Unit Testing in C

There are a number of unit test frameworks for C, but they all seem to lack something.

Sure, each can be used to perform automated testing, and some will even generate test stubs for you. But all of them are limited to API testing; that is, the only functions that can be unit-tested are the public API exported in the header files.

C differs from object-oriented languages in that most of the work -- the units to be tested, as it were -- is done in static functions that are not exported. This makes unit-testing, as it stands, all but useless from a C programmer's perspective; there is no compelling reason to use unit tests over the usual "test programs running on test data and returning 0 or 1 to the makefile" approach.

With some small work, however, it is possible to apply a unit test framework to a C project, and to perform actual unit tests (i.e. on static functions). It is a bit ugly, it is a bit invasive (naturally each .c file must contain its own unit tests), but it is scalable and maintainable.


Unit Testing with CHECK

check is a unit testing framework for C (manual). It is primarily designed to be used with the GNU autotools (autoconf, automake, etc), and the documentation is not clear on integrating check with a standard Makefile-based project. The unit tests developed here will serve as an example.

To begin, assume a project with the following directory structure:

Makefile

stuff.c

stuff.h

tests/
tests/Makefile

The main program, the shared library libstuff.so, has its API in stuff.h, its code in stuff.c, and is built by the Makefile. The unit tests run by check will be in the tests director.y

stuff.h

#ifndef STUFF_H

#define STUFF_H

double do_stuff( double i );

#endif

stuff.c

#include <math.h>
#include "stuff.h"

static double step1( double i ) { return i * i; }
static double step2( double i ) { return i + i; }
static double step3( double i ) { return pow( i, i ); }

double do_stuff( double i ) { return step3( step2( step1( i ) ) ); }

Makefile

# Library Makefile

# -------------------------------------------------------------------

NAME        =    stuff

LIBNAME     =    lib$(NAME).so

ARCHIVE     =    lib$(NAME).a

DEBUG       =    -DDEBUG -ggdb

OPT         =    -O2

ERR         =    -Wall

INC_PATH    =    -I.

LIB_PATH    =   

#----------------------------------------------------------------

CC          =     gcc

LD          =    ld

AR          =    ar rc

RANLIB      =    ranlib

RM          =    rm -f

LIBS        =    -lm

CC_FLAGS    =     $(INC_PATH) $(DEBUG) $(OPT) $(ERR) -fPIC

LD_FLAGS    =    $(LIB_PATH) $(LIBS) -shared -soname=$(LIBNAME)

           

SRC         =     stuff.c

OBJ         =     stuff.o

#---------------------------------------------------------- Targets

all: $(LIBNAME)

.PHONY: all clean check

$(ARCHIVE): $(OBJ)

    $(AR) $(ARCHIVE) $^

    $(RANLIB) $(ARCHIVE)

$(LIBNAME): $(ARCHIVE)

    $(LD) $(LD_FLAGS) --whole-archive $< --no-whole-archive -o $@

.c.o: $(SRC)

    $(CC) $(CC_FLAGS) -o $@ -c $<

clean:

    [ -f $(LIBNAME) ] && $(RM) $(LIBNAME)|| [ 1 ]

    [ -f $(ARCHIVE) ] && $(RM) $(ARCHIVE)|| [ 1 ]

    [ -f $(OBJ) ] && $(RM) $(OBJ) || [ 1 ]

    cd tests && make clean

check: $(LIBNAME)

    cd tests && make && make check

So far, this is all pretty straightforward stuff. The Makefile in the tests directory will contain all of the check-specific settings.

tests/Makefile

# Unit-test Makefile

#--------------------------------------------------------- Definitions

TGT_NAME    =    stuff

TGT_SRC     =    ../stuff.c

OPT         =    -O2 -fprofile-arcs -ftest-coverage

ERR         =    -Wall

INC_PATH    =    -I. -I..

LIB_PATH    =    -L..

LD_PATH     =     ..

