Writing rules

At the bare minimum, a package in contrib must provide two Makefile targets in src/foo/rules.mak:

• .foo to build and install the package, and
• .sum-foo to fetch or create a source tarball and verify it, where foo the package name.

Tarball

.sum-foo typically depends on a separate target that fetches the source code. In that case, .sum-foo needs only verify that the tarball is correct, e.g.:

$(TARBALLS)/libfoo-$(FOO_VERSION).tar.bz2:
	$(call download,$(FOO_URL))

# This will use the default rule: check SHA-512
.sum-foo: libfoo-$(FOO_VERSION).tar.bz2

NOTE: contrary to the previous VLC contribs, this system always uses a source tarball, even if the source code is downloaded from a VCS. This serves two purposes:

• offline builds (or behind a firewall),
• source code requirements compliance.

Compilation

Similarly, .foo typically depends on the source code directory. In this case, care must be taken that the directory name only exists if the source code is fully ready. Otherwise Makefile dependencies will break (this is not an issue for files, only directories).

libfoo: libfoo-$(FOO_VERSION).tar.bz2 .sum-foo
	$(UNPACK) # to libfoo-$(FOO_VERSION)
	### apply patches here ###
	# last command: make the target directory
	$(MOVE)

.foo: libfoo
	cd $< && $(HOSTVARS) ./configure $(HOSTCONF)
	cd $< && $(MAKE) install
	touch $@

Conditional builds

As far as possible, build rules should determine automatically whether a package is useful (for VLC media player) or not. Useful packages should be listed in the PKGS special variable. See some examples:

# FFmpeg is always useful
PKGS += ffmpeg

# DirectX headers are useful only on Windows
ifdef HAVE_WIN32
PKGS += directx
endif

# x264 is only useful when stream output is enabled
ifdef BUILD_ENCODERS
PKGS += x264
endif

If a package is a dependency of another package, but it is not a direct dependency of VLC, then it should NOT be added to PKGS. The build system will automatically build it via dependencies (see below).

Some packages may be provided by the target system. This is especially common when building natively on Linux or BSD. When this situation is detected, the package name should be added to the PKGS_FOUND special variable. The build system will then skip building this package:

# Asks pkg-config if foo version 1.2.3 or later is present:
ifeq ($(call need_pkg,'foo >= 1.2.3'),)
PKGS_FOUND += foo
endif

Dependencies

If package bar depends on package foo, the special DEPS_bar variable should be defined as follow:

DEPS_bar = foo $(DEPS_foo)

Note that dependency resolution is unfortunately not recursive. Therefore $(DEPS_foo) really should be specified explicitly as shown above. (In practice, this will not make any difference insofar as there are no pure second-level nested dependencies. For instance, libass depends on FontConfig, which depends on FreeType, but libass depends directly on FreeType anyway.)

Also note that DEPS_bar is set "recursively" with =, rather than "immediately" with :=. This is so that $(DEPS_foo) is expanded correctly, even if DEPS_foo it is defined after DEPS_bar.

Implementation note:

If you must know, the main.mak build hackery will automatically
emit a dependency from .bar onto .dep-foo:

	.bar: .dep-foo

...whereby .dep-foo will depend on .foo:

	.dep-foo: .foo
		touch $@

...unless foo was detected in the target distribution:

	.dep-foo:
		touch $@

So you really only need to set DEPS_bar.