libakgl

A C library for building 2D games on SDL3. Not an engine: no editor, no scripting layer, no inheritance, and no runtime malloc. Behaviour attaches to a struct as function pointers, objects come from fixed pools, and every call reports failure through an error context the caller cannot silently ignore.

The manual

docs/ is the manual — twenty-one chapters covering every subsystem, plus two complete tutorial games. Start at Chapter 3, Getting started if you want a window on screen, or Chapter 2, Design philosophy if you want to know why the library is shaped the way it is.

The two tutorials build real, running programs that live under examples/:

A 2D sidescroller	Gravity, a jump, collision you write yourself, coins and hazards
A top-down JRPG	A town map, NPCs spawned from map objects, four-way animation, a text box, a follower

For per-function reference, build the Doxygen output with doxygen Doxyfile; it is a CI gate, so an undocumented symbol fails the build.

This README covers the development process only — hooks, mutation testing, benchmarks and memory checking. Everything about using the library is in docs/.

The task-oriented FAQ that used to live here has moved into docs/, corrected. It was not merely incomplete: its examples did not compile. Two unbalanced PASS() calls, a sprite->frameids = [0, 1, 2, 3]; that is not C in any dialect, a stray 9 inside a bitmask expression, an int screenwidth = NULL, and — in the first snippet a reader ever saw — the exact strncpy call AGENTS.md forbids. The prose had drifted with it: its claim that the engine "ONLY supports TilED TMJ tilemaps with tileset external references" is backwards, and the loader accepts only embedded tilesets. Writing the manual turned up twenty-seven such claims across the headers. Every example in docs/ is now compiled, linked, run or matched against the source tree by ctest, so that class of drift fails a build instead of reaching a reader.

Documentation examples

Every fenced example in docs/ carries an info string saying what it is, and the docs_examples test acts on it: c blocks are compiled, c run= blocks are linked against the library and executed headless with their output compared byte for byte, c excerpt= blocks must still appear verbatim in the file they quote, json blocks are loaded through the real akgl_*_load_json, and c screenshot= blocks generate the figures in docs/images/. A block with no info string is a hard error, because the failure mode the harness exists to prevent is passing while checking nothing.

ctest --test-dir build -R docs_examples --output-on-failure
ctest --test-dir build -R docs_screenshots --output-on-failure
cmake --build build --target docs_screenshots   # regenerate the figures

While editing one chapter, run it directly rather than the whole suite:

./tests/docs_examples.sh --root . \
    --cflags-file build/docs_cflags.txt \
    --ldflags-file build/docs_ldflags.txt \
    --checkjson build/akgl_docs_checkjson \
    docs/14-physics.md

docs/MAINTENANCE.md is the reference for the info strings, the preludes and the fixtures. There is deliberately no docs-path filter in CI: documentation goes stale because the code moved, not because somebody edited a chapter.

Git hooks

The repository ships a pre-commit hook that keeps committed C sources in the project's canonical format (Emacs cc-mode "stroustrup"; see AGENTS.md for the full style guide). The hook lives in scripts/hooks/ and is version controlled, but Git configuration is not cloned, so every clone has to be pointed at it once:

git config core.hooksPath scripts/hooks

Confirm it took effect:

git rev-parse --git-path hooks     # should print: scripts/hooks

That is the whole installation. The hook is already committed with its executable bit set, so nothing needs chmod.

What the hook does

On each commit it looks at the staged content of any added, copied, modified, or renamed .c/.h file under src/, include/, tests/, or util/, and reindents it if it does not already match the canonical style. Checking the staged content rather than the working tree means what lands in the commit is what was actually verified.

When a file needs reindenting, the hook fixes it in the working tree and re-stages it — but only when the index and working tree agree for that file. If they differ, you have staged part of a file with git add -p, and re-staging would sweep your unstaged work into the commit. Rather than do that silently the hook stops and prints the commands to run yourself:

scripts/reindent.sh path/to/file.c
git add path/to/file.c

include/akgl/SDL_GameControllerDB.h is generated and always skipped.

Requirements

The hook drives Emacs in batch mode, because cc-mode's indentation engine is the definition of the style and clang-format cannot reproduce it exactly. If emacs is not on PATH the hook prints a warning and allows the commit — a hook that refuses to run without an optional tool only teaches people to reach for --no-verify. Install Emacs to get the check; without it, formatting is on you.

