Parallel Installability

All public libraries should be designed to be parallel installed to ease API breaks later in the life of the library. If a library is used by multiple projects, and wants to break API, either all of the projects must be ported to the new API in parallel, or some of them will no longer be installable at the same time as the others, due to depending on conflicting versions of this library.

This is unmaintainable, and asking all the projects to port to a new API at the same time is hard to organize and demoralizing, as most API breaks do not bring large new features which would motivate porting.

The solution is to ensure that all libraries are parallel installable, allowing the old and new versions of the API to be installed and compiled against at the same time, without conflicts. Building in support for this kind of parallel installation is much easier to do at the start of a project than it is to do retroactively.

This eliminates the ‘chicken and egg’ problem of porting a collection of applications from one version of a library to the next, and makes breaking API a lot simpler for library maintainers, which can allow for more rapid iteration and development of new features if they desire.

The alternative, and equally valid, solution is for the library to never break API — the approach taken by libc.

The solution to the problem is essentially to rename the library, and in most cases the nicest way to do so is to include the version number in the path of every file it installs. This means multiple versions of the library can be installed at the same time.

For example, say that library Foo traditionally installs these files:

You might modify Foo version 4 to install these files instead:

It could then be parallel installed with version 5:

This is easily supported using pkg-config: foo-4.pc would add /usr/include/foo-4 to the include path and libfoo-4.so to the list of libraries to link; foo-5.pc would add /usr/include/foo-5 and libfoo-5.so.

The version number that goes in filenames is an ABI/API version. It should not be the full version number of your package — just the part which signifies an API break. If using the standard major.minor.micro scheme for project versioning, the API version is typically the major version number.

Minor releases (typically where API is added but not changed or removed) and micro releases (typically bug fixes) do not affect API backwards compatibility so do not require moving all the files.

The examples in the following sections assume that the API version and soname are exported from configure.ac using the following code:

# Before making a release, the LIBRARY_LT_VERSION string should be modified.
# The string is of the form c:r:a. Follow these instructions sequentially:
#
#  1. If the library source code has changed at all since the last update,
#     then increment revision (‘c:r:a’ becomes ‘c:r+1:a’).
#  2. If any interfaces have been added, removed, or changed since the last update,
#     increment current, and set revision to 0.
#  3. If any interfaces have been added since the last public release,
#     then increment age.
#  4. If any interfaces have been removed or changed since the last public release,
#     then set age to 0.
AC_SUBST([LIBRARY_LT_VERSION],[1:0:0])

AC_SUBST([LIBRARY_API_VERSION],[4])

Header files should always be installed in a versioned subdirectory that requires an -I flag to the C compiler. For example, if my header is foo.h, and applications do this:

#include <foo/foo.h>

then I should install these files:

Applications should pass the flag -I/usr/include/foo-4 or -I/usr/include/foo-5 to the C compiler. Again, this is facilitated by using pkg-config.

Note the extra foo/ subdirectory. This namespaces the #include to avoid file naming collisions with other libraries. For example, if two different libraries install headers called utils.h, which one gets included when you use #include <utils.h>?

There’s some temptation to keep one of the header files outside of any subdirectory:

The problem there is that users are always accidentally getting the wrong header, since -I/usr/include seems to find its way onto compile command lines with some regularity. If you must do this, at least add a check to the library that detects applications using the wrong header file when the library is initialized.

Versioned header files can be installed from automake using the following code:

libraryincludedir = $(includedir)/liblibrary-@LIBRARY_API_VERSION@/library
library_headers = \
	liblibrary/example1.h \
	liblibrary/example2.h \
	$(NULL)

# The following headers are private, and shouldn't be installed:
private_headers = \
	liblibrary/example-private.h \
	$(NULL)
# The main header simply #includes all other public headers:
main_header = liblibrary/library.h
public_headers = \
	$(main_header) \
	$(library_headers) \
	$(NULL)

libraryinclude_HEADERS = $(public_headers)

As well as correct versioning, all APIs in installed headers should be namespaced correctly.

Library object files should have a versioned name. For example:

This allows applications to get exactly the one they want at compile time, and ensures that versions 4 and 5 have no files in common.

Versioned libraries can be built and installed from automake using the following code:

lib_LTLIBRARIES = liblibrary/liblibrary-@LIBRARY_API_VERSION@.la

liblibrary_liblibrary_@LIBRARY_API_VERSION@_la_SOURCES = \
	$(private_headers) \
	$(library_sources) \
	$(NULL)
liblibrary_liblibrary_@LIBRARY_API_VERSION@_la_CPPFLAGS = …
liblibrary_liblibrary_@LIBRARY_API_VERSION@_la_CFLAGS = …
liblibrary_liblibrary_@LIBRARY_API_VERSION@_la_LIBADD = …
liblibrary_liblibrary_@LIBRARY_API_VERSION@_la_LDFLAGS = \
	-version-info $(LIBRARY_LT_VERSION) \
	$(AM_LDFLAGS) \
	$(NULL)

pkg-config files should have a versioned name. For example:

Since each pkg-config file contains versioned information about the library name and include paths, any project which depends on the library should be able to switch from one version to another simply by changing their pkg-config check from foo-4 to foo-5 (and doing any necessary API porting).

Versioned pkg-config files can be installed from autoconf and automake using the following code:

AC_CONFIG_FILES([
liblibrary/library-$LIBRARY_API_VERSION.pc:liblibrary/library.pc.in
],[],
[LIBRARY_API_VERSION='$LIBRARY_API_VERSION'])

# Note that the template file is called library.pc.in, but generates a
# versioned .pc file using some magic in AC_CONFIG_FILES.
pkgconfigdir = $(libdir)/pkgconfig
pkgconfig_DATA = liblibrary/library-$(LIBRARY_API_VERSION).pc

DISTCLEANFILES += $(pkgconfig_DATA)
EXTRA_DIST += liblibrary/library.pc.in

From a user standpoint, the best approach to configuration files is to keep the format both forward and backward compatible (both library versions understand exactly the same configuration file syntax and semantics). Then the same configuration file can be used for all versions of the library, and no versioning is needed on the configuration file itself.

If you can’t do that, the configuration files should simply be renamed, and users will have to configure each version of the library separately.

If you use gettext for translations in combination with autoconf and automake, normally things are set up to install the translations to /usr/share/locale/lang/LC_MESSAGES/package. You’ll need to change package. The convention used in GNOME is to put this in configure.ac:

GETTEXT_PACKAGE=foo-4
AC_SUBST([GETTEXT_PACKAGE])
AC_DEFINE_UNQUOTED([GETTEXT_PACKAGE],["$GETTEXT_PACKAGE"])

Then use GETTEXT_PACKAGE as the package name to pass to bindtextdomain(), textdomain(), and dgettext().

A D-Bus interface is another form of API, similar to a C API except that resolution of the version is done at runtime rather than compile time. Versioning D-Bus interfaces is otherwise no different to C APIs: version numbers must be included in interface names, service names and object paths.

For example, for a service org.example.Foo exposing interfaces A and B on objects Controller and Client, versions 4 and 5 of the D-Bus API would look like this:

Desktop applications generally do not need to be versioned, as they are not depended on by any other modules. Daemons and utility programs, however, interact with other parts of the system and hence need versioning.

Given a daemon and utility program:

these should be versioned as:

You may want to install a symbolic link from /usr/bin/foo-lookup-utility to the recommended versioned copy of the utility, to make it more convenient for users to use.