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 product
		goods=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 product
		goods=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 buffer
	if((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 process
	if((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 index
	if((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 process
	if((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 semaphores
	if((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 1024	

union semun
{
	int val;
	struct semid_ds *buf;
	unsigned short *array;
};

//create a semaphore set
int 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 semaphore
	arg.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 set
int 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 set
	if((semid=semget(key,0,IPC_CREAT|0666))==-1)
	{
		perror("semget error:");
		return -1;
	}

	return (semid);
}

//P operation
int 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 opeation
int 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 set
int deletesem(int semid)
{
	return (semctl(semid,0,IPC_RMID,0)==-1);
}

//waiting for the semaphore is equal to 1
int 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 memory
int 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 memory
int 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 buffer
	if((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 process
	if((shmaddr=shmat(shmid_goods,(char*)0,0))==(char*)-1)
	{
		perror("attach shared memory error\n");
		exit(1);
	}

	//create a shared memory as index
	if((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 process
	if((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 semaphores
	if((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 product
		goods=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 buffer
	if((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 process
	if((shmaddr=shmat(shmid_goods,(char*)0,0))==(char*)-1)
	{
		perror("attach shared memory error\n");
		exit(1);
	}

	//create a shared memory as index
	if((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 process
	if((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 semaphores
	if((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 product
		goods=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);
	}

}


 

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