Get relative include path for file with clang tooling

I'm using clang tooling to auto generate some code from an existing code base.

In some cases I have an enum type that I want to use, so I want to include it. I'm able to get the path to the file to include by doing:

const EnumType *enumType = ...;
auto defineFileLoc =
        enumType->getDecl()->getDefinition()->getLocation();
const auto defineFilePath =
        sourceManager.getFilename(sourceManager.getExpansionLoc(defineFileLoc))
            .str();

This produces the full path of the file. I can include the full path, but this is ugly and not portable, so I was wondering if there is a way to generate the relative include path of the file.

I was thinking one way would be to get a list of the directories in the translation unit include path and then see if I can find something relative to that, but I am not able to find a way to get this with clang tooling. I use marchers, so the entire context I have for the specific match is a clang::ast_matchers::MatchFinder::MatchResult object.

Is there a way to get the list of directories in the include path for a specific translation unit? Or, is there a better way to do what I want?

The approach suggested in the question is right:

get a list of the directories in the translation unit include path and then see if I can find something relative to that

The header search path is stored in the HeaderSearchOptions class, specifically the UserEntries field. That has a vector of HeaderSearchOptions::Entry, each of which has a std::string Path.

The HeaderSearchOptions can be retrieved from the CompilerInvocation using getHeaderSearchOpts.

Getting the CompilerInvocation

The procedure for getting a CompilerInvocation varies depending on exactly how your tool starts up.

In my code, I use createInvocation() to get a CompilerInvocation, then use ASTUnit::LoadFromCompilerInvocation to run the parser and get an AST.

If you are using ClangTool::buildASTs, you'll have an ASTUnit and can call its getHeaderSearchOpts method.

If you are using ClangTool::runInvocation then your callback is handed a CompilerInvocation (which has getHeaderSearchOpts, mentioned above).

If however you are also using newFrontendActionFactory (which is what the tutorial does), then you need to modify the call site to also pass a SourceFileCallbacks whose handleBeginSource retrieves the CompilerInvocation from the CompilerInstance by calling getInvocation.

For example, define a class like:

// Intercept `handleBeginSource` so we can get the invocation.
class MySourceFileCallbacks : public clang::tooling::SourceFileCallbacks {
public:
  virtual bool handleBeginSource(clang::CompilerInstance &ci)
  {
    llvm::outs() << "in handleBeginSource, hasInvocation: "
                 << ci.hasInvocation() << "\n";

    clang::CompilerInvocation &invoc = ci.getInvocation();
    clang::HeaderSearchOptions &hso = invoc.getHeaderSearchOpts();
    for (clang::HeaderSearchOptions::Entry const &entry : hso.UserEntries) {
      llvm::outs() << "  " << entry.Path << "\n";
    }

    return true;
  }
};

then modify the call to newFrontendActionFactory to be:

MySourceFileCallbacks sfCallbacks;  // <--- add
return Tool.run(newFrontendActionFactory(&finder, &sfCallbacks).get());
                                               // ^^^^^^^^^^^^ add

Constructing a relative path

Given a file name as a std::string and a HeaderSearchOptions, the procedure for creating a relative path is mostly straightforward. However, you may want to take account of which Group each Entry is in, reflecting how it arose as a search path.

Below is the code I use to do this, producing as output a complete #include directive:

string ClangUtil::getIncludeSyntax(
  clang::HeaderSearchOptions const &headerSearchOptions,
  string const &fname,
  int * NULLABLE userEntryIndex)
{
  // Avoid having to awkwardly check the pointer below.
  int dummy = 0;
  if (!userEntryIndex) {
    userEntryIndex = &dummy;
  }

  // Use naive ordered search.  This isn't 100% correct in certain
  // scenarios (e.g., involving -iquote), but should suffice for my
  // purposes.
  *userEntryIndex = 0;
  for (auto const &e : headerSearchOptions.UserEntries) {
    if (beginsWith(fname, e.Path)) {
      // Get the name without the path prefix.  The path is assumed to
      // end with a directory separator, and that is stripped from
      // 'fname' too.
      string relative = fname.substr(e.Path.size() + 1);

      if (e.Group == clang::frontend::Angled) {
        // Perhaps ironically, paths specified with the -I option are
        // classified by clang as "angled", but conventionally the
        // names in such directories are referenced using quotes.
        return string("\"") + relative + "\"";
      }
      else {
        return string("<") + relative + ">";
      }
    }

    ++(*userEntryIndex);
  }

  if (beginsWith(fname, "./")) {
    *userEntryIndex = -1;

    // Drop the "." path component.
    string trimmed = fname.substr(2);
    return string("\"") + trimmed + "\"";
  }

  // Not found among the search paths, use the absolute name with quotes.
  *userEntryIndex = -2;
  return string("\"") + fname + "\"";
}

The above code is part of my print-clang-ast repository, whose original purpose was to print AST details, but also functions as a collection of clang utilities.