LLVM how to map custom function call to printf - c++

Question

I just started with LLVM. I am using C++ to create a compiler for Pascal-like language. Given that print functions are system calls, how do I map from my language writeln(5) to the system function printf ("%d",5)? I thought that code below should do it.
NamedValues.clear();
vector<Type *> Doubles(1, Type::getInt32Ty(TheContext));
FunctionType * FT = FunctionType::get(Type::getInt32Ty(TheContext), Doubles, false);
Function * function = Function::Create(FT, Function::ExternalLinkage, "writeln", TheModule.get());
// create function body
BasicBlock * BB = BasicBlock::Create(TheContext, "entry", function);
Builder.SetInsertPoint(BB);
// get param
Function::arg_iterator AI = function->arg_begin();
Value * val = AI;
AI->setName("val");
// get format string
string format = "%d";
Constant * ConstStr = ConstantDataArray::getString(TheContext, format.c_str());
ConstStr = new GlobalVariable(*TheModule, ConstStr->getType(), true, GlobalValue::InternalLinkage, ConstStr, format);
Constant * Idxs[] = {ConstantInt::get(Type::getInt32Ty(TheContext), 0), 0};
Idxs[1] = Idxs[0];
// make params
vector<Type *> params;
params.push_back(PointerType::getUnqual(Type::getInt8Ty(TheContext)));
// get printf function
FunctionCallee PrintF = TheModule->getOrInsertFunction("printf", FunctionType::get(Type::getVoidTy(TheContext), params, true));
// call
/*
* CallInst *CreateCall(FunctionCallee Callee, ArrayRef<Value *> Args = None,
const Twine &Name = "", MDNode *FPMathTag = nullptr)
*/
Builder.CreateCall(PrintF.getFunctionType(), ConstantExpr::getGetElementPtr(ConstStr->getType(), ConstStr, Idxs), val);
Builder.CreateRet(Builder.getInt32(0));
main_loop();
Error I got:
Assertion failed: (Ty == cast<PointerType>(C->getType()->getScalarType())->getElementType()), function getGetElementPtr, file /Users/ustynov/Documents/Study/HOMEWORKS/semestralwork/llvm/lib/IR/Constants.cpp, line 2012.