fcntl_lock.c

《LINUX C编程实战》（作者：童永清

#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <string.h>

/*自定义的错误处理函数*/
void my_err(const char * err_string, int line)
{
	fprintf(stderr, "line:%d  ", line);
	perror(err_string);
	exit(1);
}

/*锁的设置或释放函数*/
int lock_set(int fd, struct flock * lock)
{
	if (fcntl(fd, F_SETLK, lock) == 0) { // 执行成功
		if (lock->l_type == F_RDLCK) {
			printf("set read lock,pid:%d\n", getpid());
		} else if (lock->l_type == F_WRLCK) {
			printf("set write lock,pid:%d\n", getpid());
		} else if (lock->l_type == F_UNLCK) {
			printf("release lock,pid:%d\n", getpid());
		}
	} else {						  // 执行失败，返回-1
		perror("lock operation fail\n");
		return -1;
	}

	return 0;
}

/*测试锁,只有当测试发现参数lock指定的锁能被设置时，返回0*/
int lock_test(int fd, struct flock * lock)
{
	if (fcntl(fd, F_GETLK, lock) == 0) {	// 执行成功
		if (lock->l_type == F_UNLCK) {	// 测试发现能按参数lock要求设置锁
			printf("lock can be set in fd\n");
			return 0;
		} else {	// 有不兼容的锁存在，打印出设置该锁的进程ID
			if (lock->l_type == F_RDLCK) {
				printf("can't set lock, read lock has been set by:%d\n", lock->l_pid);
			} else if (lock->l_type == F_WRLCK) {
				printf("can't set lock, write lock has been set by:%d\n", lock->l_pid);
			}
			return -2;
		}
	} else {								// 执行失败，返回-1
		perror("get incompatible locks fail");
		return -1;
	}
}

int main()
{
	int		fd;
	int		ret;
	struct flock	lock;
	char		read_buf[32];

	/*打开或创建新文件文件*/
	if ((fd = open("example_65", O_CREAT|O_TRUNC|O_RDWR, S_IRWXU)) == -1) {
		my_err("open", __LINE__);
	}
	if (write(fd, "test lock", 10) != 10) {
		my_err("write", __LINE__);
	}
	/*初始化lock结构*/
	memset(&lock, 0, sizeof (struct flock));
	lock.l_start = SEEK_SET;
	lock.l_whence = 0;
	lock.l_len = 0;
	
	/*设置读锁*/
	lock.l_type = F_RDLCK;
	if (lock_test(fd, &lock) == 0) { // 测试可以设置锁
		lock.l_type = F_RDLCK;
		lock_set(fd, &lock);
	}
	
	/*读数据*/
	lseek(fd, 0, SEEK_SET);
	if ((ret = read(fd, read_buf, 10)) < 0) {
		my_err("read", __LINE__);
	}
	read_buf[ret] = '\0';
	printf("%s\n", read_buf);

	/*等待任意按键*/
	getchar();

	/*设置写锁*/
	lock.l_type = F_WRLCK;
	if (lock_test(fd, &lock) == 0) { // 测试可以设置锁
		lock.l_type = F_WRLCK;
		lock_set(fd, &lock);
	}
		
	/*释放锁*/
	lock.l_type = F_UNLCK;
	lock_set(fd, &lock);
	close(fd);
	return 0;
}