[cfe-dev] [RFC] clang support for API information generation in JSON

Zixu Wang via cfe-dev cfe-dev at lists.llvm.org
Mon Sep 6 17:30:08 PDT 2021

> On Sep 1, 2021, at 11:46 PM, Petr Hosek <phosek at chromium.org> wrote:
> In Fuchsia, we have been using clang-doc (https://clang.llvm.org/extra/clang-doc.html <https://clang.llvm.org/extra/clang-doc.html>) for this purpose (using the YAML output format). Would it be possible to use clang-doc for your purposes?

We’ve looked into clang-doc when planning and designing clang-extract-api. Our conclusion is that though clang-doc looks great and works by extracting information from the clang AST, it still does not fit the purpose of clang-extract-api 100%, as it’s focus is on documentation generation. And we think it’s okay to have two separate tools in this case. In the future more fields might become of-interest for one tool but not the other and it’s good to have the input/output conventions separated.
Another reason is that we would like to implement the core functionalities of clang-extract-api in libclang so that it could enable more use cases. Clang-doc lives in clang-tools-extra and is more like an endpoint tool. One possibility is that we could refactor clang-doc to re-use the AST visitor implemented by clang-extract-api (in libclang) and have the two tools process and serialize the extracted information in their own ways.


> You might need to extend the output format to include additional information but that should be quite straightforward.
> On Wed, Sep 1, 2021 at 11:19 AM Zixu Wang via cfe-dev <cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org>> wrote:
> Hi All!
> I’m writing to propose clang-extract-api, a tool to collect and serialize API information from header files, for example function signatures, Objective-C interfaces and protocols, and inline documentation comments. We hope it could help future tools to understand clang-based language APIs without needing to dig into the AST themselves.
> Background
> Motivation
> Library and SDK providers may find it useful to be able to create and inspect a “snapshot” of APIs they expose, for example, to check for API/ABI-breaking changes between two versions, or to automate generating documentation for the APIs. Here is a list of examples of information we want to extract from APIs:
> 	• the name (spelling/mangled) of the symbol;
> 	• the unique identifier of the symbol, for example the Unified Symbol Resolution (USR);
> 	• the source location of the API declaration (file, line, column);
> 	• access control of the API (public/private/protected);
> 	• availability (available/unavailable/deprecated);
> 	• function signatures (return/parameters);
> 	• documentation comments attached to a symbol;
> 	• relations with other symbols (class methods, typedef relations, struct data fields, enum constants, etc.)
> Since these API information is available in the header files, which declare and distribute the APIs, we can implement a tool to extract them without invoking a compilation of the whole project to enable easy access to the information for tooling.
> Existing solutions
> While there are some existing solutions in clang to dump symbols or AST information, they either expose unnecessary low-level details or fail to provide enough information of APIs. For example, clang -ast-dump dumps low-level details for all declarations for debug purposes and the output is not machine-parsable. Doxygen also extracts documentation comments and other information from API declarations, but its output is rendered documentation in web formats which is not flexible for other uses and tools.
> Proposal
> We propose to implement this tool as a new frontend action invoked by `clang -extract-api` as show in the example below.
> clang -extract-api \
>   header.h [more_header.h ...] or a filelist\
>   -isysroot <SDK> \
>   -target <TARGET> \
>   -isystem <SYS INCLUDE PATH> \
>   ...
>   -o output.json
> It takes in the header file(s) or a filelist file containing paths to the header file(s) as the input. The header files will be parsed by clang and the extract-api action will visit the AST to extract needed information and serialize to a JSON output. Please find an example input and output attached.
> The example output is based on the symbol graph format that's already used by Swift for serializing symbol information and their relations. This format can represent the required API information and is flexible and extendable as demonstrated in the example so we think it's a good starting point.
> We are excited about this idea and its potential uses, and we’d love to hear feedback and suggestions!
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