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);
}
}