A C unit testing library in 1 file. No dependencies, no dynamic allocation. ISC licensed.
Clone or download
Pull request Compare This branch is 87 commits behind silentbicycle:master.
Fetching latest commit…
Cannot retrieve the latest commit at this time.
Failed to load latest commit information.



A testing system for C, contained in 1 file.

Key Features

  • Small, Portable, Lightweight

    greatest doesn't depend on anything beyond ANSI C89, and the test scaffolding should build without warnings under -Wall -pedantic. It is under 800 LOC (SLOCCount), and does no dynamic allocation.

  • Easy To Set Up

    To use, just #include "greatest.h" in your project. There is very little boilerplate. Most features are optional.

  • Un-Opinionated

    When a command-line test runner is useful, greatest can provide one, but it can also run as part of other programs. It doesn't depend on a particular build system or other tooling, and should accommodate a variety of testing approaches. It actively avoids imposing architectural choices on code under test. While greatest was designed with C in mind, it attempts to be usable from C++.

  • Modular

    Tests can be run individually, or grouped into suites. Suites can share common setup, and can be in distinct compilation units.

  • Low Friction

    Specific tests or suites can be run by name, for focused and rapid iteration during development. greatest adds very little startup latency.

There are some compile-time options, and slightly nicer syntax for parametric testing (running tests with arguments) is available if compiled with -std=c99.

I wrote a blog post with more information.

theft, a related project, adds property-based testing.

Basic Usage

#include "greatest.h"

/* A test runs various assertions, then calls PASS(), FAIL(), or SKIP(). */
TEST x_should_equal_1(void) {
    int x = 1;
    ASSERT_EQ(1, x);                              /* default message */
    ASSERT_EQm("yikes, x doesn't equal 1", 1, x); /* custom message */
    /* printf expected and actual values as "%d" if they differ */
    ASSERT_EQ_FMT(1, x, "%d");

/* Suites can group multiple tests with common setup. */
SUITE(the_suite) {

/* Add definitions that need to be in the test runner's main file. */

int main(int argc, char **argv) {
    GREATEST_MAIN_BEGIN();      /* command-line arguments, initialization. */

    /* Individual tests can be run directly. */
    /* RUN_TEST(x_should_equal_1); */

    /* Tests can also be gathered into test suites. */

    GREATEST_MAIN_END();        /* display results */


$ make simple && ./simple
cc -g -Wall -Werror -pedantic    simple.c   -o simple

* Suite the_suite:
1 test - 1 passed, 0 failed, 0 skipped (5 ticks, 0.000 sec)

Total: 1 test (47 ticks, 0.000 sec), 3 assertions
Pass: 1, fail: 0, skip: 0.

Test cases should call assertions and then end in PASS(), SKIP(), FAIL(), or one of their message variants (e.g. SKIPm("TODO");). If there are any test failures, the test runner will return 1, otherwise it will return 0. (Skips do not cause a test runner to report failure.)

Tests and suites are just functions, so normal C scoping rules apply. For example, a test or suite named "main" will have a name collision.

(For more examples, look at example.c and example-suite.c.)

Available Assertions

Note that all assertions have a "message" form, which takes an additional first argument, a custom string to include in the test failure message. This form adds an 'm' suffix to the ASSERT name. For example, ASSERT_EQ(foo, bar); could also be used as ASSERT_EQm("theese should match", foo, bar). If the "message" form is not used, greatest will attempt to create a reasonable default message.


Assert that COND evaluates to a true value.


Assert that COND evaluates to a false value.


Assert that EXPECTED == ACTUAL. To compare with a custom equality test function, use ASSERT_EQUAL_T instead. To print the values if they differ, use ASSERT_EQ_FMT.


Assert that EXPECTED == ACTUAL. If they are not equal, print their values using FORMAT as the printf format string.

For example: ASSERT_EQ_FMT(123, result, "%d");


Assert that ACTUAL is within EXPECTED +/- TOLERANCE, once the values have been converted to a configurable floating point type (GREATEST_FLOAT).


Assert that the strings are equal (i.e., strcmp(EXPECTED, ACTUAL) == 0).


Assert that the first SIZE bytes of the strings are equal (i.e., strncmp(EXPECTED, ACTUAL, SIZE) == 0).


Assert that the first SIZE bytes of memory pointed to by EXPECTED and ACTUAL are equal. If the memory differs, print a hexdump and highlight the lines and individual bytes which do not match.


Assert that the enum value EXPECTED is equal to ACTUAL. If not, convert each enum value to a string using ENUM_STR_FUN before printing them.

ENUM_STR_FUN should have a type like:

const char *some_enum_str(enum some_enum x);


Assert that EXPECTED and ACTUAL are equal, using the greatest_equal_cb function pointed to by TYPE_INFO->equal to compare them. The function's UDATA argument can be used to pass in arbitrary user data (or NULL). If the values are not equal and the TYPE_INFO->print function is defined, it will be used to print an "Expected: X, Got: Y" message.


Assert that COND evaluates to a true value. If not, then use longjmp(3) to immediately return from the test case and any intermediate function calls. (If built with GREATEST_USE_LONGJMP set to 0, then all setjmp/longjmp-related functionality will be compiled out.)


Because of how PASS(), ASSERT(), FAIL(), etc. are implemented (returning a test result enum value), calls to functions that use them directly from test functions must be wrapped in CHECK_CALL:

TEST example_using_subfunctions(void) {

This is only necessary if the called function can cause test failures.

Command Line Options

Test runners build with the following command line options:

Usage: (test_runner) [-hlfv] [-s SUITE] [-t TEST]
  -h        print this Help
  -l        List suites and their tests, then exit
  -f        Stop runner after first failure
  -v        Verbose output
  -s SUITE  only run suite w/ name containing SUITE substring
  -t TEST   only run test w/ name containing TEST substring

If you want to run multiple test suites in parallel, look at parade.

These command line options are processed by GREATEST_MAIN_BEGIN();.


All the macros have unprefixed and prefixed forms. For example, SUITE is the same as GREATEST_SUITE.

Checkout the source for the entire list.

These aliases can be disabled by #define-ing GREATEST_USE_ABBREVS to 0.

Color Output

If you want color output (PASS in green, FAIL in red, etc.), you can pipe the output through the included greenest script in contrib/:

$ ./example -v | greenest

(Note that greenest depends on a Unix-like environment.)

greatest itself doesn't have built-in coloring to stay small and portable.