while(1){p(semid,2);sleep(1);p(semid,0);//consumer is consuming a productgoods=shmaddr[indexaddr[1]];printf("consumer:%d consumes a product[%d]:%d\n",getpid(),indexaddr[1],goods);indexaddr[1]=(indexaddr[1]+1)%num;v(semid,0);sleep(1);v(semid,1);}}