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.
the section called “Limits on File System Capacity”, for information about the PIPE_BUF parameter.