Notes on writing a GCC plugin

Here are notes on some of my discoveries while working on a plugin for GCC.

Callback arguments

I have been toying around with a plugin to GCC to generate a Fortran module source with the appropriate bind(c) settings from a C header file. According to the GCC internals manual, the basic structure is:

void callback(void * gcc_data, void * user_data)
{
    ...
};

int plugin_init(...)
{
    struct PACKAGE_NAME_data data;
    ...
    register_callback(..., callback, &data)
};

However, all of the tutorials online punt on how to actually pass arguments through the user_data by using NULL. This is not helpful if the plugin will be run in concurrent processes. Initially, I could not get the casting correct. After a bit of playing, I finally stumbled on the proper casting:

void callback(void * gcc_data, void * user_data)
{
    struct PACKAGE_NAME_data * data;
    data = reinterpret_cast<struct PACKAGE_NAME_data *>(user_data);
    ...
};

int plugin_init(...)
{
    static struct PACKAGE_NAME_data data;
    ...
    register_callback(..., callback, &data)
};

(You could just use C style casting, but this is C++ so you shouldn’t)

When to walk the AST

All the tutorials I could find online worked at the gate pass or at the final stage. I want to walk the tree immediately after the parsing is done and the AST has been assembled. I originally thought I wanted the PLUGIN_PRE_GENERICIZE, but that is not appropriate for C/C++ parsing (the manual says this in the section on GENERIC). It looks like the GIMPLE pass is where we get the full tree before push down. The PLUGIN_OVERRIDE_GATE appears to run too soon, but I’m still working on that.

Working with autotools

We need to undefine the macros set by autotools because the plugin framework defines them before we can. This simply suppresses the warnings about that.

#undef PACKAGE_BUGREPORT
#undef PACKAGE_NAME
#undef PACKAGE_STRING
#undef PACKAGE_TARNAME
#undef PACKAGE_VERSION

I just put it in a header ‘package-config.h’ with the appropriate #include to use in additional files.

Edit 2020-03-29

The original purpose for writing a plugin was to concoct a way to generate the Fortran 2003 interface module given a C header. It sort of worked, but not well enough to put it out in the world. I came back to it (again) the other night to take another stab. But then I was googling a different problem and came across this article. It looks like they beat me to the punch. Oh well. But at least I have a new project to keep an eye on!

Keith F. Prussing, Ph.D. on 2018-01-18