llvm dev - Aug 2009 - [LLVMdev] Having JIT resolve extern "C" functions declared in executible

2009/7/4 Albert Graef <Dr.Graef at t-online.de>:
> This is all I ever needed to interface to C functions using LLVM. It's
> really easy. Of course you still need a prototype of the external
> function (function definition without body) in your IR, but that's it.
Hi Albert,

I'm having a similar problem and I found I can't declare the function
and use it, most likely because my syntax is wrong.

I have the function extern'd and am creating only the signature on the
IR code (attached). The idea is that it creates something like this:

My variable is DoubleTy and the expected parameter is also, but I'm
getting this error:

Assertion `(0 == FTy->getNumParams() || FTy->getParamType(0)
=Actual->getType()) && "Calling a function with a bad
signature!"'

cheers,
--renato

Reclaim your digital rights, eliminate DRM, learn more at
http://www.defectivebydesign.org/what_is_drm
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2009/8/22 Bill Wendling <isanbard at gmail.com>:
> I think you might have to provide an empty list if your function
doesn't
> take parameters. Maybe using an irbuilder would help?
It does take one parameter. Here's the important bits:

// My Function
extern "C"
void print(double X) {
    printf("%f\n", X);
}

// Args type
std::vector<const Type*> args(1, Type::getDoubleTy(getGlobalContext()));
// return void, 1 arg double, no varargs
FT = FunctionType::get(Type::getVoidTy(getGlobalContext()), args, false);
// creating stub for print function
Function* printFunction = Function::Create(FT,
Function::ExternalLinkage, "print", module);

By doing this (and the variable declaration below), my IR is:

; ModuleID = 'example'

define void @main() {
entry:
  %var = alloca double                            ; <double*> [#uses=1]
  store double 1.000000e+01, double* %var
}

declare void @print(double)
; ================== END

The function call is, then:
// Number 10
Value* value = ConstantFP::get(getGlobalContext(), APFloat(10.0));
// Variable var is double
Value* var = builder.CreateAlloca(Type::getDoubleTy(getGlobalContext()),
0, "var");
// Assign it value 10
builder.CreateStore(value, var);

// Create a call to the function, passing a double var
builder.CreateCall(printFunction, var, "print");


If var is DoubleTy and the arg is expecting DoubleTy, why is the assert failing?

I've tried creating a vector with the var inside and create the
function as below, but got the same error:
builder.CreateCall(printFunction, vec.begin(), vec.end(), "print");


cheers,
--renato

Reclaim your digital rights, eliminate DRM, learn more at
http://www.defectivebydesign.org/what_is_drm