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

[Zixu Wang via cfe-dev]

> On Sep 1, 2021, at 11:26 AM, Tom Stellard <tstellar at redhat.com> wrote:
> 
> On 9/1/21 11:18 AM, Zixu Wang via cfe-dev 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.
> 
> Would this tool be able to provide the same functionality as tools
> like abi-compliance-checker[1] and libabigail[2], that extra ABI/API
> information from debuginfo?
> 
> -Tom
> 
> [1] https://github.com/lvc/abi-compliance-checker
> [2] https://sourceware.org/libabigail/

Hi Tom!

I’m not really familiar with those tools but a brief look seems to suggest that they operate on compiled binaries. We propose clang-extract-api to directly work on and extract information from the parsed AST of the header source files so I’m not quite sure how to compare these tools. Debuginfo lives in the compiled binary so clang-extract-api doesn’t look into it. However, anything about the API described in the header file could potentially be extracted.

Do you have a specific example of information you want to get from abi-compliance-checker and libabigail?

Zixu

> 
>> *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> \
>>   -I <INCLUDE PATH> \
>>   -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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>