Module sections

A module is made up of several sections, some of which you'll be interacting with through the JavaScript API:

  • Imports (import) are elements that can be accessed within the module and can be one of the following:
    • Function, which can be called inside the module using the call operator
    • Global, which can be accessed inside the module via the global operators
    • Linear Memory, which can be accessed inside the module via the memory operators
    • Table, which can be accessed inside the module using call_indirect
  • Exports (export) are elements that can be accessed by the consuming API (that is, called by a JavaScript function)
  • Module start function (start) is called after the module instance is initialized
  • Global (global) contains the internal definition of global variables
  • Linear memory (memory) contains the internal definition of linear memory with an initial memory size and optional maximum size
  • Data (data) contains an array of data segments which specify the initial contents of fixed ranges of a given memory
  • Table (table) is a linear memory whose elements are opaque values of a particular table element type:
    • In the MVP, its primary purpose is to implement indirect function calls in C/C++
  • Elements (elements) is a section that allows a module to initialize the elements of any import or internally defined table with any other definition in the module
  • Function and code:
    • The function section declares the signatures of each internal function defined in the module
    • The code section contains the function body of each function declared by the function section

Some of the keywords (import, export, and so on) should look familiar; they're present in the contents of the Wat file in the previous section. WebAssembly's components follow a logical mapping that directly correspond to the APIs  (for example, you pass a memory and table instance into JavaScript's WebAssembly.instantiate() function). Your primary interaction with a module in binary format will be through these APIs.

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