Defining Binaries for the Build System

One part of what the build system does is compile C/C++ and link the resulting objects to produce executables and/or libraries. This document describes the basics of defining what is going to be built and how. All the following describes constructs to use in files.

Source files

Source files to be used in a given directory are registered in the SOURCES and UNIFIED_SOURCES variables. UNIFIED_SOURCES have a special behavior in that they are aggregated by batches of 16, requiring, for example, that there are no conflicting variables in those source files.

SOURCES and UNIFIED_SOURCES are lists which must be appended to, and each append requires the given list to be alphanumerically ordered.



SOURCES and UNIFIED_SOURCES can contain a mix of different file types, for C, C++, and Objective C.

Static Libraries

To build a static library, other than defining the source files (see above), one just needs to define a library name with the Library template.


The library file name will be libfoo.a on UNIX systems and foo.lib on Windows.

If the static library needs to aggregate other static libraries, a list of Library names can be added to the USE_LIBS variable. Like SOURCES, it requires the appended list to be alphanumerically ordered.

USE_LIBS += ['bar', 'baz']

If there are multiple directories containing the same Library name, it is possible to disambiguate by prefixing with the path to the wanted one (relative or absolute):


Note that the leaf name in those paths is the Library name, not an actual file name.

Note that currently, the build system may not create an actual library for static libraries. It is an implementation detail that shouldn’t need to be worried about.

As a special rule, USE_LIBS is allowed to contain references to shared libraries. In such cases, programs and shared libraries linking this static library will inherit those shared library dependencies.

Intermediate (Static) Libraries

In many cases in the tree, static libraries are built with the only purpose of being linked into another, bigger one (like libxul). Instead of adding all required libraries to USE_LIBS for the bigger one, it is possible to tell the build system that the library built in the current directory is meant to be linked to that bigger library, with the FINAL_LIBRARY variable.


The FINAL_LIBRARY value must match a unique Library name somewhere in the tree.

As a special rule, those intermediate libraries don’t need a Library name for themselves.

Shared Libraries

Sometimes, we want shared libraries, a.k.a. dynamic libraries. Such libraries are defined similarly to static libraries, using the SharedLibrary template instead of Library.


When this template is used, no static library is built. See further below to build both types of libraries.

With a SharedLibrary name of foo, the library file name will be libfoo.dylib on OSX, on ELF systems (Linux, etc.), and foo.dll on Windows. On Windows, there is also an import library named foo.lib, used on the linker command line. libfoo.dylib and are considered the import library name for, resp. OSX and ELF systems.

On OSX, one may want to create a special kind of dynamic library: frameworks. This is done with the Framework template.


With a Framework name of foo, the framework file name will be foo. This template however affects the behavior on all platforms, so it needs to be set only on OSX.


Executables, a.k.a. programs, are, in the simplest form, defined with the Program template.


On UNIX systems, the executable file name will be foobar, while on Windows, it will be foobar.exe.

Like static and shared libraries, the build system can be instructed to link libraries to the executable with USE_LIBS, listing various Library names.

In some cases, we want to create an executable per source file in the current directory, in which case we can use the SimplePrograms template


Contrary to Program, which requires corresponding SOURCES, when using SimplePrograms, the corresponding SOURCES are implied. If the corresponding sources have an extension different from .cpp, it is possible to specify the proper extension:

], ext='.c')

Please note this construct was added for compatibility with what already lives in the mozilla tree ; it is recommended not to add new simple programs with sources with a different extension than .cpp.

Similar to SimplePrograms, is the CppUnitTests template, which defines, with the same rules, C++ unit tests programs. Like SimplePrograms, it takes an ext argument to specify the extension for the corresponding SOURCES, if it’s different from .cpp.

Linking with system libraries

Programs and libraries usually need to link with system libraries, such as a widget toolkit, etc. Those required dependencies can be given with the OS_LIBS variable.

OS_LIBS += [

This expands to foo.lib bar.lib when building with MSVC, and -lfoo -lbar otherwise.

For convenience with pkg-config, OS_LIBS can also take linker flags such as -L/some/path and -llib, such that it is possible to directly assign LIBS variables from CONFIG, such as:


(assuming CONFIG['MOZ_PANGO_LIBS'] is a list, not a string)

Like USE_LIBS, this variable applies to static and shared libraries, as well as programs.

Libraries from third party build system

Some libraries in the tree are not built by the build system, and there is no Library corresponding to them.

However, USE_LIBS allows to reference such libraries by giving a full path (like when disambiguating identical Library names). The same naming rules apply as other uses of USE_LIBS, so only the library name without prefix and suffix shall be given.


Note that /path/from/topsrcdir/to/third-party and ../relative/third-party/baz must lead under a subconfigured directory (a directory with an AC_OUTPUT_SUBDIRS in, or security/nss.

Building both static and shared libraries

When both types of libraries are required, one needs to set both FORCE_SHARED_LIB and FORCE_STATIC_LIB boolean variables.


But because static libraries and Windows import libraries have the same file names, either the static or the shared library name needs to be different than the name given to the Library template.

The STATIC_LIBRARY_NAME and SHARED_LIBRARY_NAME variables can be used to change either the static or the shared library name.


With the above, on Windows, foo_s.lib will be the static library, foo.dll the shared library, and foo.lib the import library.

In some cases, for convenience, it is possible to set both STATIC_LIBRARY_NAME and SHARED_LIBRARY_NAME. For example:


This allows to use mylib in the USE_LIBS of another library or executable.

When referring to a Library name building both types of libraries in USE_LIBS, the shared library is chosen to be linked. But sometimes, it is wanted to link the static version, in which case the Library name needs to be prefixed with static: in USE_LIBS

   STATIC_LIBRARY_NAME = 'mylib_s'
   USE_LIBS += [


The SONAME variable declares a “shared object name” for the library. It defaults to the Library name or the SHARED_LIBRARY_NAME if set. When linking to a library with a SONAME, the resulting library or program will have a dependency on the library with the name corresponding to the SONAME instead of the Library name. This only impacts ELF systems.

   SONAME = 'foo'
   USE_LIBS += [

On e.g. Linux, the above myprog will have DT_NEEDED markers for and instead of and if there weren’t a SONAME. This means the runtime requirement for myprog is instead of