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.

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