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This article will explain in detail what is the difference between select, poll and epoll. The content of the article is of high quality, so the editor will share it with you for reference. I hope you will have a certain understanding of the relevant knowledge after reading this article.

Fd refers to the file descriptor

Select

Time complexity O (n)

Indiscriminately poll all streams to find streams that can read or write data, and operate on them

Poll

Then query the device status corresponding to each fd

Time complexity O (n)

Poll is essentially no different from select.

It copies the array passed in by the user to the kernel space

But it has no limit on the maximum number of connections because it is stored based on linked lists.

Epoll

At this point, our operations on these streams are meaningful.

Time complexity O (1)

It can be understood as event poll.

Epoll is actually event-driven (each event is associated with a fd)

But select,poll,epoll is essentially synchronous iBandO

Because they all need to be responsible for reading and writing themselves after the read and write events are ready.

That is to say, the reading and writing process is blocked.

On the other hand, the implementation of asynchronous Iripple O is responsible for copying data from the kernel to user space without having to read and write on its own.

AIO is also based on epoll.

Copy data from the kernel to user space

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