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Snow is a header-only unit testing library for C. Just include the file snow/snow.h.

IRC channel: #snow on Freenode. If you have any questions, or just want to chat, just ping me (@mort) :)


Snow 2

Snow 2 is a complete rewrite of Snow. Here are the highlights:

  • Blocks have moved from inside of macro arguments (i.e describe(foo, { ... })) to outside of macro arguments (i.e describe(foo) { ... }). This applies to describe, subdesc, it/test, before_each, and after_each.
    • This means that it's possible to show line numbers, that compiler error messages are nicer, and syntax highlighters and auto indenters should be more happy.
  • asserteq and assertneq works slightly differently, but most code which worked before should continue to work.
  • All assertion macros have gotten an extra, optional argument, which is an explanation of what the assertion means. For example, you can now write asserteq(foo, bar, "Some explanation").
  • You can select what tests to run with glob-style matches, not just filter based on the name of the top-level describe.


Some miscellaneous points:

  • Snow uses some GNU extensions, so it might not work with all ISO C compatible compilers. It's confirmed to work with at least GCC and Clang. It should even work on GCC and Clang versions too old to support C11 (or even C99), but the convenience asserteq and assertneq macros require C11.
  • I really recommend running the test executable with valgrind. That will help you find memory issues such as memory leaks, out of bounds array reads/writes, etc.
  • Windows is supported through MinGW or cygwin, with the caveat that it assumes your terminal supports UTF-8. CMD.exe and Powershell will print mangled ✓ and ✕ characters. (Git Bash and Cygwin's terminal should be fine though)
    • Windows also generally doesn't have the <fnmatch.h> header. Snow defaults to compile without fnmatch under MinGW (and instead uses plain strcmp). You can control this with -DSNOW_USE_FNMATCH=1 or -DSNOW_USE_FNMATCH=0. Gnulib implements fnmatch, and supports Windows under Cygwin.


When creating the main function using the snow_main macro, your executable will take these arguments. The --no- prefixed arguments will disable the relevant function:

  • --version, -v: Show the current version and exit.
  • --help, -h: Show usage and exit.
  • --list, -l: List available tests and exit.
  • --color, -c, or --no-color: Enable the use of color. Default: on when output is a TTY, off otherwise.
  • --quiet, -q, or --no-quiet: Suppress most messages, only test faulures and the 'Total: Passed X/Y tests' line will still print. Default: off.
  • --log <file>: Output to a log file instead of stdout.
  • --timer, -t, or --no-timer: Print the number of miliseconds CPU time spent on each test alongside its success message. Default: on.
  • --maybes, -m, or --no-maybes: Print out messages when beginning a test rather than just when it completed. Default: on when output is a TTY, off otherwise.
  • --cr, or --no-cr: Print a \r after maybe messages instead of \n. This will override them with successes or failures as they are printed out. Default: on when output is a TTY, off otherwise.


Here's a simple example which tests a couple of filesystem functions, and has a subdescription for testing fread-related stuff.

  • Compile: gcc -Isnow -DSNOW_ENABLED -g -o test example.c
    • -Isnow: add snow to our include path, to make #include <snow/snow.h> work. (That assumes snow/snow/snow.h exists, like if you clone this repo.)
    • -DSNOW_ENABLED: Enable snow (otherwise describe(...) would be compiled down to nothing).
    • -g: Add debug symbols for valgrind.
  • Run: valgrind --leak-check=full --show-leak-kinds=all --track-origins=yes --error-exitcode=1 ./test
#include <stdio.h>
#include <snow/snow.h>

describe(files) {
	it("opens files") {
		FILE *f = fopen("test", "r");
		assertneq(f, NULL);

	it("writes to files") {
		FILE *f = fopen("testfile", "w");
		assertneq(f, NULL);

		char str[] = "hello there";
		asserteq(fwrite(str, 1, sizeof(str), f), sizeof(str));

	subdesc(fread) {
		it("reads 10 bytes") {
			FILE *f = fopen("/dev/zero", "r");
			assertneq(f, NULL);

			char buf[10];
			asserteq(fread(buf, 1, 10, f), 10);

		it("reads 20 bytes") {
			FILE *f = fopen("/dev/zero", "r");
			assertneq(f, NULL);

			char buf[20];
			asserteq(fread(buf, 1, 20, f), 20);


Compile options

  • SNOW_ENABLED: Define to enable Snow.
  • SNOW_USE_FNMATCH: Set to 0 to not use fnmatch for test name matching, and instead just compare literal strings. (Useful for systems without fnmatch)
  • SNOW_COLOR_SUCCESS: The escape sequence before printing success.
  • SNOW_COLOR_FAIL: The escape sequence before printing failure.
  • SNOW_COLOR_MAYBE: The escape sequence before printing maybes.
  • SNOW_COLOR_DESC: The escape sequence before printing the test description.
  • SNOW_COLOR_BOLD: The escape sequence for bold text.
  • SNOW_COLOR_RESET: The escape sequence to reset formatting.
  • SNOW_DEFAULT_ARGS: A comma seperated list of strings to pass as arguments to snow before the command-line arguments.

