Which don't behave like you may expect them to; unprimed, I expected the natural extension of either: files (being a filesystem object), always returning 0 if no data, or sockets (being an IPC mechanism), always EAGAINing if no data. The pipe semantics make sense of course ‒ pipes can be modelled as sockets if there aren't writers, but files if there are; indeed, this makes sense as the writer continuously appending a sliding "window" over a file ‒ but they're unique amongst the UNIX file types, but arriving at that specific interaction table is non-obvious, especially to a user. Quoth Issue 8 Draft 3: 60746 When attempting to read from an empty pipe or FIFO: 60747 • If no process has the pipe open for writing, read( ) shall return 0 to indicate end-of-file. 60748 • If some process has the pipe open for writing and O_NONBLOCK is set, read( ) shall return 60749 −1 and set errno to [EAGAIN]. 60750 • If some process has the pipe open for writing and O_NONBLOCK is clear, read( ) shall 60751 block the calling thread until some data is written or the pipe is closed by all processes that 60752 had the pipe open for writing. Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@xxxxxxxxxxxxxxxxxx> --- man7/pipe.7 | 9 ++++++++- 1 file changed, 8 insertions(+), 1 deletion(-) diff --git a/man7/pipe.7 b/man7/pipe.7 index c3e06bdab..91554fa3c 100644 --- a/man7/pipe.7 +++ b/man7/pipe.7 @@ -56,12 +56,19 @@ .SS I/O on pipes and FIFOs .BR write (2) blocks until sufficient data has been read from the pipe to allow the write to complete. +.PP Nonblocking I/O is possible by using the .BR fcntl (2) .B F_SETFL operation to enable the .B O_NONBLOCK -open file status flag. +open file status flag or by opening a +.BR fifo (7) +with +.BR O_NONBLOCK . +If any process has the pipe open for writing, reads fail with +.BR EAGAIN ; +otherwise\[em]with no potential writers\[em]reads succeed and return empty. .PP The communication channel provided by a pipe is a .IR "byte stream" : -- 2.39.2
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