Path: news.eternal-september.org!eternal-september.org!.POSTED!not-for-mail
From: Lawrence D'Oliveiro <ldo@nz.invalid>
Newsgroups: comp.os.linux.misc
Subject: O_RDWR On Named Pipes
Date: Tue, 11 Mar 2025 01:48:01 -0000 (UTC)
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Typically when you open a file descriptor on a pipe, it’s either for 
reading or writing, but not both.

However, when you open a named pipe, it is possible to specify the mode 
O_RDWR; but does this work, or return an error? And if it doesn’t return 
an error, what exactly does it do?

I checked some relevant man pages <https://manpages.debian.org/fifo(7)>
<https://manpages.debian.org/pipe(7)>, but they completely avoid any 
mention of O_RDWR mode.

So I tried it. And it works without error. You get back a single file 
descriptor, that you can use for both writing to the pipe and reading from 
it. So if no other processes open the same named pipe, you can write 
something to it (I suppose until the kernel buffer fills up), and read the 
same thing back again.

Not very useful, on the face of it. But I guess it means you can (with 
appropriate coordination from both ends) switch the direction of data flow 
at any point, without having to close and reopen the named pipe. Basically 
O_RDWR gives you a half-duplex communication channel, where O_RDONLY and 
O_WRONLY will give you the ends of a simplex (unidirectional) channel.