LLVM: "Export" class

Question

I'd like to call this code from my program using LLVM:

#include <string>
#include <iostream>
extern "C" void hello() {
        std::cout << "hello" << std::endl;
}

class Hello {
public:
  Hello() {
    std::cout <<"Hello::Hello()" << std::endl;
  };

  int hello() {
    std::cout<< "Hello::hello()" << std::endl;
    return 99;
  };
};

I compiled this code to llvm byte code using clang++ -emit-llvm -c -o hello.bc hello.cpp and then I want to call it from this program:

#include <llvm/ExecutionEngine/ExecutionEngine.h>
#include <llvm/ExecutionEngine/GenericValue.h>
#include <llvm/ExecutionEngine/JIT.h>
#include <llvm/LLVMContext.h>
#include <llvm/Module.h>
#include <llvm/Target/TargetSelect.h>
#include <llvm/Support/MemoryBuffer.h>
#include <llvm/Support/IRReader.h>

#include <string>
#include <iostream>
#include <vector>
using namespace std;
using namespace llvm;

void callFunction(string file, string function) {
  InitializeNativeTarget();
  LLVMContext context;
  string error;

  MemoryBuffer* buff = MemoryBuffer::getFile(file);
  assert(buff);
  Module* m = getLazyBitcodeModule(buff, context, &error);
  ExecutionEngine* engine = ExecutionEngine::create(m);    
  Function* func = m->getFunction(function);

  vector<GenericValue> args(0);    
  engine->runFunction(func, args);

  func = m->getFunction("Hello::Hello");
  engine->runFunction(func, args);
}

int main() {
  callFunction("hello.bc", "hello");
}

(compiled using g++ -g main.cpp 'llvm-config --cppflags --ldflags --libs core jit native bitreader')

I can call the hello() function without any problems. My question is: how can I create a new instance of the Hello class using LLVM? I'm getting a segmentation fault when I call Hello::Hello()

Thanks for any hints!!

Manuel

servn · Accepted Answer · 2011-05-28 02:08:56Z

3

Running clang++ -emit-llvm on the given source won't emit Hello::Hello, and m->getFunction("Hello::Hello") wouldn't find it even if it were emitted. I would guess it's crashing because func is null.

Trying to directly call functions which aren't extern "C" from the LLVM JIT is generally not recommended... I'd suggest writing a wrapper like the following, and compiling it with clang (or using the clang API, depending on what you're doing):

extern "C" Hello* Hello_construct() {
  return new Hello;
}

answered May 28, 2011 at 2:08

servn

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