I have a module written in C. In which I have API defined as follow.
int ReadData(void* data, int data_size, char* filename, int offset)
When performing unit test in order to cover full coverage I need to hit at error condition for fseek and fread system call. Can somebody explain me How can I override fseek/fread during particular test case so instead of system call it will call mock fseek/fread for that test case ?
ReadAPI.c
#include<stdio.h>
#include<stdlib.h>
int readdata(void* data,int size, int offset, char* filename)
{
if(data == NULL || size == 0 || filename == NULL)
{
return -1;
}
FILE* fp = fopen(filename,"rb");
if(fp == NULL)
{
return -1;
}
if(fseek(fp,offset,SEEK_SET) != 0)
{
fclose(fp);
return -1;
}
fread(data,size,1,fp);
if(ferror(fp))
{
fclose(fp);
return -1;
}
fclose(fp);
return 1;
}
readapi.h
#include<stdio.h>
#include<stdlib.h>
int readdata(void* data,int size, int offset, char* filename)
Test.cpp
#include<gtest/gtest.h>
#include"readapi.h"
TEST(test, Test1fseek)
{
// When fseek called in readdata API call mock fseek to hit fseek fail
// fseek(){return -1;}
char data[10] = {0};
int status = readdata(data,sizeof(data),0,"test.txt");
EXPECT_EQ(status, -1);
}
TEST(test, Test2fread)
{
//When fread called in readdata API call mock fread to hit read fail
// fread(){return -1;}
char data[10] = {0};
int status = readdata(data,sizeof(data),0,"test.txt");
EXPECT_EQ(status, -1);
}
int main()
{
return RUN_ALL_TEST();
}
You can refer to mocking-free-functions in Gmock's documentation.
Unfortunately, using the recommended method means you will have to change your code, which may not work if your code need to strictly be a C code.
However, if you are willing to accept this, based on the documentation, you will have to create a wrapper around all the system functions that you are using and then mock that wrapper.
Also, don't forget that you will have to add EXPECT_CALL or ON_CALL for all functions that are expected to be called and return a non-default (i.e. 0) value. For example, in your first test, you should also provide EXPECT_CALL or ON_CALL for fopen.
Here is an example implementation:
// System wrapper interface class.
class MySystemWrapperInterface {
public:
virtual FILE* fopen(const char* filename, const char* mode) = 0;
virtual int fseek(FILE* stream, long int offset, int whence) = 0;
virtual size_t fread(void* ptr, size_t size, size_t nmemb, FILE* stream) = 0;
virtual int ferror(FILE* stream) = 0;
virtual int fclose(FILE* stream) = 0;
};
// System wrapper actual class used in production code.
class MySystemWrapperClass : public MySystemWrapperInterface {
public:
FILE* fopen(const char* filename, const char* mode) {
return ::fopen(filename, mode);
}
int fseek(FILE* stream, long int offset, int whence) {
return ::fseek(stream, offset, whence);
}
size_t fread(void* ptr, size_t size, size_t nmemb, FILE* stream) {
return ::fread(ptr, size, nmemb, stream);
}
int ferror(FILE* stream) { return ::ferror(stream); }
int fclose(FILE* stream) { return ::fclose(stream); }
};
// Mocked System wrapper used for testint.
class MySystemWrapperMockClass : public MySystemWrapperInterface {
public:
MOCK_METHOD(FILE*, fopen, (const char*, const char*), (override));
MOCK_METHOD(int, fseek, (FILE*, long int, int), (override));
MOCK_METHOD(size_t, fread, (void*, size_t, size_t, FILE*), (override));
MOCK_METHOD(int, ferror, (FILE*), (override));
MOCK_METHOD(int, fclose, (FILE*), (override));
};
// Wrapper class for your own readdata function.
class MyClass {
// The system wrapper passed by dependency injection through constructor.
MySystemWrapperInterface* system_wrapper_;
public:
// Initialize the system wrapper in constructor.
MyClass(MySystemWrapperInterface* system_wrapper)
: system_wrapper_(system_wrapper) {}
int readdata(void* data, int size, int offset, char* filename) {
if (data == NULL || size == 0 || filename == NULL) {
return -1;
}
FILE* fp = system_wrapper_->fopen(filename, "rb");
if (fp == NULL) {
return -1;
}
if (system_wrapper_->fseek(fp, offset, SEEK_SET) != 0) {
system_wrapper_->fclose(fp);
return -1;
}
system_wrapper_->fread(data, size, 1, fp);
if (system_wrapper_->ferror(fp)) {
system_wrapper_->fclose(fp);
return -1;
}
system_wrapper_->fclose(fp);
return 1;
}
};
TEST(test, Test1fseek) {
// Create the mock object and inject it into your class.
MySystemWrapperMockClass mock_system_wrapper;
MyClass my_object(&mock_system_wrapper);
// When fseek called in readdata API call mock fseek to hit fseek fail
// fseek(){return -1;}
// IMPORTANT: Don't forget to add EXPECT_CALL or ON_CALL for all functions
// that are expected to be called.
EXPECT_CALL(mock_system_wrapper, fopen)
.Times(1)
.WillOnce(Return(reinterpret_cast<FILE*>(0x1)));
EXPECT_CALL(mock_system_wrapper, fseek).Times(1).WillOnce(Return(1));
EXPECT_CALL(mock_system_wrapper, fclose).Times(1).WillOnce(Return(1));
char data[10] = {0};
int status = my_object.readdata(data, sizeof(data), 0, "test.txt");
EXPECT_EQ(status, -1);
}
TEST(test, Test2fread) {
// Create the mock object and inject it into your class.
MySystemWrapperMockClass mock_system_wrapper;
MyClass my_object(&mock_system_wrapper);
// When fread called in readdata API call mock fread to hit read fail
// fread(){return -1;}
// IMPORTANT: Don't forget to add EXPECT_CALL or ON_CALL for all functions
// that are expected to be called.
EXPECT_CALL(mock_system_wrapper, fopen)
.Times(1)
.WillOnce(Return(reinterpret_cast<FILE*>(0x1)));
EXPECT_CALL(mock_system_wrapper, fseek).Times(1).WillOnce(Return(0));
EXPECT_CALL(mock_system_wrapper, fread).Times(1).WillOnce(Return(-1));
EXPECT_CALL(mock_system_wrapper, ferror).Times(1).WillOnce(Return(-1));
EXPECT_CALL(mock_system_wrapper, fclose).Times(1).WillOnce(Return(1));
char data[10] = {0};
int status = my_object.readdata(data, sizeof(data), 0, "test.txt");
EXPECT_EQ(status, -1);
}
Live example: https://godbolt.org/z/qxf74fWGh