How do I use google mock in C?

I found a way to be able to mock bare C functions in google-mock.

The solution is to declare foobar to be a weak alias that maps to foobarImpl. In production code you do not implement foobar() and for unit tests you provide an implementation that calls a static mock object.

This solution is GCC specific but there are other compilers/linkers that provide weak aliasing.

  • rename the function void foobar(); to void foobarImpl();
  • add an attribute to the function foobar like: void foobar() __attribute__((weak, alias("foobarImpl") ));
  • if you want to have a non weak alias use a preproessor directive to remove the weak from the attributes.


#pragma once
void foobar();


// header.h
#pragma once

void foobar();    
void foobarImpl(); // real implementation


extern "C" {
#include "header.h"
// code.c
void foobarImpl() {
  /* do sth */
void foobar() __attribute__(( weak, alias ("foobarImpl") )); // declare foobar to be a weak alias of foobarImpl

This will tell the gnu linker to link calls of foobar() with foobarImpl() whenever there is no symbol called foobar()

then add the testing code

struct FooInterface {
   virtual ~FooInterface() {}
   virtual void invokeFoo() const { }

class MockFoo : public FooInterface {
  MOCK_CONST_METHOD0(invokeFoo, void());

struct RealFoo : public FooInterface {
   virtual ~RealFoo() {}
   virtual void invokeFoo() const { foobarImpl(); }

MockFoo mockFoo;
RealFoo realFoo;
void foobar() {

if this code is compiled and linked it will replace foobar with the mock call.
if you really want to call foobar() you can still do add a default invocation.

ON_CALL(mockFoo, invokeFoo())

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