Bypassing and uninstalling

Skip the hook for a single commit:

git commit --no-verify

Remove it entirely:

git config --unset core.hooksPath

If you already have local hooks

core.hooksPath replaces the hooks directory outright — once it is set, Git stops reading .git/hooks/ altogether, so any hooks you keep there will silently stop firing. If that matters, leave core.hooksPath unset and symlink just this one hook instead:

ln -s ../../scripts/hooks/pre-commit .git/hooks/pre-commit

Formatting without the hook

The hook is a convenience, not the source of truth. The same check is available directly, and is what you would run in CI:

scripts/reindent.sh --check     # list non-conforming files; exit 1 if any
scripts/reindent.sh             # reindent every tracked C source in place
scripts/reindent.sh src/game.c  # reindent specific files

--check exits 0 when everything conforms, 1 when a file needs reindenting, and 2 if Emacs is missing or the script cannot run — so a CI job that treats any non-zero status as failure will not mistake a broken toolchain for a clean tree.

Mutation testing

The mutation harness makes one deliberate source-code change at a time in a scratch copy, then rebuilds and runs the passing CTest suite to measure whether tests detect the change. The known-failing character test is excluded by default.

cmake --build build --target mutation
scripts/mutation_test.py --target src/tilemap.c --list
scripts/mutation_test.py --target src/tilemap.c --max-mutants 10
scripts/mutation_test.py --threshold 40 --junit mutation-junit.xml

The default run covers all libakgl-owned files under src/. Use repeated --target options to narrow the scope. A surviving mutant identifies behavior that the current tests do not verify; the script prints its file, line, operator, and exact edit. The real working tree is never mutated.

Performance testing

tests/perf.c and tests/perf_render.c are benchmarks rather than unit tests: they drive the hot paths — pool acquire and release, the registry, the physics sweep, the per-actor update and render, the tilemap draw, text, and asset loading — and print what each one costs per operation.

ctest --test-dir build -L perf --output-on-failure   # benchmarks only, about 30 seconds
ctest --test-dir build -LE perf                      # everything except the benchmarks
AKGL_BENCH_SCALE=0.1 ctest --test-dir build -L perf  # a tenth of the iterations

Each measurement is the best of five runs and is held to a budget set at roughly ten times the recorded baseline, so a suite that turns red means an algorithmic regression rather than a busy machine. Budgets are enforced only in an optimized build at full scale; below AKGL_BENCH_SCALE=1.0, and in a coverage build, they are reported without failing.

PERFORMANCE.md carries the recorded baseline, the frame budget it adds up to, and what the numbers say — including the raw-SDL control rows that separate what libakgl costs from what the rasterizer costs. The six defects the stress tests turned up, and the targets the numbers are measured against, are in TODO.md under Performance.

Memory checking

memcheck runs the suites that already exist under valgrind rather than adding suites of its own. The perf binaries carry most of it: they are the only programs in the tree that load assets, draw a scene, and run a frame loop in one process, and tests/benchutil.h cuts their iteration counts by three orders of magnitude when it detects valgrind — a benchmark walks one path a hundred thousand times, a leak check wants every path walked once.

cmake --build build --target memcheck   # everything, about 30 seconds
scripts/memcheck.sh -R tilemap          # one suite; ctest selection flags pass through
scripts/memcheck.sh -LE perf            # skip the benchmarks

The run forces SDL_VIDEO_DRIVER=dummy, SDL_RENDER_DRIVER=software and SDL_AUDIO_DRIVER=dummy so the vendor GPU stack is never loaded — that removes thousands of unfixable findings inside the driver without suppressing anything. Definite leaks, invalid accesses and uninitialised reads are counted and set the exit status; "still reachable" is not, because that is what SDL and FreeType keep for the process lifetime. Suppressions for genuine third-party findings are in scripts/valgrind.supp.

What it found the first time it ran is in TODO.md under Memory checking.

What is compiled in, and the notice it carries

libccd is compiled into libakgl.so. It is the collision narrowphase -- GJK, EPA and MPR, the same code vendored in ODE, FCL and Bullet -- and it is BSD-3-Clause. Its licence is deps/libccd/BSD-LICENSE, and cmake --install puts a copy at share/doc/akgl/BSD-LICENSE.libccd. If you redistribute a binary built from this tree, that notice has to travel with it.

It is the only dependency in that position. SDL3, SDL3_image, SDL3_mixer, SDL3_ttf, jansson, libakerror and libakstdlib are all linked as separate shared objects and carry their own notices; deps/semver is a single header, MIT, installed alongside ours; and deps/tg is vendored but compiled into nothing -- see TODO.md, "tg is vendored and has no consumer".

libccd is given a static arena rather than the allocator it ships with, so "libakgl does not call malloc at runtime" stays literally true with it linked in. That is src/collision_arena.c, and Chapter 15 explains why.