Sendfile(2)是一种避免缓冲区复制开销的技术，并且它直接将数据位从文件系统转移到网络。

Sendfile (2) is a technique that avoids buffer copy overhead, and it directly pushes bits from the file system onto the network.

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步骤二：数据从内核空间缓冲区复制到用户空间缓冲区，之后系统调用read返回，这导致了从内核空间向用户空间的上下文切换。

Step two: data is copied from the kernel buffer into the user buffer, and the read system call returns. The return from the call caused a context switch from kernel back to user mode.

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步骤二：write系统调用导致内核将数据从内核缓冲区复制到与socket相关联的内核缓冲区中。

Step two: the write system call causes the kernel to copy the data from the original kernel buffers into the kernel buffers associated with sockets.

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