By default, exception catching is disabled in Emscripten.
For example, if you compile the following program:
#include <stdio.h>
int main() {
try {
puts("throw...");
throw 1;
puts("(never reached)");
} catch(...) {
puts("catch!");
}
return 0;
}
The first throw will abort the program and you’ll see something like this in the output:
throw...
exception thrown: 5246024 - Exception catching is disabled, this exception cannot be caught. Compile with -sNO_DISABLE_EXCEPTION_CATCHING or -sEXCEPTION_CATCHING_ALLOWED=[..] to catch.
If you want to opt-in, you have two following options.
First, you can enable exceptions via Emscripten’s JavaScript-based support.
To enable it, pass -fexceptions at both compile time and link time.
When you rebuild the example above with this flag, the output will change to:
throw...
catch!
Note that this option has relatively high overhead, but it will work on all JavaScript
engines with WebAssembly support. You can reduce the overhead by specifying a
list of allowed functions in which exceptions are enabled, see the
EXCEPTION_CATCHING_ALLOWED setting.
Alternatively, you can opt-in to the native WebAssembly exception handling proposal.
To enable it, pass -fwasm-exceptions at both compile time and link time.
Rebuilding the example with this flag will result in the same output as with
-fexceptions above:
throw...
catch!
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. However, it’s still brand-new and not yet supported by default in most engines.