When did directories stop being readable as files?

Question

I have the 1st edition of The Unix Programming Environment book. In chapter 2, the authors state that directories are readable as files, and mention some facts about the format of these files. They give some usage examples such as cat . (in exercise 2-2).

At least in Darwin, directories are no longer readable as files. At least, they appear to be zero-length files when read.

When did this change occur, and is there any official documentation about it?

Answer 1

The error number reference from the latest POSIX specification (POSIX.1-2008) states:

[EISDIR]

Is a directory. An attempt was made to open a directory with write mode specified.

This means that, on a POSIX compliant OS, you should be able to read() a directory if you opened it read only (O_RDONLY).

I've just tried this on a NetBSD (which really cares about POSIX) box, and works as expected, while it fails on GNU/Linux with EISDIR (which shouldn't happen).

A quick look at Linux shows this is intended (http://lxr.free-electrons.com/source/fs/libfs.c#L189):

ssize_t generic_read_dir(struct file *filp, char __user *buf, size_t siz, loff_t *ppos)
{
        return -EISDIR;
}

While a concrete filesystem implementation can override it (like CEPH does: http://lxr.free-electrons.com/source/fs/ceph/dir.c#L1142), the default behavior is to return EISDIR whenever someone is trying to read() a directory, even if it's open read only.

I've traced this change back to 2.0.x, and at least for the ext2 filesystem, this was still the case.

So, yes, on a POSIX compliant OS you should be able to read a directory, but some kernels (like Linux and, apparently, others) just ignore this condition and break the standard.