[LLVMdev] Calling C-language variadic functions

[=?utf-8?Q?=C3=93scar_Fuentes?=]

Gregory Junker <gjunker at dayark.com> writes:

> Such as printf, etc., from IR created using the API (IRBuilder).
>
> Google hasn't provided much help, and I can't find anything relevant in 
> the docs (the docs talk about how to do varargs in LLVM ASM, but not how 
> to call an external vararg function that exists in a library that gets 
> linked to the LLVM module).
>
> Is there something special I need to do?

Declare the function with the right type:

FunctionType* FuncTy_7 = FunctionType::get(
  /*Result=*/IntegerType::get(mod->getContext(), 32),
  /*Params=*/FuncTy_7_args,
  /*isVarArg=*/true);

Function* func_foo = Function::Create(
  /*Type=*/FuncTy_7,
  /*Linkage=*/GlobalValue::ExternalLinkage,
  /*Name=*/"foo", mod); // (external, no body)
 func_foo->setCallingConv(CallingConv::C);

Note the `` /*isVarArg=*/true " part.

These are excerpts taken from the online LLVM demo, which is very useful
as a quick method for seeing how the C++ API is used (it doesn't use
IRBuilder, though).

> Simply calling 
> IRBuilder::CreateCall() isn't working for me. I see the call to the 
> variadic function in the asm dump, and also in the generated code, but 
> the code (x86_64) doesn't seem to have enough activity going on at the 
> call site (and I get a possibly predictable "illegal instruction" abort 
> when I try to run the code).

I guess that you are not using `verifyModule' before generating the
native code, because the sort of declaration/call mismatch you are
experiencing is detected there. Or you are calling the external vaarg
function with the same number of parameters listed on the faulty
declaration, in which case `verifyModule' has no reason for complaning.