Understanding the undefine flags passed to gcc when linking

Simon Marlow simonmarhaskell at gmail.com
Thu Mar 13 16:47:07 EDT 2008

Bruce, Joseph R (Joe) wrote:
> Thanks Simon.  That is the best answer I've received to this question in
> many askings.
> I will try to go the shared library route.

Shared libraries are currently experimental - you'll probably experience 
problems, and parts of the infrastructure are still in flux.  I wouldn't 
recommend relying on shared libraries just yet.

> A question about you last
> comment: when you say link RTS and base, you mean extract the object
> files and package in a single .a?  Am I going to run into namespace
> collisions?

Yes, put the whole of the RTS and base into a single .a.  You shouldn't 
run into namespace collisions, as the symbol namespace is global anyway.


> Joe
>> -----Original Message-----
>> From: Simon Marlow [mailto:simonmarhaskell at gmail.com] 
>> Sent: Wednesday, March 12, 2008 3:18 PM
>> To: Bruce, Joseph R (Joe)
>> Cc: glasgow-haskell-users at haskell.org
>> Subject: Re: Understanding the undefine flags passed to gcc 
>> when linking
>> Bruce, Joseph R (Joe) wrote:
>>> When I run 'ghc -v ...', the linking is done via gcc->ld 
>> with a large 
>>> list of -u flags passed in.  I'm hoping to find a way to link my 
>>> object files without those flags, but first I need to 
>> understand what 
>>> they are doing.  Can someone explain it to me?  Or point me to 
>>> documentation that explains it?
>>> My ultimate goal is to build a static Haskell library and make it 
>>> available to c programmers ignorant of Haskell.  The undefines 
>>> described above are a major obstacle.  Does anyone know a 
>> way around them?
>> The -u flags arise because there is a circular dependency 
>> between the RTS (libHSrts.a) and the base package 
>> (libHSbase.a).  The RTS refers to a few datatypes and 
>> functions in the base package, for instance the constructors 
>> for True and False, I#, C# and so on, and some exceptions 
>> that the RTS automatically raises for certain events (e.g. 
>> stack overflow, non-termination).
>> One way to avoid needing the -u flags would be to link the 
>> base package twice, but that would slow down linking, and 
>> twice might not be enough. 
>>   With shared libraries there's no problem, because we always 
>> just link the whole library, this is only an issue with 
>> static libraries.
>> So for building a static Haskell library, can't you just link 
>> the RTS and base package together in a single .a file?
>> Cheers,
>> 	Simon

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