实例9-14.c

Linux下C语言编程例子

#include <linux/sem.h>
#include <stdio.h>
#include <errno.h>
#define SEM_PATH "/UNIX/my_sem"
#define max_tries 3 
int semid;
main()
{
int flag1,flag2,key,i,init_ok,tmperrno;
struct semid_ds sem_info;
struct seminfo sem_info2;
union semun arg; 			
struct sembuf askfor_res, free_res;
flag1=IPC_CREAT|IPC_EXCL|00666;
flag2=IPC_CREAT|00666;
key=ftok(SEM_PATH,'a');
init_ok=0;
semid=semget(key,1,flag1);//create a semaphore set that only includes one semphore.
if(semid<0)
{
	tmperrno=errno;
	perror("semget");
if(tmperrno==EEXIST)
		{
		semid=semget(key,1,flag2);
/*flag2 只包含了IPC_CREAT标志, 参数nsems(这里为1)必须与原来的信号灯数目一致*/
		arg.buf=&sem_info;
		for(i=0; i<max_tries; i++)
		{
			if(semctl(semid, 0, IPC_STAT, arg)==-1)
			{	perror("semctl error"); i=max_tries;}
			else
			{ 
				if(arg.buf->sem_otime!=0){ i=max_tries;  init_ok=1;}
				else	 sleep(1);	
			}
		}
		if(!init_ok)
		{
			arg.val=1;
			if(semctl(semid,0,SETVAL,arg)==-1) perror("semctl setval error");
		} 
	}
	else
	{perror("semget error, process exit");	exit();	}
}
else /*semid>=0; */ 	
{
	arg.val=1;
	if(semctl(semid,0,SETVAL,arg)==-1)
		perror("semctl setval error");
}
	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);
	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);
	printf("now free the resource\n");	
		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");	
	if(semctl(semid, 0, IPC_RMID)==-1)
		perror("semctl IPC_RMID");
	else printf("remove sem ok\n");
}