Structure Macros

describe(testname) <block>

A top-level description of a component, which can contain subdescs and its. A describe(testname, block) will define a function void test_##testname(), which the main function created by snow_main will call automatically.

subdesc(testname) <block>

A description of a sub-component, which can contain nested subdescs and its. It's similar to describe, but doesn't define a function.

it(description) <block>

A particular test case. It can contain asserts and defers, as well as just regular code. A failing assert (or direct call to fail(...)) will mark the test as failed, but if it completes normally, it's marked as successful.

test(description) <block> is an alias, for cases where using it would read awkwardly.


defer is used for tearing down, and is inspired by Go's defer statement.

Once the test case completes, each deferred expression will be executed, in the reverse order of their definitions (i.e defer(printf("World")); defer(printf("Hello ")); will print "Hello World"). If the test case fails, only deferred expressions defined before the point of failure will be executed.

before_each() <block>

Code to run before each test case.

after_each() <block>

Code to run after each test case.


This macro expands to a main function which handless stuff like parsing arguments and freeing memory allocated by Snow. All described functions will automatically be called by the main functions.

If you want more control over the main function, you can use the snow_main_decls macro to create the necessary global variables and functions, and then call the snow_main_function(int argc, char **argv) function.

This is essentially how snow_main() works:

int main(int argc, char **argv) {
	return snow_main_function(argc, argv);

Assert Macros

fail(fmt, ...)

Just directly fail the test case. The arguments are a printf-style format, optionally followed by arguments, just like printf.

assert(x [, explanation])

Fail if the expression x returns 0. explanation is an optional string which will be printed if the assertion fails, and can be used to provide some context.

asserteq(a, b [, explanation])

Fail unless a equals b. If b is a string, strcmp will be used to check for equality; otherwise, == will be used.

asserteq requires C11. If you can't use C11, or want to explicitly state what type your arguments are (say you want to compare strings by pointer instead of by content), you can use the asserteq_int, asserteq_ptr, asserteq_dbl, and asserteq_str macros instead of asserteq.

assertneq(a, b [, explanation])

Fail if a equals b. If b is a string, strcmp will be used to check for equality; otherwise, == will be used.

assertneq requires C11. If you can't use C11, or want to explicitly state what type your arguments are (say you want to compare strings by pointer instead of by content), you can use the assertneq_int, assertneq_ptr, assertneq_dbl, and asserteq_str macros instead of assertneq.

asserteq_buf(a, b, n [, explanation])

Fail unless the first n bytes of a and b are the same.

assertneq_buf(a, b, n [, explanation])

Fail if the first n bytes of a and b are the same.

snow_fail(fmt, ...), snow_fail_update()

snow_fail_update saves the current file/line, while snow_fail fails the currently executing test case and prints the saved file/line from the last snow_fail_update. This allows for implementing new checks to fail tests. All assertion functions from Snow are implemented using snow_fail and snow_fail_update.

How to test

Exactly how to test your code might not be as obvious with C as it is for other languages. I haven't yet used Snow for any big projects, but here's how I would do it

Testing a library's interface

When testing a library's interface, you can just create a test folder which is completely decoupled from the library's source code, and just compile your test code with a flag to link against your library.

Testing a program or library internals

Testing anything that's not exposed as a library's public API is is possible because unless SNOW_ENABLED is defined, describe will just be an empty macro, and all uses of it will be removed by the preprocessor. Therfore, you can include tests directly in your C source files, and regular builds won't use snow, while builds with -DSNOW_ENABLED will be your test suite.

Since all test macros are compiled down to nothing, this will have no runtime performance or binary size impact.

Note that since snow.h defines macros with pretty general names (it, describe, assert), it's probably a good idea to put your tests at the bottom of the source and only include snow.h right above the test code, to avoid name conflicts.

The exampleproject directory is an example of a program tested this way.