Can I get all custom section from a WebAssembly in Node.JS?

user_11191860 0
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I see there is way i can get a specific custom section with given name.

var nameSections = WebAssembly.Module.customSections(module, "sec_name");

Is there any way I can get all the custom sections in a given WebAssembly module?

webassembly rust node.js
1 Answers

The JavaScript interface at this time does not offer that. The customSection function was discussed and approved here. There is currently an opened issue to address the question you asked here. Generally this issue status at this time is that providing such an API would bring too much complexities (not so much about the sections as a list, but about the content of each). That makes sense to me, because the WebAssembly standard is currently in very active development (many post-MVP features are in progress).

That said, it seems that you are left off to parse the binary by yourself. The binary format is documented here. Basically, after you parse the magic number and the version there is a list of sections: 1 byte ID, one u32 (LEB128 format with allowed trailing zeroes) for the section's byte content length and then are the bytes of the section's content (with that length). After this header and the content bytes, is the next section, if any, till the end.

The custom sections have an id of 0. The content of these sections starts with a name and after it are the section "user" bytes. This name is a vector of bytes. Each vector has a length (again u32) followed by that number of bytes that must be the UTF-8 encoded string.

So the pseudo-code is something like this:

parse the magic 
parse the version
loop till there are bytes
  read the section id
  read the section u32 length
  if id = 0 then  
    read the name u32 length
    skip the name bytes
    use the user bytes till the end of the section
  jump to the end of the section
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