[LLVMdev] How to call a pointer that points to a C function (original) (raw)

David Blaikie dblaikie at gmail.com
Mon Oct 27 10:06:49 PDT 2014

Easy way to answer questions like this is to write the obvious c code and run it through ckang -emit-llvm to see what the IR looks like. On Oct 27, 2014 10:05 AM, "CK Tan" <cktanx at gmail.com> wrote:

Does anyone have a sample code on calling a c function thru pointer? Thanks.

> On Oct 27, 2014, at 1:37 AM, David Chisnall <David.Chisnall at cl.cam.ac.uk> wrote: > > There is at least one error here, possibly two. > >> On 27 Oct 2014, at 07:46, C K Tan <cktanx at gmail.com> wrote: >> >> I have a pointer to a function that I need to invoke without going >> through llvm::Module::getOrInsertFunction. This example does not work: >> >> static int add(int x, int y); >> llvm::Value *one, *two; >> >> llvm::Constant* addfn >> = llvm::ConstantInt::get(JB->getIntPtrTy(DataLayout), (intptrt)add); >> llvm::Type* args[] = { Int32Ty, Int32Ty }; >> llvm::FunctionType* ftype = llvm::FunctionType::get(Int32Ty, args); > > The type that you want is not the function type, but the function pointer type. ftype->getPointerTo() should give you the type that you need. > >> addfn = llvm::ConstantExpr::getPointerCast(addfn, ftype); > > I don't remember if this will just do casts between pointers or if it can also create inttoptrs, but you might need to be more explicit and ask for an inttoptr. > > David

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