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, depending on the exception mode. wasm if -fwasm-exception is used, emscripten otherwise.
If native Wasm exceptions
are used, SUPPORT_LONGJMP defaults to wasm, and if JavaScript-based
exceptions are used or no exception
support is used, it defaults to emscripten.
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
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.
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.
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, which defaults to emscripten or wasm depending on
the exception mode, 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
}