(libc.info.gz) Pipe Atomicity

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

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