h是声明errno所需要的，errno是全局系统调用错误号变量(下面会有更多介绍)。
Errno.h is needed for the declaration of errno, the global system call error number variable (more on that below).
您会记起在非 devfs 系统上， register_blkdev()和 register_chrdev() 调用（连同提供的主设备号）正是用于这一目的。
You'll recall that on a non-devfs system, the register_blkdev() and register_chrdev() calls (along with supplied major numbers) are used for this purpose.
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