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Atomicity of Pipe I/O

Reading or writing pipe data is atomic if the size of data written is less 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.

See section Limits on File System Capacity, for information about the PIPE_BUF parameter.

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