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(libc.info)Pipe Atomicity

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Atomicity of Pipe I/O
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   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.

   Note: Limits for Files, for information about the `PIPE_BUF'
parameter.