[Caml-list] Delimcc.0 OPAM Package Difficulty on Mac OS X

[oleg@okmij.org]

Perhaps the following comment from delimcc:stacks.c will help:

/* This function is defined in stacks.h and implemented in the ocamlrun.
   We re-define it here with the weak attribute: although 
   the function is implemented by ocamlrun and so will be available
   at run-time, it is not exported by ocamlc. Unfortunately, ocamlc, 
   when linking the bytecode executable and checking dlls requires 
   that all symbols imported by dlls be satisfied at link time.
*/
void caml_realloc_stack (asize_t required_size)
  __attribute__ ((weak));


To elaborate: caml_realloc_stack is part of OCaml run-time, that is,
byte code interpreter. On my FreeBSD system:

nm -D /usr/local/bin/ocamlrun 
...
0000000000409ae0 T caml_realloc_stack
...

So, caml_realloc_stack exists and it is global (that is, exported and
available for linking). 

So, when testd0 is invoked, the bytecode interpreter ocamlrun starts,
loads eventually dlldelimcc.so. The latter needs caml_realloc_stack,
and it can find it in ocamlrun. Everything should work...

You might have noticed the strange thing that the error is reported
before testd0 gets to run -- by the compiler itself. This is because
ocamlc is a little bit too helpful. When it links the bytecode
executable, it loads all referenced dll -- at link time -- and checks
that all needed symbols _will_ be satisfied at run-time. (More
precisely, it does ldopen with the flag RTLD_NOW). Normally it
is not ordinary linker's job to simulate run-time linking. After all, the
run-time environment may be different from link-time
environment. And in our case, it is different indeed. If your ocamlc
is natively compiled, it does not use ocamlrun. And natively compiled
ocamlc does _not_ export caml_realloc_stack. So, the premature, too
helpful check really goes wrong here: caml_realloc_stack will be found
at run-time but cannot be found at link time (where it is not really
needed). 

The weak subterfuge was sufficient to keep ocamlc happy. 

nm -D dlldelimcc.so 
                 w caml_realloc_stack

The weak reference should never cause an error (even if it remains
unresolved) -- according to my man pages.  Apparently MacOS thinks
different. Or findlib may set some flag that cause dlopen
to behave differently. I don't have access to MacOS. I guess I would
first check that dlldelimcc.so  does indeed refers to
caml_realloc_stack as a weak symbol, and then ask around as to why it
causes the problems on MacOS specifically.

Of course I'd be happier if ocamlc didn't use the RTLD_NOW flag.