clang rewriter: add structure definitions to invalid program

Question

I need to rewrite a body of C/C++ code to inject structure definitions automatically when they are used. Specifically, I need to recognize function bodies of the form:

int func(struct STRUCT_x_y *args) { /* access args->field here */ }

... and generate the structure declaration prior to the function's body, e.g.:

struct STRUCT_x_y {
    int field;
};

int func(struct STRUCT_x_y *args) { /* access args->field here */ }

When trying to use Clang's rewriter to insert the structure declaration (e.g. following this skeleton program), I get errors because the original program text doesn't compile without these declarations -- the function attempts to access fields on the undefined structure.

Is it possible to use Clang's rewriter on a program that isn't valid C++? I can even place a further restriction -- the function body is invalid, but the rest of the program is fine.

(I can, of course, hack together some ugly solution that uses regular expressions to sort-of-detect the function signature and generate the structure, but I'd really rather harness the power of Clang's parser and rewriter.)

Answer 1

What I ended up doing is just making two passes through the program. During the first pass, I used a null diagnoser so that all errors and warnings are ignored. I collected the methods whose signatures indicated that a struct needs to be generated. After the first pass was done, I generated all the structures and ran the program through a second pass, this time with the diagnosers.

Generally speaking, Clang's AST is immutable and doesn't really allow tree modifications other than the pure text-based rewriting. But the approach above worked reasonably well for us. If you are curious about context, this is part of the IOVisor BCC project.