apue_常用文件锁

最新推荐文章于 2022-08-29 14:30:53 发布

HuygensCheng

最新推荐文章于 2022-08-29 14:30:53 发布

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分类专栏： apue 学习记录文章标签： c语言

原文链接：www.apuebook.com

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这段代码展示了如何使用fcntl函数在Unix/Linux系统中进行文件锁定操作。lock_reg和lock_test函数分别用于设置锁（读写锁）和检查锁状态。read_lock、readw_lock、write_lock、writew_lock及un_lock为锁操作的快捷方式。lock_test函数返回0表示可以读取，返回进程ID表示有其他进程持有写锁。lockfile则确保文件被写锁定。

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int
lock_reg(int fd, int cmd, int type, off_t offset, int whence, off_t len)
{
	struct flock	lock;

	lock.l_type = type;		/* F_RDLCK, F_WRLCK, F_UNLCK */
	lock.l_start = offset;	/* byte offset, relative to l_whence */
	lock.l_whence = whence;	/* SEEK_SET, SEEK_CUR, SEEK_END */
	lock.l_len = len;		/* #bytes (0 means to EOF) */

	return(fcntl(fd, cmd, &lock));
}
pid_t
lock_test(int fd, int type, off_t offset, int whence, off_t len)
{
	struct flock	lock;

	lock.l_type = type;		/* F_RDLCK or F_WRLCK */
	lock.l_start = offset;	/* byte offset, relative to l_whence */
	lock.l_whence = whence;	/* SEEK_SET, SEEK_CUR, SEEK_END */
	lock.l_len = len;		/* #bytes (0 means to EOF) */

	if (fcntl(fd, F_GETLK, &lock) < 0)
		err_sys("fcntl error");

	if (lock.l_type == F_UNLCK)
		return(0);		/* false, region isn't locked by another proc */
	return(lock.l_pid);	/* true, return pid of lock owner */
}
#define	read_lock(fd, offset, whence, len) \
			lock_reg((fd), F_SETLK, F_RDLCK, (offset), (whence), (len))
#define	readw_lock(fd, offset, whence, len) \
			lock_reg((fd), F_SETLKW, F_RDLCK, (offset), (whence), (len))
#define	write_lock(fd, offset, whence, len) \
			lock_reg((fd), F_SETLK, F_WRLCK, (offset), (whence), (len))
#define	writew_lock(fd, offset, whence, len) \
			lock_reg((fd), F_SETLKW, F_WRLCK, (offset), (whence), (len))
#define	un_lock(fd, offset, whence, len) \
			lock_reg((fd), F_SETLK, F_UNLCK, (offset), (whence), (len))

#define	is_read_lockable(fd, offset, whence, len) \
			(lock_test((fd), F_RDLCK, (offset), (whence), (len)) == 0)
#define	is_write_lockable(fd, offset, whence, len) \
			(lock_test((fd), F_WRLCK, (offset), (whence), (len)) == 0)

int
lockfile(int fd)
{
	struct flock fl;

	fl.l_type = F_WRLCK;
	fl.l_start = 0;
	fl.l_whence = SEEK_SET;
	fl.l_len = 0;
	return(fcntl(fd, F_SETLK, &fl));
}
#define lockfile(fd) write_lock((fd),0,SEEK_SET,0)