Made of Bugs

Implementing a Declarative Mini-language in the C Preprocessor

Last time, I announced Check Plus, a declarative language for defining Check tests in C. This time, I want to talk about the tricks I used to implement a declarative minilanguage using the C preprocessor (and some GCC extensions).

The Problem

We want to write some toplevel declarations that look like:

#define SUITE_NAME example
BEGIN_SUITE("Example test suite");

#define TEST_CASE core
BEGIN_TEST_CASE("Core tests");
…

and so on, and somehow translate them into code that does the equivalent of:

Suite *s = suite_create ("Example test suite");
TCase *tc_core = tcase_create ("Core tests");
…

First steps

Instead of having our macros somehow lay down code directly, we'll use them to define some global data structures, which we will then loop across at runtime, and make the appropriate Check library calls. We'll define some structs that we're going to use for this purpose:

typedef void (*test_func)(int);
typedef void (*test_hook)(void);

struct test_case {
    test_func  *funcs, *end_funcs;
    const char *test_name;
    test_hook *setup_hooks, *end_setup_hooks;
    test_hook *teardown_hooks, *end_teardown_hooks;
};

struct test_suite {
    struct test_case *cases, *end_cases;
    const char *suite_name;
};

Since tests and test fixtures are code, we're going to need to keep function pointers to them. We use typedefs to keep thos readable, and then we define structs to represent a test suite, and a test case. We're going to represent arrays (of test cases, fixture hooks, or test functions) using a pair of pointers, to the start and end of the array. There's a specific reason for this representation, which I'll get to shortly.

ELF Sections

We can easily define global instances of these structs, but the problem is how to get the pointers to the child arrays correctly. These macros are going to be interspersed with other code, so we can't just use a literal array, like

struct test_suite s = {
 .suite_name = "Example test suite",
 .cases = {
   …
 }
};

because that "…" is going to have arbitrary code shoved in it.

The solution is to take advantage of a feature of the ELF object file format, and some GCC extensions that let us take advantage of it.

ELF object files can contain an arbitary set of named sections, each of which can contain code or data (or both, but that's not typical). GCC allows us to specify that a given variable should live in a given section using __attribute__((section("section_name")). So, by placing all our struct test_cases for a given test_suite into a separate section, we can cause them to get placed contiguously, which means we just need to get pointers to the start and end. That we can accomplish by declaring a zero-length array in the appropriate section, which (being of size zero) won't generate any data, but will have the appropriate address.

So, the code we're going to generate for the above will look something like:

struct test_case _begin_example_cases[];
struct test_case _end_example_cases[];
struct test_suite _suite_example = {
  .cases = _begin_example_cases,
  .end_cases = _end_example_cases,
  .suite_name = "Example test suite"
};
struct test_case _begin_example_cases[0]
  __attribute__((section(".data.example.cases")));

struct test_case _test_example_core
  __attribute__((section(".data.example.cases"))) = {
  .test_name = "Core tests",
  …
};

…

struct test_case _end_example_cases[0]
  __attribute__((section(".data.example.cases")));

Preprocessor tricks

So far, I haven't actually talked about the preprocessor at all. I've just shown the code we want our macros to generate. So now let's get to the business of actually generating this code. I'll walk through the definition of `BEGIN_SUITE', which expands to the code above. The other definitions are all essentially analagous.

My first version of this library didn't use the SUITE_NAME and TEST_CASE macros, instead requiring those arguments to be passed to every macro. I like the decreased repetition that this method gives, but we'll start with the explicit version, since it requires less trickery to understand.

From the above example, we can see that BEGIN_SUITE(example, "Example")' is given theexample' token, and needs to construct both symbols (_suite_example) and strings (".data.example.cases") containing that symbol. To do the first, we need to the processor operator ##, which concatenates two tokens into one. For example, given

#define PASTE(x,y) x##_##y

`PASTE(foo, bar)' is equivalent to just having written "foo_bar" literally in your code.

Secondly, to construct strings, we need the #x' preprocessor operator. This operator, when applied to an argument to a macro, returns a string literal containing the tokens passed to the macro. A common usage of this is for anassert` macro, that can print the failed expression:

#define assert(x) if (!(x)) { die("Assertion failed: %s\n", #x); }

And finally, we're going to need the implicit concatenation feature of the C language, which lets you type "foo" "bar" with the same meaning as "foobar".

Putting it all together, and we get:

#define BEGIN_SUITE(sym, name)                                          \
    struct test_case _begin_ ## sym ## _cases[];                        \
    struct test_case _end_ ## sym ## _cases[];                          \
    struct test_suite _suite_ ## sym = {                                \
        .cases       = (struct test_case*)_begin_ ## sym ## _cases,     \
        .end_cases   = (struct test_case*)_end_ ## sym ## _cases,       \
        .suite_name  = name,                                            \
    };                                                                  \
    struct test_case _begin_ ## sym ## _cases[0]                        \
    __attribute__((section(".data." #sym ".cases")))

Two last things to note: we deliberately leave off a trailing semicolon, requiring the user to provide one, and we need to end every line with a \, to include the entire body in the macro definition.

Another layer of indirection

The one final bit is to get rid of that explicit sym argument everywhere, in favor of the SUITE_NAME define. Our first try might look something like:

#define BEGIN_SUITE(name)                                               \
    struct test_case _begin_ ## SUITE_NAME ## _cases[];                 \
    …

You'll quickly realize, however, that since ## is applied at the same time as function-like macro expansion, this will always give you the symbol _begin_SUITE_NAME_cases. The solution is to force the preprocessor to perform multiple phases of macro expansion, to cause the ## to be applied only after SUITE_NAME is expanded. In this case, we require three layers of macros to get the order right:

#define BEGIN_SUITE(name) _BEGIN_SUITE(SUITE_NAME, name)
#define _BEGIN_SUITE(sym, name) __BEGIN_SUITE(sym, name)
#define __BEGIN_SUITE(sym, name)                                        \
    struct test_case _begin_ ## sym ## _cases[];                        \

This causes the following sequence of expansions for BEGIN_SUITE("Example");:

BEGIN_SUITE("Example");                     // (a)
_BEGIN_SUITE(SUITE_NAME, "Example");        // (b)
__BEGIN_SUITE(sym, "Example");              // (c)

The key here is that when expanding a function-like macro, the preprocessor performs the following steps, in order:

  1. Apply any # or ## operators.
  2. Macro-expand any arguments to the call
  3. Perform the substitution
  4. Macro-expand the result of the substitution

In going from (a) to (b), only step (3) applies. When going from (b) to (c), rule (2) means that SUITE_NAME gets expanded first, leaving (c). Rule (1) means that the _BEGIN_SUITE macro is necessary, since a call to __BEGIN_SUITE(SUITE_NAME, "Example") will get the # and ## applied to SUITE_NAME before it can be expanded.

Conclusions

So, that's basically all there is to the Check-Plus implementation. If you're comfortable with that, you can try your hand at unravelling the implementation of the Linux kernel's tracing macros, next (sample usage)!