Question

#include<iostream>
#include<cstdlib>
#include<cstring>
#include<cstdio>
using namespace std;

class Book{
    public:
        int a;
        int b;
};

int main()
{
    Book b1;
    b1.a = 10;
    b1.b = 20;
    cout<< b1.a << " " <<b1.b;
}

when we compile the above code with

clang++ test.cc -o a.exe

and run a.exe works perfectly. But when we compile the same program with

clang++ test.cc -emit-llvm -S -o a.exe

and now when we run it, the program goes crazy by launching ntvdm.exe (can be seen in process explorer) and command prompt starts behaving weird.

Software stack:

clang version 2.9 (tags/RELEASE_29/final)
Target: i386-pc-mingw32
Thread model: posix

Answer 1

By adding ‘-emit-llvm -S’ you are not generating machine code, but LLVM bytecode. To run that, you need to use lli.

As ntvdm.exe is virtual machine for running real-mode DOS programs, it might mean that windows interprets executable in LLVM bytecode as 16-bit DOS program and tries to run it as one.