How to build and use DLLs on Windows
I’ve recently been involved with a couple of discussions about Windows’ dynamic linking. One was Joe Nelson in considering how to make libderp accessible on Windows, and the other was about w64devkit, my Mingw-w64 distribution. I use these techniques so infrequently that I need to figure it all out again each time I need it. Unfortunately there’s a whole lot of outdated and incorrect information online which gets in the way every time this happens. While it’s all fresh in my head, I will now document what I know works.
If all you care about is the GNU toolchain then DLLs are straightforward, working mostly like shared objects on other platforms. To illustrate, let’s build a “square” library with one “exported” function, square, that returns the square of its input (square.c):
Given a DLL, printing a list of the exported functions of a DLL is not so straightforward. For ELF shared objects there’s nm -D, but despite what the internet will tell you, this tool does not support DLLs. objdump will print the exports as part of the “private” headers (-p). A bit of awk can cut this down to just a list of exports. Since we’ll need this a few times, here’s a script, exports.sh, that composes objdump and awk into the tool I want:
