Re: Fwd: Simple changes to select(2) and pipe(7) - example program

[Alejandro Colomar <alx.manpages@gmail.com>]

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-------- Forwarded Message --------
Subject: Simple changes to select(2) and pipe(7)
Date: Sun, 6 Nov 2022 19:53:35 +0100
From: J.H. vd Water <henri.van.de.water@xs4all.nl>
To: mtk.manpages@gmail.com
CC: alx.manpages@gmail.com

Hi,

I am following the instructions from here:

  - https://www.kernel.org/doc/man-pages/reporting_bugs.html

"Simple one-line or two-line errors ... can be reported by sending an email
  message to the maintainers ..."

man 2 select  ... DESCRIPTION reads:

"select() allows a program to monitor multiple file descriptors, waiting
  until one or more of the file descriptors become "ready" for some class of
  I/O operation (e.g., input possible). A file descriptor is considered
  ready if it is  possible to perform a corresponding I/O operation
  (e.g., read(2), or a sufficiently small write(2)) without blocking."

I suggest to add the following line:

"However, note that select(2) will block on the read end of a pipe/fifo, if
  the write end of the pipe/fifo has never been opened before, unlike read(2)
  (read(2) will always return with zero if the write end of the pipe/fifo is
  closed - see pipe(7) where the text starts with I/O on pipes and fifos).

---
man 7 pipe ... where the paragraph start with "I/O on pipes and FIFOs":

"If a process attempts to read from an empty pipe, then read(2) will block
  until data is available."

I suggest to change the above line as follows:

"If a process attempts to read from an empty pipe while the write end is open,
  then read(2) will block (in case the O_NONBLOCK openfile status flag has been
  disabled) until data is available; however, if the write end of the pipe is
  closed and the pipe is empty, then read(2) will return with zero."

Regards,
Henri

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