Calling C++ Application Code from C library?

Question

Folks, Assuming I have a c++ application / library running which implements say

/* Alloc API */
void* my_alloc(int size) {
    return malloc(sizeof(size));
}

This is not under "extern c".

I have a C dynamic library from which I need to call my_alloc, can I directly call that API?

Like,

int test_my_alloc (int size) {
    int *x;

    x = (int*)my_alloc(size);
    if (x == NULL) {
        return 0;
    } else {
        return 1;
    }
}

Answer 1

You need to create a stub, e.g.

Stub header:

// stub.h

#ifdef __cplusplus
extern "C" {
#endif

void* my_alloc_c(int size);

#ifdef __cplusplus
}
#endif

Stub implementation:

// stub.cpp

#include "stub.h"
#include "header_where_my_alloc_is_declared.h"

void* my_alloc_c(int size)
{
    return my_alloc(size);
}

Your C code (example):

// my_code.c

#include "stub.h"

int main()
{
    void * p = my_alloc_c(42);
    return 0;
}

Then compile your stub and link it with your C code:

g++ -Wall -c stub.cpp          # compile stub.cpp
gcc -Wall my_code.c stub.o     # compile your C code and link with stub.o