How to implement google mock to mock the C fopen function - c

Question

I am writing a unit test for embedded C codes using google test/mock. The C function code looks like this.
int readGPIOSysfs(int ioport){
...
FILE *f = fopen("/sys/class/gpio/export", "w");
..
fclose(f);
}
How can I implement a google mock on fopen function call?
Thank you in advance.

Answer 1

I do it using CppuTest instead of Gtest :
//io.c -> imlementation
#include "drivers/io.h"
#include <string.h>
#include <stdio.h>
bool read_io()
{
FILE *fp_driver = fopen("/dev/io/my_device", "rw");
char msg[255] = {0};
if (fp_driver == NULL)
return false;
fread(msg, sizeof(char), 255, fp_driver);
if (strcmp(msg, "This is a test"))
return false;
return true;
}
//io.cpp -> Tests functions
#include "CppUTest/CommandLineTestRunner.h"
#include "CppUTest/TestHarness.h"
#include "CppUTestExt/MockSupport.h"
#include <stdio.h>
#include <string.h>
extern "C"
{
#include "drivers/io.h"
}
TEST_GROUP(IO)
{
void setup() {
}
void teardown() {
mock().clear();
}
};
// Mocking function
FILE *fopen (const char *__restrict __filename, const char *__restrict __modes)
{
void * r = mock().actualCall(__func__).returnPointerValue();
return (FILE *)r;
}
TEST(IO, simpleTest)
{
/* Create a temp file with a test string inside to match with
implementation function expectation */
FILE * tmp_log_file = tmpfile();
char str[] = "This is a test";
fwrite(str, sizeof(char), strlen(str), tmp_log_file);
rewind(tmp_log_file);
/* Return or temp file pointer for the next call of fopen */
mock().expectOneCall("fopen").andReturnValue(tmp_log_file);
bool r = read_io();
CHECK_TRUE(r);
mock().checkExpectations();
}
int main(int ac, char** av)
{
return CommandLineTestRunner::RunAllTests(ac, av);
}