Linking C libraries with DMD

[Mike Parker via Digitalmars-d-learn]

On Friday, 22 January 2016 at 02:39:33 UTC, jmh530 wrote:

>
> The LearningD book says that you should compile the libraries 
> with DMC on Windows, but I can't figure out how to generate a 
> shared library on DMC. I didn't get the implib error for what I 
> was working on before.
>
> I feel like getting stuff to work with Windows is always such a 
> hassle, but that's the only way I'll be able to use this stuff 
> at work.

Your confusion appears to be coming from a lack of understanding 
of what's going on under the hood. When working with a system 
language like D, it is imperative to understand what the compiler 
and linker are doing. The same issues you are having can arise 
when using C and C++, they are just less common as you tend to 
use the same compiler toolchain for both your executable and your 
libraries.

First of all, understand that DMD does not use just one linker on 
Windows. The default is OPTLINK, which only works with 32-bit 
object files (and by extension, library files, as they are just 
archives of objects) in the OMF format. When compiling with 
-m32mscoff or -m64, DMD uses the Microsoft linker, which deals 
with objects in the COFF format. This matters at *link time*, not 
at runtime. So it  *generally* (see below) doesn't matter which 
format your DLL is in, as it is loaded at runtime no matter how 
you compile.

Second, understand that when you choose to link with an import 
library rather than loading the DLL manually, then it is the 
format of the import library that's important. It needs to be in 
the OMF format if you are compiling with vanilla DMD and in the 
COFF format if not. OMF import libraries can be generated from 
COFF DLLS with implib. Import libraries generated by the MinGW 
toolchain are actually in the COFF format, but they are not 
always compatible with the Microsoft toolchain. You are likely 
going to have issues even when compiling with -m32mscoff or -m64. 
Your implib difficulties may actually be arising because the DLL 
was compiled with MinGW, despite it being in COFF.

Third, understand that passing -L to DMD tells it that the 
succeeding flag should be passed to the linker. On Windows, -L-L 
has no meaning, as neither OPTLINK nor the MS linker accept the 
-L switch. -L is used with GCC to specify the library path, so in 
the command line -L-L/path/to/libs, the first -L tells DMD that 
the next part is for the linker and the second -L tells the 
linker where to find libraries. Again, this is only for the GCC 
toolchain. For DMD on Windows, how you specify the library path 
depends on whether you are linking with OPTLINK or the MS linker. 
As for the libraries themselves, you don't need to don't actually 
need the -L flag on Windows. In fact, you can save yourself some 
trouble and just pass the full path to any libraries you need 
with no flags at all:

dmd myapp.d C:\path\to\libs\mylib.lib

As long as the library is in the appropriate format, this command 
line will do the right thing.

I strongly recommend that you compile your DLL and generate the 
import library with the Microsoft tools. Then you should be able 
to use the 32-bit version with -m32mscoff and the 64-bit version 
with -m64. This should /just work/.

Development on Windows is not any more difficult than on Linux. 
It's annoying, sure, but not difficult. You just need to make 
sure that all of the tools you are using are compatible.