Linking D code into existing C programs

[David Nadlinger via Digitalmars-d]

On Tuesday, 27 September 2016 at 10:46:23 UTC, Johan Engelen 
wrote:
> On Monday, 26 September 2016 at 23:32:05 UTC, Walter Bright 
> wrote:
>> Linking C libraries and object code into D programs has always 
>> worked easily in D. The other way around, not so well.
>>
>> [snip]
>>
>> How much of an issue is this with D? Is it something we need 
>> to address?
>
> We've been toying with this in setting up LDC's build such that 
> it works on different platforms. […]
> We've now moved to using the system linker separately to do the 
> linking.

To clarify: What we are doing in LDC is of course not what was 
suggested in the initial post, i.e. linking D object files into C 
executables without any (explicit) druntime dependencies and 
expecting this to work.

The problem we were facing in particular is that many C/C++ 
libraries (such as LLVM) come with a list of linker flags to use 
in the form of pkg-config or a similar tool. These will usually 
be formatted for the default way of linking C on the system (i.e. 
through a gcc-compatible interface). Thus, you can't just prefix 
them with -L and forward them to the D compiler since that 
forwards them directly to the low-level linker (-Xlinker, …).

This is a problem which *every* D project that wants to 
seamlessly link against system C libraries on Linux needs to 
solve.

There are two obvious ways to improve the situation:
  - Like Johan suggested, add a D compiler flag that prints the 
default linker flags used to create D executables.
  - Add a D compiler flag to forward flags directly to the linker 
driver used, without using -Xlinker.

Unfortunately, the LDC build system of course needs to work with 
existing host compilers anyway, so these wouldn't have helped us 
with shipping the first DDMD-based version. The most robust 
solution I could come up with was to extract the linker command 
line used from `$DMD -v some_test_executable.d` to then pass the 
same flags directly to the system gcc when building the D program.

  — David