Question

Off late I’d been hearing that applications written in different languages can call each other’s functions/subroutines. Now, till recently I felt that was very natural – since all, yes all – that’s what I thought then, silly me! – languages are compiled into machine code and that should be same for all the languages. Only some time back did I realise that even languages compiled in ‘higher machine code’ – IL, byte code etc. can interact with each other, the applications actually. I tried to find the answer a lot of times, but failed – no answer satisfied me – either they assumed I knew a lot about compilers, or something that I totally didn’t agree with, and other stuff…Please explain in an easy to understand way how this works out. Especially how languages compiled into ‘pure’ machine code have different something called ‘calling conventions’ is what is making me clutch my hair.

Answer 1

This is actually a very broad topic. Languages compiled to machine code can often call each others’ routines, though usually not without effort; e.g., C++ code can call C routines when properly declared:

// declare the C function foo so it can be called by C++ code
extern "C" {
    void foo(int, char *);
}

This is about as simple as it gets, because C++ was explicitly designed for compatibility with C (it includes support for calling C++ routines from C as well).

Calling conventions indeed complicate the picture in that C routines compiled by one compiler might not be callable from C compiled by another compiler, unless they share a common calling convention. For example, one compiler might compile

foo(i, j);

to (pseudo-assembly)

PUSH the value of i on the stack
PUSH the value of j on the stack
JUMP into foo

while another might push the values of i and j in reverse order, or place them in registers. If foo was compiled by a compiler following another convention, it might try to fetch its arguments off the stack in the wrong order, leading to unpredictable behavior (consider yourself lucky if it crashes immediately).

Some compilers support various calling conventions for this purpose. The Wikipedia article introduces calling conventions; for more details, consult your compiler’s documentation.

Finally, mixing bytecode-compiled or interpreted languages and lower-level ones in the same address space is still more complicated. High-level language implementations commonly come with their own set of conventions to extend them with lower-level (C or C++) code. E.g., Java has JNI and JNA.