On 29/04/2009 23:48, Philip Hölzenspies wrote:
Dear GHC-ers,
From earlier questions, it seems no one wants to get down and dirty with linker tricks in ghc. I'm now looking over an alternative solution.
I'm currently trying to use SDL. There's a wrapper on hackage, which works fine under Linux, but it takes some tinkering under Mac OS X, because OS X needs instantiated Cocoa stuff for the SDL entry point. Long story short, to use SDL with ghc on the OS X, a little wrapper c-file needs to be written, because the linker will find a 'main' function in libSDLmain.a which expects a function 'SDL_main' as an entry point into the user code.
I have written a small c file that can be used to wrap around any haskell module (see below), based on the macro (cpp) definition of MODNAME. I can compile this c file with gcc, for, let's say a module Foo with entry point Foo.bar (so, if we could still have ghc make the entry point, we would say ghc -main-is Foo.bar):
gcc -c -DMODNAME=Foo -D"ENTRYPOINT=bar();" -o Foo_entry.o entry.c
However, this only works after I explicitly add ghc's "include" dir to my search path so that HsFFI.h can be found. Before I had this pre-processor trick, I could simply feed the c file to ghc, together with all my haskell files. Ghc then simply adds the correct includes and feeds the c file to the c compiler. Unfortunately, giving ghc the above -D options doesn't do much good.
The problem seems to lie with the suffix recognition ghc does. I took this from [1]. With the -cpp switch, the c pre-processor is used, but only when the file has a .hs suffix. The c compiler is only run when the file has a .c suffix. Using -x .hs puts the file correctly through the c pre-processor, but then fails to compile it, because it's not haskell code. Using -x .c doesn't do pre-processing (even with -cpp). I tried to compile this wrapper using this command (assume now that we have Foo.hsMain as an entry point):
ghc -cpp -pgmP cc -optP-E -DMODNAME=Foo -o Foo_entry.o entry.c
(Notice that you can't just use cpp, because on OS X that's a shell script that doesn't provide CPPs full strength, so you want "cc -E".) This doesn't work:
entry.c:27:2: error: #error MODNAME undefined!
Trying to be more explicit as to where the -D should go:
ghc -cpp -pgmP cc -optP-E -optP-DMODNAME=Foo -o Foo_entry.o entry.c
does exactly the same. Now faking .hs:
ghc -cpp -pgmP cc -optP-E -optP-DMODNAME=Foo -o Foo_entry.o -x hs entry.c
which produces the error (coming from stdio.h inclusion):
/usr/include/i386/_types.h:37:28: parse error on input `;'
So, I guess my question is: Is it possible to let a c input file of ghc pass through *both* the c pre-processor *and* the c compiler?
Maybe I'm missing something, but what's wrong with ghc -c -optc-DMODNAME=Foo -o Foo_entry.o entry.c Cheers, Simon