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### 15.4 Atomicity of Pipe I/O

Reading or writing pipe data is atomic if the size of data written is not greater than `PIPE_BUF`. This means that the data transfer seems to be an instantaneous unit, in that nothing else in the system can observe a state in which it is partially complete. Atomic I/O may not begin right away (it may need to wait for buffer space or for data), but once it does begin it finishes immediately.

Reading or writing a larger amount of data may not be atomic; for example, output data from other processes sharing the descriptor may be interspersed. Also, once `PIPE_BUF` characters have been written, further writes will block until some characters are read.

See Limits for Files, for information about the `PIPE_BUF` parameter.