Re: [MirageOS-devel] Compiling gmp-xen with nixpkgs

[Anil Madhavapeddy <anil@xxxxxxxxxx>]

On 15 Jun 2015, at 17:48, Thomas Leonard <talex5@xxxxxxxxx> wrote:
> 
> On 15 June 2015 at 13:58, Tim Cuthbertson <tim@xxxxxxxxxxx> wrote:
>> Hi all,
>> 
>> After getting acquainted with some of the sample applications, I've
>> been trying to get the server component of a relatively small project
>> of mine[0] running under mirage.
>> 
>> After refactoring the server code to use the mirage interfaces rather
>> than Unix & friends directly, I've got it compiling to the xen
>> backend.
>> 
>> I've been using nix[1] to build my project, so while getting stuff
>> working I have been using opam instead to maximise my chance of it
>> working. But now that it does, I'd like to get it working under `nix`
>> as well. Most of it is just grunt work, as the mirage libraries &
>> dependencies seem to build without issue (yay!), but when it comes to
>> linking stub libraries I'm having some trouble.
>> 
>> As I understand it, some (most?) c stubs will need cross-compilation
>> for a xen (minios?) target before they can be linked into a xen
>> unikernel. You've made *-xen opam package variants for the libraries
>> needed for nocrypto - zarith, gmp, etc.
>> 
>> In trying to recreate these I feel like I'm doing just the same as the
>> opam packages - based on the opam repo I'm applying the exact same
>> patches, running the same build script (modulo $PKG_CONFIG_PATH), etc.
>> But I still can't get gmp to build. It gets to the linking stage, and
>> I get a screenful of undefined symbols, primarily "_ctype" and
>> "printk". I've uploaded the full log[2]. if that helps.
>> 
>> I can't really tell which side of the problem this is - should gmp not
>> be referencing these symbols (maybe I'm missing some compile flags)?
>> or should the symbols be found somewhere, and I'm failing to link in
>> some important library?
>> 
>> In either case it's hard to tell where to go from here, since I can't
>> see anything too suspicious in the logs, and I'm trying my best to do
>> the same thing as `opam` would. Any pointers?
> 
> Hi Tim,
> 
> I don't know much about the gmp build process, but if you have a good
> (opam) build and a bad (nix) one, I guess you can just compare the two
> environments. e.g. are the missing symbols present in the good build?
> Does the nix one build if you point PKG_CONFIG_PATH at the opam
> installation, etc?
> 
> Many of the errors are coming from libtests.a - should it even be
> building the tests for a cross-compile (I don't know)?
> 

One common reason this breaks under Nix (or similar non-traditional
paths) is that there is an rpath which is used by the linker to find
the library in a standard environment, but not when Nix is explicitly
managing paths.

It's easiest to compare the raw build commands and bisect them for
differences to track this down, as Thomas notes.

-anil

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