Fd is the file descriptor of the device file.
fd是设备文件的文件描述符。
fd is the file descriptor, a pointer to the file. For this example, use the following
fd是文件描述符，是指向文件的一个指针。
If the optional fd parameter is specified, it is interpreted as an integral file descriptor to be sent over the stream.
如果指定了可选参数fd，它将被作为一个文件描述符通过流传送。
If the optional fd parameter is specified, it is interpreted as an integral file descriptor to be sent over the stream.
如果指定了可选参数fd，它将被作为一个文件描述符通过流传送。
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