Wrapping C   functions with GNU linker
//number.cpp
int giveMeANumber() {
  return rand() % 6 + 1;
}

//test.cpp
extern "C" int __wrap_rand(void) {
return 4;
}

void unitTest() {
  assert giveMeANumber() == 5;
}

$ g++ test.cpp -o test number.o -Xlinker --wrap=rand

//number.cpp
int foo() {
  //some complex calculations I would like to mock
}
int giveMeANumber() {
  return foo() % 6 + 1;
}

//test.cpp
extern "C" int __wrap_foo(void) {
return 4;
}

$ g++ test.cpp -o test number.o -Xlinker --wrap=foo

$ cat x.cc
#include <iostream>
using namespace std;

int giveMeANumber();

int main() {
    cerr << giveMeANumber() << endl;
    return 0;
}

$ cat y.cc
int giveMeANumber() {
    return 0;
}

extern "C" int __wrap__Z13giveMeANumberv() {
    return 10;
}

$ g++ -c x.cc y.cc && g++ x.o y.o -Wl,--wrap=_Z13giveMeANumberv && ./a.out
10