Josef “Jeff” Sipek

bool bitfield:1

This is the first of hopefully many posts related to interesting pieces of code I’ve stumbled across in the dovecot repository.

Back in 1999, C99 added the bool type. This is old news. The thing I’ve never seen before is what amounts to:

struct foo {
	bool	a:1;
	bool	b:1;
};

Sure, I’ve seen bitfields before—just never with booleans. Since this is C, the obvious thing happens here. The compiler packs the two bool bits into a single byte. In other words, sizeof(struct foo) is 1 (instead of 2 had we not used bitfields).

The compiler emits pretty compact code as well. For example, suppose we have this simple function:

void set(struct foo *x)
{
	x->b = true;
}

We compile it and disassemble:

$ gcc -c -O2 -Wall -m64 test.c
$ dis -F set test.o
disassembly for test.o

set()
    set:     80 0f 02           orb    $0x2,(%rdi)
    set+0x3: c3                 ret

Had we used non-bitfield booleans, the resulting code would be:

set()
    set:     c6 47 01 01        movb   $0x1,0x1(%rdi)
    set+0x4: c3                 ret

There’s not much of a difference in these simple examples, but in more complicated structures with many boolean flags the structure size difference may be significant.

Of course, the usual caveats about bitfields apply (e.g., the machine’s endian matters).

Working @ Dovecot

It’s been a hectic couple of weeks, and so this post is a bit delayed. Oh well.

A couple of months ago, I decided that it was time for me to move on work-wise. As a result, four weeks ago, I joined Dovecot Oy (a part of Open-Xchange).

As you may have guessed from the name of the company, I get to spend my time making the Dovecot email server code better, more featureful, and otherwise more excellent. It is certainly a significant but fun change—going from kernel hacking on a fairly unknown operating system to hacking on the world’s most popular IMAP server. Not a day goes by where I’m not surprised just how much functionality is in the Dovecot codebase, or when I get to consult an RFC related to some IMAP extension I didn’t even know existed.

So, with this said, you should expect to see some posts related to Dovecot, Dovecot code, and email in general.

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