[cfe-dev] Clang C interface `clang_visitChildren` problems

[Alex L via cfe-dev]

Hi,


On 15 March 2017 at 03:24, freya zhou via cfe-dev <cfe-dev at lists.llvm.org>
wrote:

>
> hi,
>
> I’m using a Clang C interface clang_visitChildren to traverse the
> abstract syntax tree in Clang. But I get following problems :
>
>    1. Get operators in the expression :
>    For expressions like a+b, I can get the current cursor information
>    (name, kind, type) and its child node info, but I don’t know how can I get
>    the operator ‘+’ ? Should I use token to get the operators in expression?
>    If so, please tell me how can I use token to get operators.
>
> I scanned the API but it seems like there's no way to get the operator
kind right now. I might be wrong though.

You could probably try to get the operator kind by looking at the tokens,
yeah. For binary operators you could try tokenizing the range from the end
of LHS to the start of RHS, and look at the first token, e.g:

CXSourceRange LHS_Range = clang_getCursorExtent(BinOp_LHS);
CXSourceRange RHS_Range = clang_getCursorExtent(BinOp_RHS);
CXToken *Tokens; unsigned NumTokens;
clang_tokenize(TU, clang_getRange(clang_getRangeEnd(BinOp_LHS),
clang_getRangeStart(BinOp_RHS)),
                                   &Tokens, &NumTokens);
clang_getTokenSpelling(Tokens[0]); // Should be '+'

Note that this will only work for simple expressions: it will not work
correctly when the operator is in a macro. It might not work in some C++
cases as well.

>
>    1. Get the value of constants :
>    For expressions like int a =10, for the node 10, I can get the result
>    by using clang_Cursor_Evaluate interface, and then call the function
>    clang_EvalResult_getAsInt to get the integer value 10. But, for
>    expressions like int a = 2+8, I got 2+8 and keep traverse down, using
>    the same approach, then I got two 0. Did get the value by a wrong
>    way？What’s the correct way to get the value?
>
> Libclang should be able to evaluate 2+8. Did you try stopping at 2+8 and
then evaluating that cursor?

>
>    1. Location order of the child node in the abstract syntax tree :
>    If I have two statement for (; ; a) {...} and for (; a; ) {...}, I use
>    clang_visitChildren to traverse them, then I got two subtree:
>    UnexposedExpr and CompoundStmt, How can I separate the two statements?
>    And why did I get UnexposedExpr, what does it means?
>
> Unexposed expression is a special type of expression that is not currently
supported by the C API. In other words, while the actual AST might be a
valid expression like an Objective-C array literal, libclang's C API can't
represent that expression type in terms of CXCursor, so it falls back to
using CXCursor_UnexposedExpr.

In your particular example I think the unexposed expression corresponds to
the 'a'. What do you mean by separating the two statements?



> I don't know how to solve these problems, I hope that someone can help.
> Thank you in advance, Freya
>
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