Stub system function in google test

Question

I am trying to use Google Test to test C code but I am encounter some problem related to write stub for system functions like: fopen,fclose,fread,fwrite, memcpy,memset,stat,...I don't known how to stub them correctly to cover all branchs in function that need to be tested.

Example , I have a function, how to test it by stub fopen, fclose, fwrite, fread? Only Stub, not Mock.

#include <stdio.h>
#include <stdlib.h>

int main(){
    FILE *f;
    //initialize the arr1 with values
    int arr1[5]={1,2,3,4,5};
    int arr2[5];
    int i=0;

    //open the file for write operation
    if((f=fopen("includehelp.txt","w"))==NULL){
        //if the file does not exist print the string
        printf("Cannot open the file...");
        exit(1);
    }
    //write the values on the file
    if((fwrite(arr1,sizeof(int),5,f))!=5){
        printf("File write error....\n");
    }
    //close the file
    fclose(f);

    //open the file for read operation
    if((f=fopen("includehelp.txt","r"))==NULL){
        //if the file does not exist print the string
        printf("Cannot open the file...");
        exit(1);
    }
    //read the values from the file and store it into the array
    if((fread(arr2,sizeof(int),5,f))!=5){
        printf("File write error....\n");
    }
    fclose(f);

    printf("The array content is-\n");
    for(i=0;i<5;i++){
        printf("%d\n",arr2[i]);
    }

    return 0;
}

Rather than stubbing out fopen, consider an alternative approach: separate logic and file IO as much as possible. Write unit tests for your logic code, and write integration tests for your logic + IO code. This way allows you to avoid the added complexity that mocking carries. — Konrad Rudolph

rveerd rveerd · Accepted Answer · 2019-12-20T16:29:43

Your file() function in sample.c calls fopen(). Defining fopen as something else in a totally different file (compilation unit) is not going to change that.

You cannot simply mock a free function.

You can change the file() function to take a pointer to the fopen() function to use. In your tests you then provide a pointer to your mock function when calling the file() function. This is a form of dependency injection.

Another option is to use conditional compilation.

An example of using dependency injection:

// Typedef for our "fopen interface". Makes our code a bit more readable.
typedef FILE *(*fopen_type)(const char *, const char *);

FILE *file(fopen_type fopen_func)
{
    FILE *f = fopen_func("abc", "r"); // Call the provided "fopen" function.
    return f; // Let's return the opened file or `NULL`.
}

And then in your test code:

TEST(OPEN_FILE, OK)
{
    ASSERT_NE(NULL, file(&my_fopen));
}

If you use many system functions that you want to mock, you can also create a struct that contains pointers to all the relevant functions.

struct system_calls {
   fopen_type fopen;
   // Add more system calls here.
};

FILE *file(struct system_calls *p)
{
    FILE *f = p->fopen("abc", "r");
    return f;
}

The premise here is that if you want to test your code, you need to write testable code. Dependency injection is one way to achieve that.

Stub system function in google test

1 Answers