using libClang or libTooling to list function declarations

Question

I am developing a windows C++ tool to analyze and mutate C code. I need help to use libClang or preferably libTooling get a list of C function declarations from a source tree of C files (translation units) with their headers. Eventually, I would like to transform the C files to add instrumentation points.

I need a good starting point example. I started with an example from AST matchers and Clang refactoring tools. This seems like a reasonable jumping off point but I don't know how to actually get the list of function names back to where I can assign them to string variables that I can use in my application.

I think ASTMatchers would be the preferred approach to tackle this problem. Shown below I created a simple clang-query example to find all function declarations in a single 'test.c' translation unit. The simple query clang-query> m functionDecl(). expression returned a list of underligned function declarations including the return type and arguments. AST matchers can be easily created from simple clang-query scripts as I understand.

Here is the simple clang-query example of what I would like to achieve in C++: Using a test file 'test.cpp':

// example code
int testFn0(const int* pa, int *pb) {
    return 3;
}

void testFn(const int* pa, int *pb) {
    int t = 0;
    bool a = true;
    bool b = true;
    bool c = true;
    bool d = true;
    if (a && b || !(c ^ d)) {
        t = 0;
    }
}

Then using clang-query

~/tools$ clang-query test.c --

clang-query> m functionDecl()

Match #1:

/home/jcoffey/tools/test.cpp:3:1: note: "root" binds here
int testFn0(const int* pa, int *pb) {
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Match #2:

/home/tools/test.cpp:7:1: note: "root" binds here
void testFn(const int* pa, int *pb) {
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2 matches.