Using .lib and .dll in D applications

[Mike Parker via Digitalmars-d-learn]

On Sunday, 19 June 2016 at 15:35:04 UTC, moe wrote:

> I am new to d and doing some small test apps at the moment to 
> learn d. Currently I must be missing something basic here. I 
> have installed dub as a project manager and I am trying to use 
> a .lib file in an app. However, I can not use a module from the 
> .lib file. I get the following error:
>
> "module barlib is in file 'barlib.d' which cannot be read"

This is a compile time error, not a link time error. Imports 
modules and linking to libraries are two different things.

>
> the dub.json for the app
> ------------------------
> {
> 	"name": "dfoo",
> 	"authors": ["moe"],
> 	"description": "A minimal D application.",
> 	"copyright": "Copyright © 2016, root",
> 	"license": "proprietary",
> 	"platforms": ["windows"],
> 	"versions": ["DesktopApp"],
> 	"targetType": "executable",
> 	"configurations": [
> 	{
> 		"name": "debug",
> 		"targetPath": "bin/debug",
> 		"buildOptions": ["debugMode", "debugInfo"]
> 	},
> 	{
> 		"name": "release",
> 		"targetPath": "bin/release",
> 		"buildOptions": ["releaseMode", "optimize", "inline"]
> 	}
> 	]
> }
>

Nowhere have you set up barlib as a dependency for your app, so 
DUB has no idea it's supposed to tell dmd that you need to link 
to the library or where to find the modules. So the easy thing to 
do is to add a dependency directive. Since your library isn't 
registered with the dub registry, you'll need to provide a path 
to the barlib dub project rather than a release tag for the 
library.

Assuming a directory tree like so:
-projects
--barlib
---dub.json
--dfoo
---dub.json

Then the path for dfoo's depencency is ../barlib. With a dub.sdl 
file, it would look like this:

dependency "barlib" path="../barlib"

You can find the equivalent JSON syntax somewhere at [1]. With 
this, dub will manage your import path and library dependency for 
you.

> I can successfully build the barlib.lib file but I can not use 
> it in the app. I thought that it should be possible to simply 
> import the module from the lib-file, but the app cannot find 
> the module. I would like to be able to build .lib and .dll 
> files and use them in other applications. Can someone tell me 
> what I am missing?

Compiling and linking are two distinct steps. The compiler passes 
any libraries you give it to the linker, but it does not do 
anything with them itself during the compile step. At that stage, 
it's only concerned with source modules. So you need to tell it: 
where to find the source modules (on the command line), which 
ones to import (with the import statement). It also needs to know 
which libraries to pass to the linker and where they can be found.

Give the directory tree above, when compiling dfoo manually, you 
might do this on Windows:

cd dfoo
dmd -I../barlib/source source/app.d ../barlib/bin/barlib.lib

On other systems:

dmd -I../barlib/source -L-L../barlib/bin -L-lbarlib source/app.d

The -I switch tells the compiler where it can find the modules 
you need to import. DRuntime and Phobos are configured in DMD's 
config files, so you don't need to every specify those. On the 
windows command line, you can pass the full path to the library 
directly and the compiler will do the right thing. This is easier 
than dealing with the different syntax the two supported Windows 
linkers use for command line switches. On Posix systems, it's 
better to use the -L switch, which tell the compiler the 
immediately following command should be passed to the system 
linker. Ex:

-L-L/some/path -> passes -L/some/path to the linker, which is the 
switch telling it where to look for libraries.
-L-lFoo (-L-l -- the second letter is a lowercase 'L') -> passes 
-lname to the linker, telling it which library to link with.

Given this, the linker will look for /some/path/libFoo.so or 
/some/path/libFoo.a, in addition to searching the normal search 
paths.

As you can see, it's easier to properly configure your dub.json 
dependecies so that DUB can handle all of this for you.

[1] http://code.dlang.org/package-format?lang=json