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+Building `bfs`
+==============
+
+Compiling
+---------
+
+`bfs` uses [GNU Make](https://www.gnu.org/software/make/) as its build system.
+A simple invocation of
+
+ $ make
+
+should build `bfs` successfully, with no additional steps necessary.
+As usual with `make`, you can run a [parallel build](https://www.gnu.org/software/make/manual/html_node/Parallel.html) with `-j`.
+For example, to use all your cores, run `make -j$(nproc)`.
+
+### Targets
+
+| Command | Description |
+|------------------|---------------------------------------------------------------|
+| `make` | Builds just the `bfs` binary |
+| `make all` | Builds everything, including the tests (but doesn't run them) |
+| `make check` | Builds everything, and runs the tests |
+| `make install` | Installs `bfs` (with man page, shell completions, etc.) |
+| `make uninstall` | Uninstalls `bfs` |
+
+### Flag-like targets
+
+The build system provides a few shorthand targets for handy configurations:
+
+| Command | Description |
+|----------------|-------------------------------------------------------------|
+| `make release` | Build `bfs` with optimizations, LTO, and without assertions |
+| `make asan` | Enable [AddressSanitizer] |
+| `make lsan` | Enable [LeakSanitizer] |
+| `make msan` | Enable [MemorySanitizer] |
+| `make tsan` | Enable [ThreadSanitizer] |
+| `make ubsan` | Enable [UndefinedBehaviorSanitizer] |
+| `make gcov` | Enable [code coverage] |
+
+[AddressSanitizer]: https://github.com/google/sanitizers/wiki/AddressSanitizer
+[LeakSanitizer]: https://github.com/google/sanitizers/wiki/AddressSanitizerLeakSanitizer#stand-alone-mode
+[MemorySanitizer]: https://github.com/google/sanitizers/wiki/MemorySanitizer
+[ThreadSanitizer]: https://github.com/google/sanitizers/wiki/ThreadSanitizerCppManual
+[UndefinedBehaviorSanitizer]: https://clang.llvm.org/docs/UndefinedBehaviorSanitizer.html
+[code coverage]: https://gcc.gnu.org/onlinedocs/gcc/Gcov.html
+
+You can combine multiple flags and other targets (e.g. `make asan ubsan check`), but not all of them will work together.
+
+### Flags
+
+Other flags are controlled with `make` variables and/or environment variables.
+Here are some of the common ones; check the [`Makefile`](/Makefile) for more.
+
+| Flag | Description |
+|----------------------------------|---------------------------------------------|
+| `CC` | The C compiler to use, e.g. `make CC=clang` |
+| `CFLAGS`<br>`EXTRA_CFLAGS` | Override/add to the default compiler flags |
+| `LDFLAGS`<br>`EXTRA_LDFLAGS` | Override/add to the linker flags |
+| `WITH_ACL`<br>`WITH_ATTR`<br>... | Enable/disable optional dependencies |
+| `TEST_FLAGS` | `tests.sh` flags for `make check` |
+| `DESTDIR` | The root directory for `make install` |
+| `PREFIX` | The installation prefix (default: `/usr`) |
+| `MANDIR` | The man page installation directory |
+
+###
+
+### Dependency tracking
+
+The build system automatically tracks header dependencies with the `-M` family of compiler options (see `DEPFLAGS` in the [`Makefile`](/Makefile)).
+So if you edit a header file, `make` will rebuild the necessary object files ensuring they don't go out of sync.
+
+We go one step further than most build systems by tracking the flags that were used for the previous compilation.
+That means you can change configurations without having to `make clean`.
+For example,
+
+ $ make
+ $ make release
+
+will build the project in debug mode and then rebuild it in release mode.
+
+A side effect of this may be surprising: `make check` by itself will rebuild the project in the default configuration.
+To test a different configuration, you'll have to repeat it (e.g. `make release check`).
+
+
+Testing
+-------
+
+`bfs` comes with an extensive test suite which can be run with
+
+ $ make check
+
+Most of the testsuite is implemented in the file [`tests.sh`](/tests.sh).
+This script contains hundreds of separate test cases.
+Most of them are *snapshot tests* which compare `bfs`'s output to a known-good copy saved under [`tests`](/tests).
+
+You can pass the name of a particular test case (or a few) to run just those tests.
+For example:
+
+ $ ./tests.sh test_basic
+
+If you need to update the reference snapshot, pass `--update`.
+It can be handy to generate the snapshot with a different `find` implementation to ensure the output is correct, for example:
+
+ $ ./tests.sh test_basic --bfs=find --update
+
+But keep in mind, other `find` implementations may not be correct.
+To my knowledge, no other implementation passes even the POSIX-compatible subset of the tests:
+
+ $ ./tests.sh --bfs=find --posix
+ ...
+ tests passed: 89
+ tests failed: 5
+
+Run
+
+ $ ./tests.sh --help
+
+for more details.
+
+### Validation
+
+A more thorough testsuite is run by the [CI](https://github.com/tavianator/bfs/actions) and to validate releases.
+It builds `bfs` in multiple configurations to test for latent bugs, memory leaks, 32-bit compatibility, etc.
+You can run it yourself with
+
+ $ make distcheck
+
+Some of these tests require `sudo`, and will prompt for your password if necessary.