That depends on the file system (and, possibly, the OS, but I think the OS should just forward the call to the relevant driver and return its result)

FreeBSD’s man page says (https://www.freebsd.org/cgi/man.cgi?sektion=2&query=read):

  [EISDIR] The file descriptor is associated with a directory
           residing on a file system that does not allow 
           regular read operations on directories (e.g. NFS)

NetBSD says something similar, adding ”The readdir() function should be used instead.”

I also am not convinced that, for example, the Linux man page (http://man7.org/linux/man-pages/man2/read.2.html) guarantees you get EISDIR when reading from a directory. It says that if you get EISDIR, fd refers to a directory, but isn’t explicit about the converse.

I certainly remember being surprised when I accidentally cat'd a directory on AIX and it worked. I can't remember exactly what you get back; I think it's a NUL-separated list of the files in it.

FreeBSD would allow this as of a few years ago (2007 at the earliest).

I know that SunOS 5.10 could read(2) a directory (that's what lore.cs.purdue.edu was running).

Imagine if unix was invented today and you would navigate a folder by reading it, and having a JSON encoded file of its content returned. Slow but so elegant and pretty.