Linking C code with D...

[Era Scarecrow via Digitalmars-d-learn]

  While trying to make a benchmark for comparing results of my 
possible compression replacement, I'm finding it quite difficult 
to link the c code with the d code.

  So I've managed to compile the compress.c file using dmc, but 
when I link the obj file to reference it I don't seem to see it...

I've linked a prototype before trying to call it

What am I hopelessly doing wrong?
unittest {
     string haystack = "...";
     auto f1 = (){
         RawBuffer rb;
         compress(rb, haystack);
     };

     auto f2 = (){
         size_t plen;
         extern (C) char *id_compress(char *id, int idlen, size_t 
*plen);
         id_compress(cast(char*) haystack.ptr, haystack.length, 
&plen);
     };

     auto memoryhungry_compress = benchmark!(f1)(100_000);
     auto original_compress = benchmark!(f2)(100_000);
     ...
}

$ dmd compress_c.obj compress.d -g -O -unittest -main

  Error 42: Symbol Undefined 
__D8compress16__unittestL366_5FZ9__lambda2MFZ11id_compressUPaiPkZPa