C setjmp-longjmp Support

setjmp-longjmp support is enabled by default in Emscripten. This is controlled by the SUPPORT_LONGJMP setting, which can take these values:

  • emscripten: JavaScript-based support

  • wasm: WebAssembly exception handling-based support

  • 0: No support

  • 1: Default support (currently emscripten) (default)

Currently -sSUPPORT_LONGJMP=1 is the same as -sSUPPORT_LONGJMP=emscripten, and turned on by default. This default will eventually be the new wasm when most browsers support it.

setjmp saves information about the calling environment into a buffer, and longjmp transfers the control back to the point where setjmp was called by using the buffer. longjmp’s call stack should contain the function from which setjmp was called.

Emscripten’s support has a restriction that indirect calls to setjmp are not supported. For example, the below does not work:

jmp_buf env;
int (*fp)(jmp_buf) = setjmp;
fp(env); // Doesn't work

JavaScript-based setjmp-longjmp Support

In this mode, Emscripten emulates setjmp-longjmp using JavaScript. This option is set by adding -sSUPPORT_LONGJMP=emscripten to the command line, but this is currently enabled by default.

Note that this option can have relatively high overhead in terms of code size, but it will work on all JavaScript engines with WebAssembly support, even if they do not yet support the new WebAssembly exception handling proposal.

WebAssembly Exception Handling-based setjmp-longjmp Support

Alternatively, you can opt-in to the new support using the WebAssembly exception handling proposal. To enable it, pass -sSUPPORT_LONGJMP=wasm at both compile time and link time.

This option leverages a new feature that brings built-in instructions for throwing and catching exceptions to WebAssembly. As a result, it can reduce code size and performance overhead compared to the JavaScript-based implementation. This option is currently supported in several major web browsers, but may not be supported in all WebAssembly engines yet.

Using Exceptions and setjmp-longjmp Together

We also have two kinds of exception handling support: JavaScript-based support and the new WebAssembly EH-based support. Our setjmp-longjmp support use the same mechanisms. Because of that, you should use the same kind of EH and setjmp-longjmp support when using exceptions and setjmp-longjmp together.

For example, to use the JavaScript-based EH and setjmp-longjmp support together:

em++ -fexceptions test.cpp -o test.js

-sSUPPORT_LONGJMP=1, which defaults to emscripten, is enabled by default, so you don’t need to pass it explicitly.

To use the WebAssembly EH and setjmp-longjmp support together:

em++ -fwasm-exceptions -sSUPPORT_LONGJMP=wasm test.cpp -o test.js

There is one specific restriction for using WebAssembly EH-based support for exceptions and setjmp-longjmp at the same time. You cannot call setjmp within a C++ catch clause. For example, the following will error out during compile time:

try {
  ...
catch (int n) {
  setjmp(buf); // Doesn't work
}

Calling setjmp within a try clause is fine. Calling another user function that calls setjmp within a catch clause is also fine.

try {
  setjmp(buf); // Works
catch (int n) {
  ...
}

try {
  ...
} catch (int n) {
  function_that_calls_setjmp(); // Works
}