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6. Using libtool with other languages

Libtool was first implemented in order to add support for writing shared libraries in the C language. However, over time, libtool is being integrated with other languages, so that programmers are free to reap the benefits of shared libraries in their favorite programming language.

This chapter describes how libtool interacts with other languages, and what special considerations you need to make if you do not use C.

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6.1 Writing libraries for C++

Creating libraries of C++ code should be a fairly straightforward process, because its object files differ from C ones in only three ways:

  1. Because of name mangling, C++ libraries are only usable by the C++ compiler that created them. This decision was made by the designers of C++ in order to protect users from conflicting implementations of features such as constructors, exception handling, and RTTI.
  2. On some systems, the C++ compiler must take special actions for the dynamic linker to run dynamic (i.e., run-time) initializers. This means that we should not call ld directly to link such libraries, and we should use the C++ compiler instead.
  3. C++ compilers will link some Standard C++ library in by default, but libtool does not know which are these libraries, so it cannot even run the inter-library dependence analyzer to check how to link it in. Therefore, running ld to link a C++ program or library is deemed to fail.

Because of these three issues, Libtool has been designed to always use the C++ compiler to compile and link C++ programs and libraries. In some instances the main() function of a program must also be compiled with the C++ compiler for static C++ objects to be properly initialized.

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6.2 Tags

Libtool supports multiple languages through the use of tags. Technically a tag corresponds to a set of configuration variables associated with a language. These variables tell libtool how it should create objects and libraries for each language.

Tags are defined at configure-time for each language activated in the package (see LT_LANG in The LT_INIT macro). Here is the correspondence between language names and tags names.

Language name

Tag name







Fortran 77




Windows Resource


libtool tries to automatically infer which tag to use from the compiler command being used to compile or link. If it can't infer a tag, then it defaults to the configuration for the C language.

The tag can also be specified using libtool's `--tag=tag' option (see section Invoking libtool). It is a good idea to do so in `Makefile' rules, because that will allow users to substitute the compiler without relying on libtool inference heuristics. When no tag is specified, libtool will default to CC; this tag always exists.

Finally, the set of tags available in a particular project can be retrieved by tracing for the LT_SUPPORTED_TAG macro (see section Libtool's trace interface).

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