信号灯.txt

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  // semop any more.
		{
			arg.val=1;
			if(semctl(semid,0,SETVAL,arg)==-1) perror("semctl setval error");
		} 
	}
	else
	{perror("semget error, process exit");	exit();	}
}
else //semid>=0; do some initializing 	
{
	arg.val=1;
	if(semctl(semid,0,SETVAL,arg)==-1)
		perror("semctl setval error");
}
//get some information about the semaphore and the limit of semaphore in redhat8.0
	arg.buf=&sem_info;
	if(semctl(semid, 0, IPC_STAT, arg)==-1)
		perror("semctl IPC STAT");		
	printf("owner's uid is %d\n", 	arg.buf->sem_perm.uid);
	printf("owner's gid is %d\n", 	arg.buf->sem_perm.gid);
	printf("creater's uid is %d\n", 	arg.buf->sem_perm.cuid);
	printf("creater's gid is %d\n", 	arg.buf->sem_perm.cgid);

	arg.__buf=&sem_info2;
	if(semctl(semid,0,IPC_INFO,arg)==-1)
		perror("semctl IPC_INFO");
	printf("the number of entries in semaphore map is %d \n",	 		arg.__buf->semmap);
	printf("max number of semaphore identifiers is %d \n", 		 	arg.__buf->semmni);
	printf("mas number of semaphores in system is %d \n",		 		arg.__buf->semmns);
	printf("the number of undo structures system wide is %d \n",	 	arg.__buf->semmnu);
	printf("max number of semaphores per semid is %d \n",		 		arg.__buf->semmsl);
	printf("max number of ops per semop call is %d \n",		 		arg.__buf->semopm);
	printf("max number of undo entries per process is %d \n", 	 		arg.__buf->semume);
	printf("the sizeof of struct sem_undo is %d \n", 	  	 			arg.__buf->semusz);
	printf("the maximum semaphore value is %d \n", 					arg.__buf->semvmx);
	
//now ask for available resource:	
	askfor_res.sem_num=0;
	askfor_res.sem_op=-1;
	askfor_res.sem_flg=SEM_UNDO;		
		
		if(semop(semid,&askfor_res,1)==-1)//ask for resource
			perror("semop error");
	
	sleep(3); //do some handling on the sharing resource here, just sleep on it 3 seconds
	printf("now free the resource\n");	
	
//now free resource	
	free_res.sem_num=0;
	free_res.sem_op=1;
	free_res.sem_flg=SEM_UNDO;

	if(semop(semid,&free_res,1)==-1)//free the resource.
		if(errno==EIDRM)
			printf("the semaphore set was removed\n");
//you can comment out the codes below to compile a different version:			
	if(semctl(semid, 0, IPC_RMID)==-1)
		perror("semctl IPC_RMID");
	else printf("remove sem ok\n");
}
注：读者可以尝试一下注释掉初始化步骤，进程在运行时会出现何种情况（进程在申请资源时会睡眠），同时可以像程序结尾给出的注释那样，把该程序编译成两个不同版本。下面是本程序的运行结果（操作系统redhat8.0）：

owner's uid is 0
owner's gid is 0
creater's uid is 0
creater's gid is 0
the number of entries in semaphore map is 32000 
max number of semaphore identifiers is 128 
mas number of semaphores in system is 32000 
the number of undo structures system wide is 32000 
max number of semaphores per semid is 250 
max number of ops per semop call is 32 
max number of undo entries per process is 32 
the sizeof of struct sem_undo is 20 
the maximum semaphore value is 32767 
now free the resource
remove sem ok
Summary：信号灯与其它进程间通信方式有所不同，它主要用于进程间同步。通常所说的系统V信号灯实际上是一个信号灯的集合，可用于多种共享资源的进程间同步。每个信号灯都有一个值，可以用来表示当前该信号灯代表的共享资源可用（available）数量，如果一个进程要申请共享资源，那么就从信号灯值中减去要申请的数目，如果当前没有足够的可用资源，进程可以睡眠等待，也可以立即返回。当进程要申请多种共享资源时，linux可以保证操作的原子性，即要么申请到所有的共享资源，要么放弃所有资源，这样能够保证多个进程不会造成互锁。Linux对信号灯有各种各样的限制，程序中给出了输出结果。另外，如果读者想对信号灯作进一步的理解，建议阅读sem.h源代码，该文件不长，但给出了信号灯相关的重要数据结构。
附录1： struct sem_array如下：

/*系统中的每个信号灯集对应一个sem_array 结构 */
struct sem_array {
	struct kern_ipc_perm	sem_perm;		/* permissions .. see ipc.h */
	time_t			sem_otime;			/* last semop time */
	time_t			sem_ctime;			/* last change time */
	struct sem		*sem_base;			/* ptr to first semaphore in array */
	struct sem_queue	*sem_pending;		/* pending operations to be processed */
	struct sem_queue	**sem_pending_last; 	/* last pending operation */
	struct sem_undo		*undo;			/* undo requests on this array */
	unsigned long		sem_nsems;		/* no. of semaphores in array */
};
其中，sem_queue结构如下：

/* 系统中每个因为信号灯而睡眠的进程，都对应一个sem_queue结构*/
 struct sem_queue {
	struct sem_queue *	next;	 	/* next entry in the queue */
	struct sem_queue **	prev;	 	/* previous entry in the queue, *(q->prev) == q */
	struct task_struct*	sleeper; 	/* this process */
	struct sem_undo *	undo;	 	/* undo structure */
	int   pid;	 					/* process id of requesting process */
	int   status;	 				/* completion status of operation */
	struct sem_array *	sma;	 		/* semaphore array for operations */
	int	id;	 						/* internal sem id */
	struct sembuf *	sops;		 	/* array of pending operations */
	int	nsops;					 	/* number of operations */
	int	alter;	 					/* operation will alter semaphore */
};
附录2：union semun是系统调用semctl中的重要参数：

union semun {
	int val;					/* value for SETVAL */
	struct semid_ds *buf;		/* buffer for IPC_STAT & IPC_SET */
	unsigned short *array;		/* array for GETALL & SETALL */
	struct seminfo *__buf;		/* buffer for IPC_INFO */   //test!!
	void *__pad;
};
struct  seminfo {
	int semmap;
	int semmni;
	int semmns;
	int semmnu;
	int semmsl;
	int semopm;
	int semume;
	int semusz;
	int semvmx;
	int semaem;
};