#---------------------------------------------------------

CC          =     gcc

RM          =    rm -f

# NOTE: check libs must be enclosed by --whole-archive directives

CHECK_LIBS  =    -Wl,--whole-archive -lcheck -Wl,--no-whole-archive
LIBS        =    -lm $(CHECK_LIBS)  

# NOTE: UNIT_TEST enables the static-function test case in stuff.c

CC_FLAGS    =     $(INC_PATH) $(OPT) $(ERR) -DUNIT_TEST

# NOTE: check libs must be enclosed by --whole-archive directives

LD_FLAGS    =    $(LIB_PATH)

# Test Definitions (to be added later)

TESTS       = 

#---------------------------------------------------------- Targets

all: $(TESTS)

.PHONY: all clean check

clean:

    $(RM) $(TESTS) *.gcno *.gcda

check:

        @for t in $(TESTS); do                          \

                LD_LIBRARY_PATH='$(LD_PATH)' ./$$t;     \

        done

There are a few things to take note of here.

First, the compiler options profile-arcs and test-coverage cause check to perform test coverage profiling. See the the manual for details.

Next, libcheck.a (there is no .so) is added to the linker options, enclosed in --whole-archive directives so that the linker will not discard what it thinks are unused object files. This last point is important; when linking a static library into a dynamic library, --whole-archive must be used.

Finally, the preprocessor definition UNIT_TEST is added to the compiler options. This will be used to ensure that unit tests do not get built into a distribution version of libstuff.so.

Simple Unit Test

The first unit test will be a simple one that ensures the library links with no errors. This is a common problem when developing a shared library; unresolved symbols will not be reported until an executable is linked to the library.

tests/test_link.c

#include <stuff.h>

int main(void) { return (int) do_stuff( 32.0 ); }

Add a make target for this first test.

tests/Makefile

# Test 1 : Simple test to ensure that linking against the library succeeds

TEST1        =    test_link

TEST1_SRC    =    test_link.c

TEST1_FLAGS  =    $(CC_FLAGS) $(LD_FLAGS)
TEST1_LIBS   =    $(LIBS) -l$(TGT_NAME)

TESTS        =    $(TEST1)

# ...

$(TEST1): $(TEST1_SRC)

    $(CC) $(TEST1_FLAGS) -o $@ $^ $(TEST1_LIBS)

Note that the lines before # ... go in the definitions (top) part of the Makefile, while the lines after it go in the targets (bottom) part of it.

This first test can now be run:

bash# make check
gcc -I.  -DDEBUG -ggdb -O2  -Wall -fPIC -o stuff.o -c stuff.c
ar rc libstuff.a stuff.o
ranlib libstuff.a
ld  -lm  -shared -soname=libstuff.so --whole-archive libstuff.a --no-whole-archive -o libstuff.so
cd tests && make && make check
make[1]: Entering directory `stuff/tests'
gcc -I. -I.. -O2 -fprofile-arcs -ftest-coverage -Wall -DUNIT_TEST -L.. -Wl,--whole-archive -lstuff -lcheck -Wl,--no-whole-archive  -o test_link test_link.c
make[1]: Leaving directory `stuff/tests'
make[1]: Entering directory `stuff/tests'
make[1]: Leaving directory `stuff/tests'

Success! Note that the link test does not involve check; make will error out if the linking fails. Pedantic unit testers may want to take the route of making the test fail first (by invoking, say, do_nothing() in test_link.c) in order to convince themselves that it works.


Testing Static Functions

Testing a static function requires embedding the test code in the file that contains the function. The UNIT_TEST preprocessor directive will prevent the unit test from being compiled in non-unit-test binaries.

First, append the test code to the end of the library code.

stuff.c

/* ================================================================= */

/* UNIT TESTS */

#ifdef UNIT_TEST

#include <check.h>
#include <stdlib.h>    /* for rand() */

START_TEST (test_step1)
{
    double d = (double) rand();
    fail_unless( step1(d) == (d * d), "Step 1 does not square" );
}
END_TEST

START_TEST (test_step2)
{
    double d = (double) rand();
    fail_unless( step2(d) == (d + d), "Step 2 does not double" );
}
END_TEST

START_TEST (test_step3)
{
    double d = (double) rand();
    fail_unless( step3(d) == pow(d, d), "Step 3 does not exponentiate" );
}
END_TEST

TCase * create_static_testcase(void) {
    TCase * tc = tcase_create("Static Functions");
    tcase_add_test(tc, test_step1);
    tcase_add_test(tc, test_step2);
    tcase_add_test(tc, test_step3);
    return tc;
}

#endif

The START_TEST and END_TEST macros are provided by check, as are the tcase_ routines. The strategy here is to define a single exported function, create_static_testcase, which the unit-test binaries will link to.

