int* p = (int*)malloc(sizeof(int));
*p = 42;  // illegal, there isn't an int

#ifdef __cplusplus
extern "C" {
#endif

int* foo(void);

#ifdef __cplusplus
}
#endif

#include "foo.h"
#include <stdlib.h>

int* foo(void) {
    return malloc(sizeof(int));
}

#include "foo.h"
#include<cstdlib>

int main() {
    int* p = foo();
    *p = 42;
    std::free(p);
}