On 6/26/23 00:07, наб wrote:
Which don't behave like you may expect them to. 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 | 10 +++++++++- 1 file changed, 9 insertions(+), 1 deletion(-) diff --git a/man7/pipe.7 b/man7/pipe.7 index c3e06bdab..fb2ed61cd 100644 --- a/man7/pipe.7 +++ b/man7/pipe.7 @@ -56,12 +56,20 @@ .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 . +There are oddities if reading and no data is available: +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.
Hi наб! I'm not convinced that this is an oddity. If there are no writers, a read at the end of a file must notify the EOF, which is done by an empty read(2); in this case, it's the end of the pipe(7). If there are writers, the pipe may get more text, and so the reader gets EAGAIN or blocks, depending on O_NONBLOCK. Are you sure we need this patch? Cheers, Alex
.PP The communication channel provided by a pipe is a .IR "byte stream" :
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