The next step is to add a unit test suite to the test directory.

tests/test_stuff.c

#include <check.h>
#include <math.h>
#include <stdlib.h>

#include <stuff.h>

#define SUITE_NAME "Stuff"

/* ----------------------------------------------------------------- */
/* TESTS */

START_TEST (test_stuff_diff)
{
    double d = (double) rand();
    fail_unless ( do_stuff(d) != d, "do_stuff doesn't!" );
}
END_TEST

START_TEST (test_stuff_rand)
{
    double d = (double) rand();
    double dd = d * d;
    double sumdd = dd + dd;
    fail_unless ( do_stuff(d) == pow(sumdd, sumdd), "Incorrect result" );
}
END_TEST

/* ----------------------------------------------------------------- */
/* SUITE */

extern TCase * create_static_testcase(void);

Suite * create_suite(void) {
    Suite *s = suite_create( SUITE_NAME );

    /* Create test cases against library API */
    TCase *tc_core = tcase_create ("Core");
    tcase_add_test(tc_core, test_stuff_diff);
    tcase_add_test(tc_core, test_stuff_rand);
    suite_add_tcase(s, tc_core);

    /* Create test cases against static functions */
    suite_add_tcase( s, create_static_testcase() );

    return s;
}

int main( void ) {
    int num_fail;
    Suite *s = create_suite();
    SRunner *sr = srunner_create(s);
    srunner_run_all (sr, CK_NORMAL);
    num_fail = srunner_ntests_failed (sr);
    srunner_free (sr);
    return (num_fail == 0) ? EXIT_SUCCESS : EXIT_FAILURE;
}

This file creates a test suite that contains two tests of the library API (defined in test_stuff.c) and three tests of the static functions (defined in stuff.c). The entire suite will be run when the unit-test binary is executed.

Finally, the new test must be added to tests/Makefile, as discussed earlier.

tests/Makefile

# Test 2 : Actual unit tests against source code

# NOTE: this cannot link against the library as it incorporates the source code

TEST2        =    test_$(TGT_NAME)

TEST2_SRC    =     $(TEST2).c \

                   $(TGT_SRC)

TEST2_FLAGS  =    $(CC_FLAGS) $(LD_FLAGS)

TEST2_LIBS   =    $(LIBS) 

TESTS        =    $(TEST1) $(TEST2)

# ...

$(TEST2): $(TEST2_SRC)

    $(CC) $(TEST2_FLAGS) -o $@ $^ $(TEST2_LIBS)

Now all of the tests will be run when the make target is invoked:

gcc -I.  -DDEBUG -ggdb -O2  -Wall -fPIC -o stuff.o -c stuff.c
ar rc libstuff.a stuff.o
ranlib libstuff.a
ld  -lm  -shared -soname=libstuff.so --whole-archive libstuff.a --no-whole-archive -o libstuff.so
cd tests && make && make check
make[1]: Entering directory `stuff/tests'
gcc -I. -I.. -O2 -fprofile-arcs -ftest-coverage -Wall -DUNIT_TEST -L.. -Wl,--whole-archive -lstuff -lcheck -Wl,--no-whole-archive  -o test_link test_link.c
gcc -I. -I.. -O2 -fprofile-arcs -ftest-coverage -Wall -DUNIT_TEST -L.. -Wl,--whole-archive -lstuff -lcheck -Wl,--no-whole-archive  -o test_stuff test_stuff.c ../stuff.c
make[1]: Leaving directory `stuff/tests'
make[1]: Entering directory `stuff/tests'
Running suite(s): Stuff
100%: Checks: 5, Failures: 0, Errors: 0
make[1]: Leaving directory `stuff/tests'