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### 23.10 Building GCC plugins

If plugins are enabled, GCC installs the headers needed to build a plugin (somewhere in the installation tree, e.g. under `/usr/local`). In particular a `plugin/include` directory is installed, containing all the header files needed to build plugins.

On most systems, you can query this `plugin` directory by invoking `gcc -print-file-name=plugin` (replace if needed `gcc` with the appropriate program path).

Inside plugins, this `plugin` directory name can be queried by calling `default_plugin_dir_name ()`.

Plugins may know, when they are compiled, the GCC version for which `plugin-version.h` is provided. The constant macros `GCCPLUGIN_VERSION_MAJOR`, `GCCPLUGIN_VERSION_MINOR`, `GCCPLUGIN_VERSION_PATCHLEVEL`, `GCCPLUGIN_VERSION` are integer numbers, so a plugin could ensure it is built for GCC 4.7 with

    #if GCCPLUGIN_VERSION != 4007
    #error this GCC plugin is for GCC 4.7
    #endif

The following GNU Makefile excerpt shows how to build a simple plugin:

    HOST_GCC=g++
    TARGET_GCC=gcc
    PLUGIN_SOURCE_FILES= plugin1.c plugin2.cc
    GCCPLUGINS_DIR:= $(shell $(TARGET_GCC) -print-file-name=plugin)
    CXXFLAGS+= -I$(GCCPLUGINS_DIR)/include -fPIC -fno-rtti -O2

    plugin.so: $(PLUGIN_SOURCE_FILES)
       $(HOST_GCC) -shared $(CXXFLAGS) $^ -o $@

A single source file plugin may be built with `g++ -I`gcc -print-file-name=plugin`/include -fPIC -shared -fno-rtti -O2 plugin.c -o plugin.so`, using backquote shell syntax to query the `plugin` directory.

When a plugin needs to use `gengtype`, be sure that both `gengtype` and `gtype.state` have the same version as the GCC for which the plugin is built.