Step four: the write system call returns, creating our fourth context switch.

步骤四：系统调用返回，导致了第4次上下文切换。

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Step three: the write system call causes a context switch from user mode to kernel mode.

send() 套接字调用引发了从用户模式到内核模式的上下文切换。

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Similarly, functions such as printf and fprintf invoke the write system call to do their work.

类似地，printf和fprintf这样的函数调用write系统调用完成其工作。

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