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Dynamic linking

WebAssembly enables load-time and run-time (dlopen) dynamic linking in the MVP by having multiple instantiated modules share functions, linear memories, tables and constants using module imports and exports. In particular, since all (non-local) state that a module can access can be imported and exported and thus shared between separate modules' instances, toolchains have the building blocks to implement dynamic loaders.

Since the manner in which modules are loaded and instantiated is defined by the host environment (e.g., the JavaScript API), dynamic linking requires use of host-specific functionality to link two modules. At a minimum, the host environment must provide a way to dynamically instantiate modules while connecting exports to imports.

The simplest load-time dynamic linking scheme between modules A and B can be achieved by having module A export functions, tables and memories that are imported by B. A C++ toolchain can expose this functionality by using the same function attributes currently used to export/import symbols from native DSOs/DLLs:

#ifdef _WIN32
#  define EXPORT __declspec(dllexport)
#  define IMPORT __declspec(dllimport)
#else
#  define EXPORT __attribute__ ((visibility ("default")))
#  define IMPORT __attribute__ ((visibility ("default")))
#endif

typedef void (**PF)();

IMPORT PF imp();
EXPORT void exp() { (*imp())(); }

This code would, at a minimum, generate a WebAssembly module with imports for:

  • the function imp
  • the heap used to perfom the load, when dereferencing the return value of imp
  • the table used to perform the pointer-to-function call

and exports for:

  • the function exp

A more realistic module using libc would have more imports including:

  • an immutable i32 global import for the offset in linear memory to place global data segments and later use as a constant base address when loading and storing from globals
  • an immutable i32 global import for the offset into the indirect function table at which to place the modules' indirectly called functions and later compute their indices for address-of

One extra detail is what to use as the module name for imports (since WebAssembly has a two-level namespace). One option is to have a single default module name for all C/C++ imports/exports (which then allows the toolchain to put implementation-internal names in a separate namespace, avoiding the need for __-prefix conventions).

To implement run-time dynamic linking (e.g., dlopen and dlsym):

  • dlopen would compile and instantiate a new module, storing the compiled instance in a host-environment table, returning the index to the caller.
  • dlsym would be given this index, pull the instance out of the table, search the instances's exports, append the found function to the function table (using host-defined functionality in the MVP, but directly from WebAssembly code in the future 🦄) and return the table index of the appended element to the caller.

Note that the representation of a C function-pointer in WebAssembly is an index into a function table, so the above scheme lines up perfectly with the function-pointer return value of dlsym.

More complicated dynamic linking functionality (e.g., interposition, weak symbols, etc) can be simulated efficiently by assigning a function table index to each weak/mutable symbol, calling the symbol via call_indirect on that index, and mutating the underlying element as needed.

After the MVP, we would like to standardize a single ABI per source language, allowing for WebAssembly libraries to interface with each other regardless of compiler. Specifying an ABI requires that all ABI-related future features (like SIMD, multiple return values and exception handling) have been implemented. While it is highly recommended for compilers targeting WebAssembly to adhere to the specified ABI for interoperability, WebAssembly runtimes will be ABI agnostic, so it will be possible to use a non-standard ABI for specialized purposes.