Andy Lutomirski [off-list ref] wrote:

quoted

Allow a buffer to be marked such that read() must return the entire buffer
in one go or return ENOBUFS.  Multiple buffers can be amalgamated into a
single read, but a short read will occur if the next "whole" buffer won't
fit.

This is useful for watch queue notifications to make sure we don't split a
notification across multiple reads, especially given that we need to
fabricate an overrun record under some circumstances - and that isn't in
the buffers.

Hmm.  I'm not totally in love with introducing a new error code like
this for read(), especially if it could affect the kind of pipe that
is bound to a file in a filesystem.  But maybe it's not a problem.

EMSGSIZE might be better?

David