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wasi-sdk/CppExceptions.md

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Support for C++ Exceptions

The released artifacts for wasi-sdk at this time do not support C++ exceptions. LLVM and Clang, however, have support for C++ exceptions in WebAssembly and this is intended to serve as documentation of the current state of affairs of using C++ exceptions. It should be noted though that the current status of C++ exceptions support is not intended to be the final state of support, and this is all continuing to be iterated on over time.

Building wasi-sdk with exceptions

When building the sysroot with wasi-sdk you can pass -DWASI_SDK_EXCEPTIONS=ON to enable support for C++ exceptions. For example:

$ cmake -G Ninja -B build/sysroot -S . \
    -DCMAKE_TOOLCHAIN_FILE=$path/to/wasi-sdk-p1.cmake \
    -DWASI_SDK_EXCEPTIONS=ON

The C++ standard library will be compiled with support for exceptions for the desired targets and the resulting sysroot supports using exceptions.

Compiling code with C++ exceptions

Currently extra compilation flags are required to fully support C++ exceptions. Without these flags programs using C++ exceptions will not work correctly:

  • -fwasm-exceptions - needed to enable the WebAssembly exception-handling proposal.
  • -mllvm -wasm-use-legacy-eh=false - indicates that the standard WebAssembly exception-handling instructions should be used.
  • -lunwind - links in support for unwinding which C++ exceptions requires.

This can be specified for example with:

$ export CFLAGS="-fwasm-exceptions -mllvm -wasm-use-legacy-eh=false"
$ export LDFLAGS="-lunwind"

Limitations

Currently C++ exceptions support in wasi-sdk does not support shared libraries. Fixing this will require resolving some miscellaneous build issues in this repository itself.

Future Plans

There are a few tracking issues with historical discussion about C++ exceptions support in wasi-sdk such as #334 and #565. The major remaining items are:

  • Figure out support for shared libraries.
  • Determine how to ship a sysroot that supports both with-and-without exceptions.
  • Figure out how to avoid the need for extra compiler flags when using exceptions.
  • Figure out if a new wasm target is warranted.