Linking D code into existing C programs

[ZombineDev via Digitalmars-d]

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.
>
> Consider the following C program:
>
>   ---- main.c ----
>   extern int foo();
>   void main(int argc, char* argv[]) {
>     foo();
>   }
>   ---- foo.c ----
>   int foo() { return 7; }
>   ---------------
>
> Compile/link in the usual way:
>
>     gcc main.c -c
>     gcc foo.c -c
>     gcc main.o foo.o
>
> No problem.
> Now replace foo.c with foo.d:
>
>   ---- foo.d ----
>   extern (C) int foo() { return 7; }
>   ---------------
>
> Compile/link with:
>
>     gcc main.c -c
>     dmd foo.d -c
>     gcc main.o foo.o
>
> Produces:
>
>   bar.o:(.eh_frame+0x13): undefined reference to 
> `__dmd_personality_v0'
>   bar.o: In function `_D3bar7__arrayZ':
>   bar.d:(.text._D3bar7__arrayZ+0x21): undefined reference to 
> `_d_arraybounds'
>   bar.o: In function `_D3bar8__assertFiZv':
>   bar.d:(.text._D3bar8__assertFiZv+0x21): undefined reference 
> to `_d_assert'
>   bar.o: In function `_D3bar15__unittest_failFiZv':
>   bar.d:(.text._D3bar15__unittest_failFiZv+0x21): undefined 
> reference to `_d_unittest'
>   bar.o: In function `__d_dso_init':
>   bar.d:(.text.d_dso_init[.data.d_dso_rec]+0x28): undefined 
> reference to `_d_dso_registry'
>   collect2: error: ld returned 1 exit status
>
> How much of an issue is this with D? Is it something we need to 
> address?

Many users that want/need to use D without druntime, which 
implies nothrow @nogc, --boundscheck=off, no unittests (and 
probably a couple of other things) and we need to support these 
use cases. In other words,
the compiler should detect if a given module does not use any 
druntime features and in such cases it should not emit calls/ 
linktime references to druntime. I don't know if in general the 
compiler can tell if certain runtime features (e.g. exception 
handling support) are not used needed, so at first we may require 
the -betterC switch to supplied.