学步园

在操作系统课程里学习过进程间通信的方式有“管道”，“信号量”，“消息”，“共享内存”，“套接字”等。自己在学习共享内存过程中写了下面的测试小程序。目的是把用到的API列到这里，在需要详细了解的时候查看相关手册。

在cycle.cc程序中，count存在于共享内存中，循环对count进行累加操作，然后打印出来。

在control.cc程序中，把count的值置为0。在执行了control.cc程序后，可以看到cycle.cc重新从1开始打印count。

一般对共享内存需要互斥访问，所以我用信号量的方式实现了一个CriticalSection的类，用来实现cycle.cc和control.cc两个程序对共享内存中count变量的互斥访问。

cycle.cc程序几乎是个无限循环，如果中断它的运行，可能导致信号量，共享内存挂住。可以用ipcs命令来查看当前创建的IPC资源。可以用ipcrm命令来删除已创建的IPC资源。

//cycle.cc<br /> //this program print count++ repeatedly, where counter is in shared memory<br /> //the key of shared memory is from command arguments<br /> #include <unistd.h><br /> #include <stdio.h><br /> #include <stdlib.h><br /> #include <errno.h><br /> #include <sys/ipc.h><br /> #include <sys/shm.h><br /> #include "criticalsection.h"<br /> void errexit()<br /> {<br /> perror("err");<br /> exit(1);<br /> }<br /> void usageexit(char* exe)<br /> {<br /> printf("usage: %s <int_key>/n",exe);<br /> exit(1);<br /> }<br /> int main(int argc,char** argv)<br /> {<br /> int *pcounter=NULL;<br /> const int max_int=0x7FFFFFFF;<br /> int shmid=-1;<br /> int res;<br /> if(argc<2) usageexit(argv[0]);<br /> int key=atoi(argv[1]);<br /> res=shmget(key,sizeof (int),IPC_CREAT | 0666);<br /> if(-1== res) errexit();<br /> shmid=res;<br /> pcounter=(int*)shmat(shmid,NULL,0);<br /> if(pcounter==(int*)-1) errexit();<br /> CriticalSection critical(key);<br /> for(*pcounter=0;*pcounter<max_int; )<br /> {<br /> critical.Enter();<br /> (*pcounter)++;<br /> sleep(3);//3 seconds<br /> printf("count=%d/n",*pcounter);<br /> critical.Leave();<br /> }<br /> res=shmdt(pcounter);<br /> if(-1==res) errexit();<br /> res=shmctl(shmid,IPC_RMID,0);<br /> if(-1==res) errexit();<br /> return 0;<br /> }<br />

//control.cc<br /> //this program change the counter value in shared memory to 0,<br /> //so that cycle.cc will restart count from 0<br /> //the key of shared memory is from command arguments<br /> #include <stdio.h><br /> #include <stdlib.h><br /> #include <errno.h><br /> #include <sys/ipc.h><br /> #include <sys/shm.h><br /> #include "criticalsection.h"<br /> void errexit()<br /> {<br /> perror("err");<br /> exit(1);<br /> }<br /> void usageexit(char* exe)<br /> {<br /> printf("usage: %s <int_key>/n",exe);<br /> exit(1);<br /> }<br /> int main(int argc,char** argv)<br /> {<br /> int *pcounter=NULL;<br /> const int max_int=0x7FFFFFFF;<br /> int shmid=-1;<br /> int res;<br /> if(argc<2) usageexit(argv[0]);<br /> int key=atoi(argv[1]);<br /> res=shmget(key,sizeof (int),0);<br /> if(-1== res) errexit();<br /> shmid=res;<br /> pcounter=(int*)shmat(shmid,NULL,0);<br /> if(pcounter==(int*)-1) errexit();<br /> CriticalSection critical(key);<br /> //for(*pcounter=0;*pcounter<max_int; )<br /> //{<br /> critical.Enter();<br /> (*pcounter)=0;<br /> // printf("count=%d/n",*pcounter);<br /> critical.Leave();<br /> //}<br /> res=shmdt(pcounter);<br /> if(-1==res) errexit();<br /> // res=shmctl(shmid,IPC_RMID,0);//who create, who release<br /> // if(-1==res) errexit();<br /> return 0;<br /> }<br />

//criticalsection.h<br /> //implement a critical section class, using semaphore<br /> #include <stdio.h><br /> #include <sys/types.h><br /> #include <sys/ipc.h><br /> #include <sys/sem.h><br /> #include <stdlib.h><br /> union semun<br /> {<br /> int val; //<= value for SETVAL<br /> struct semid_ds *buf; //<= buffer for IPC_STAT & IPC_SET<br /> unsigned short int *array; //<= array for GETALL & SETALL<br /> struct seminfo *__buf; //<= buffer for IPC_INFO<br /> };<br /> class CriticalSection<br /> {<br /> private:<br /> int mSemid;<br /> bool mOwner;<br /> public:<br /> CriticalSection(int key)<br /> {<br /> mSemid=semget(key,1,IPC_CREAT|0600|IPC_EXCL);<br /> mOwner=true;<br /> if(-1==mSemid)<br /> {<br /> //already exist<br /> if (errno==EEXIST)<br /> {<br /> mSemid=semget(key,1,0);<br /> mOwner=false;<br /> }<br /> }<br /> if(-1==mSemid) throw 1;<br /> //new created, initialize it<br /> if(mOwner)<br /> {<br /> union semun semarg;<br /> semarg.val=1;<br /> int res=semctl(mSemid,0,SETVAL,semarg);<br /> if (-1==res) throw 2;<br /> }</p> <p> }//constructor<br /> ~CriticalSection()<br /> {<br /> if (mSemid==-1) return;<br /> if(!mOwner) return;//owner release the semaphore<br /> int res=semctl(mSemid,0,IPC_RMID);<br /> if(-1==res) throw 1;<br /> }<br /> void Enter()<br /> {<br /> struct sembuf buf;<br /> buf.sem_num=0;//the first sem in set<br /> buf.sem_op=-1;<br /> buf.sem_flg=0;<br /> if (-1==semop(mSemid,&buf,1)) throw 1;<br /> }<br /> void Leave()<br /> {<br /> struct sembuf buf;<br /> buf.sem_num=0;//the first sem in set<br /> buf.sem_op=1;<br /> buf.sem_flg=0;<br /> if (-1==semop(mSemid,&buf,1)) throw 1;<br /> }<br /> };<br />