Function mocking in C?

Question

I'm writing a unit-test to check some API calls. I am using check to test. My module is build with CMake (idk if it matters).

My test calls a function (which I need to test) and this function makes a call to another binary.

Simplified version of it looks like this.

/* unitTest.c */

#include "libraryAPI.h"
void letsMakeACall(void)
{
   ck_assert_eq(foo("water"), 0);
}

-- Module I am working on---
/*libraryAPI.c*/
#include "legacyLib.h"

void foo(const char *drink )    
{

    if (checkDrink(drink)!=0)
    {
       return 1;
    }else
    {
       return 0;
    }
}


----LEGACY BINARY---
/*legacyLib.c*/

static const char* expected = "water";

void checkDrink(const char *drink)
{
    if(drink == expected)
     {
        /*There are also a dozen functions being called which depend on legacy module initialisation*/
        return 0;
     }else{
        return 1;
     }
}

I'd like to mock response from legacyLib, because otherwise it call dozens of functions and breaks. My initial idea was to add some ifdef conditions when tests are being run, but it is against guidelines. Because it is basically a call interception I don't know what it a best(or working) solution. What can I use to solve it?

Answer 1

I am also unsure how to solve this generally, I have posted a similar question, but in some cases you can do the following (presuming you are testing individual functions):

1. Include the .c file instead of the header .h, but after you "rename" your mocked function using a define directive:

   #define checkDrink checkDrink_mocked
   // preprocessor will now replace all occurrences of "checkDrink"
   // with "checkDrink_mocked"
   
   int checkDrink_mocked(const char *drink); 
   #include "legacyLib.c"
   #undef checkDrink
   

2. Implement the renamed function:

   int checkDrink_mocked(const char *drink)
   {
       return 15;  
   }