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export_native_api.md: Partially revert "Relax the ground rule" (#1588)

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And add some clarifications
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YAMAMOTO Takashi 2022-10-13 19:33:06 +09:00 committed by GitHub
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@ -189,12 +189,22 @@ void foo2(wasm_exec_env_t exec_env,
The runtime builder should ensure not broking the memory sandbox when exporting the native function to WASM. The runtime builder should ensure not broking the memory sandbox when exporting the native function to WASM.
A ground rule:
- Do the pointer address conversion in the native API if "$\*" is not used for the pointer in the function signature
A few recommendations: A few recommendations:
- Never pass any structure/class object pointer to native (do data serialization instead) - Never pass any structure/class object pointer to native (do data serialization instead)
- Do the pointer address conversion in the native API if "$\*" is not used for the pointer in the function signature
- Never pass a function pointer to the native - Never pass a function pointer to the native
Note: while not recommended here, nothing prevents you from passing
structure/function pointers as far as the native API is aware of
and careful about the ABI used in the wasm module. For example,
C function pointers are usually represented as table indexes which
the native API can call with wasm_runtime_call_indirect() or similar.
However, in this document, we don't recommend to implement your native
API that way unless necessary because it needs extra carefulness.
## Pass structured data or class object ## Pass structured data or class object