Testing in C

Recently I had the opportunity to help out a friend who was working on an assignment using C and found some nice patterns to do TDD-style development in a language that traditionally makes that a nightmare. So I thought I'd write down my thoughts in case I need to do anything similar in the future.

The first job is to create a test.c file which can be compiled into an executable and run.

The main() function consists of one function call, the call to our test runner. The test runner will go through all our tests, run them, and then print to the screen whether it was a success or not.

The general skeleton looks like this:

#define TRUE 1
#define FALSE 0

void run_tests() {
  test("constructor", test_new_list);
  test("push", test_push);
  test("pop", test_pop);
  test("len", test_len);
  test("destructor", test_destructor);

int main()
  return 0;

If you are familiar with C, then you'll probably know that something's a little odd here. The test() "function" is actually taking in the name of a test and then a reference to a function, however C doesn't actually allow referencing functions and passing them around (not easily anyway).

test() is actually a macro who's definition looks like this:

// Make a `test` macro which will automatically run our tests and print
// a tick if it was successful. Otherwise it prints a cross.
#define test(name, function_call) \
          printf("testing %s... ", name); \
          if(function_call()) { \
            printf("passed \u2714\n"); \
          } else { \
            printf("failed \u2718\n"); \

All the test() macro does is print "testing foo...", then it'll run the test and as long as its return value is true, it'll print "passed ✔". If it fails, you'll see "failed ✘".

When we run the test executable, this is the output you get:

testing constructor... passed ✔
testing push... passed ✔
testing pop... passed ✔
testing len... passed ✔
testing destructor... passed ✔

The key part here is how each test is constructed. Basically, you'll set up your inputs, run whatever function you are testing, then assert() that the outputs are what you expect. If the function returns TRUE then the test passed and if the function returns FALSE then it failed.

The assert() macro is a really simple macro that just expands to check if the expression passed into it is FALSE. If it is, then return FALSE, otherwise just continue like normal.

// Define an assert macro which will test an expression and return FALSE
// if it is not true.
#define assert(expr) \
if((expr) == FALSE) { \
  return FALSE; \

Here's an example where we are checking that a len() function for finding the length of a linked list works correctly.

int test_len() {
  Node \*head = new_list(0);

  assert(len(head) == 1);

  // Add a bunch of stuff to the list and make sure length changes
  // appropriately
  int values[] = {1, 2, 3, 4, 5};
  for (int i=0; i < 5; i++) {
    head = push(head, values[i]);
  assert(len(head) == 6);

  return TRUE;

Invoking all of this is actually pretty easy to do. What you'll do is pull out all of your functions and other bits and pieces into their own library, then when compiling test, you just link that in and you've got access to everything you need for testing.

Then just add the corresponding section to your Makefile and you're done.

Along the way, I found that I was often needing to repeat boring setup code, so I would pull this out into its own function. For example, here's a function which takes in an array and its length, and returns a linked list with all the array's elements added.

// Given an array of values, create a new list from them (note: list is
// reversed)
Node* make_list(int values[], int length) {
  Node \*head = new_list(values[0]);
  for (int i=1; i<length; i++) {
    head = push(head, values[i]);

  return head;

Little helper functions like these often make a world of difference when doing testing and TDD.