Directly compiling a D program with other libraries

[Mike Parker]

On Monday, 13 March 2023 at 05:05:27 UTC, Jeremy wrote:
> Hello, I am new to this forum and to D.
>
> I am trying to compile a basic D program with libraries 
> (`requests` which requires `cachetools` and `automem`) without 
> using dub. I have never used dub before, only a compiler.
>
> The folders containing the libraries are in the same folder as 
> main.d, the file I am trying to compile, and the command I am 
> using to compile is `ldc2 -I. main.d`.
>
> When I compile my program, I just get linker errors such as:
> ```
> /usr/lib/gcc/x86_64-pc-linux-gnu/12/../../../../x86_64-pc-linux-gnu/bin/ld: main.o: in function `_Dmain':
> main.d:(.text._Dmain+0x2f): undefined reference to 
> `_D8requests10getContentFNcAyaZSQBd7streams__T6BufferThZQk'
> ```
>
> Does anyone have any advice on how to solve my problem?

That's a linker error, meaning the missing symbol isn't available 
to link into the executable. You need to compile the source of 
all the libraries you use and make sure the resultant binaries 
are available for the linker to link into the executable.

The -I switch you've passed tells the compiler where to find 
imported modules. The compiler needs parse them to know which 
symbols are available for you to use when it's compiling your 
code. (The current working directory is the default anyway, so 
you don't need to pass `-I.` for that.)

By default, the compiler does not compile imported modules. If 
you add `-i` to the command line, then it will compile all of the 
modules you import (as long as they're in the `-I` path), 
excluding the DRuntime and Phobos modules. It will then also pass 
all of the compiled object files to the linker, so then your 
linker error should go away.

However, when you choose not to use dub, you need to also ensure 
that you are accounting for any special compiler flags the 
libraries you use may require (for example, specific `-version` 
values). If they're configured to compile as static or shared 
libraries, it may be easier just to store the source for each of 
them outside of your project's source tree, use dub to build each 
of them, and then pass the compiled libraries to the compiler 
when you build your program. In that case, you wouldn't use `-i`. 
Just make sure that `-I` is correctly configured in that case.