Add helper function for C++ exception formatting

emcc.py

@@ -2074,6 +2074,15 @@ def default_setting(name, new_default):
   else:
     settings.JS_LIBRARIES.append((0, 'library_pthread_stub.js'))
 
+  if (
+      settings.FORMAT_EXCEPTION_SUPPORT or
+      "$formatException" in settings.DEFAULT_LIBRARY_FUNCS_TO_INCLUDE or
+      settings.INCLUDE_FULL_LIBRARY
+  ):
+    settings.DEFAULT_LIBRARY_FUNCS_TO_INCLUDE += ["$formatException"]
+    settings.EXPORTED_FUNCTIONS += ["_emscripten_format_exception", "_free"]
+    settings.EXPORTED_RUNTIME_METHODS += ["formatException"]
+
   if settings.FORCE_FILESYSTEM and not settings.MINIMAL_RUNTIME:
     # when the filesystem is forced, we export by default methods that filesystem usage
     # may need, including filesystem usage from standalone file packager output (i.e.

src/library_exceptions.js

@@ -428,6 +428,20 @@ var LibraryExceptions = {
     catchInfo.free();
     {{{ makeThrow('ptr') }}}
   },
+
+#if !DISABLE_EXCEPTION_CATCHING
+  $formatException__deps: ["emscripten_format_exception", "$withStackSave", "free"],
+  $formatException: function(excPtr){
+      return withStackSave(function(){
+      var result_ptr = stackAlloc({{{ POINTER_SIZE }}});
+      _emscripten_format_exception(result_ptr, excPtr);
+      var utf8_addr = {{{ makeGetValue('result_ptr', '0', '*') }}};
+      var result = UTF8ToString(utf8_addr);
+      _free(utf8_addr);
+      return result;
+    });
+  },
+#endif
 };
 
 // In LLVM, exceptions generate a set of functions of form __cxa_find_matching_catch_1(), __cxa_find_matching_catch_2(), etc.

src/settings.js

@@ -339,6 +339,13 @@ var EXCEPTION_DEBUG = 0;
 // [link]
 var DEMANGLE_SUPPORT = 0;
 
+// If 1, expose a formatException function that can be used to format a top
+// level C++ exception. If the thrown exception was
+// throw new std::runtime_error("a big problem occurred")
+// It returns a string like: "Cpp Exception std::runtime_error: A big problem occured."
+// [link]
+var FORMAT_EXCEPTION_SUPPORT = 0;
+
 // Print out when we enter a library call (library*.js). You can also unset
 // Runtime.debug at runtime for logging to cease, and can set it when you want
 // it back. A simple way to set it in C++ is

system/lib/libcxxabi/src/format_exception.cpp

@@ -0,0 +1,47 @@
+#include "cxa_exception.h"
+#include "cxxabi.h"
+#include <stdio.h>
+#include <typeinfo>
+
+#ifdef __USING_EMSCRIPTEN_EXCEPTIONS__
+
+extern "C" {
+#define DEMANGLED_BUF_SIZE 100
+
+int __cxa_can_catch(const std::type_info* catchType,
+                    const std::type_info* excpType,
+                    void** thrown);
+
+int emscripten_format_exception(char** result, void* exc_ptr) {
+  __cxxabiv1::__cxa_exception* exc_info =
+    (__cxxabiv1::__cxa_exception*)exc_ptr - 1;
+  std::type_info* exc_type = exc_info->exceptionType;
+  const char* exc_name = exc_type->name();
+
+  int status = 0;
+  char* demangled_buf = __cxxabiv1::__cxa_demangle(exc_name, 0, 0, &status);
+  if (status == 0 && demangled_buf) {
+    exc_name = demangled_buf;
+  }
+
+  int can_catch = __cxa_can_catch(&typeid(std::exception), exc_type, &exc_ptr);
+  int ret;
+  if (can_catch) {
+    const char* exc_what = ((std::exception*)exc_ptr)->what();
+    ret = asprintf(result, "Cpp Exception %s: %s", exc_name, exc_what);
+  } else {
+    ret = asprintf(result,
+                   "Cpp Exception: The exception is an object of type '%s' at "
+                   "address %p which does not inherit from std::exception",
+                   exc_name,
+                   exc_ptr);
+  }
+
+  if (demangled_buf) {
+    free(demangled_buf);
+  }
+  return ret;
+}
+}
+
+#endif // __USING_EMSCRIPTEN_EXCEPTIONS__

tests/test_core.py

@@ -1543,6 +1543,65 @@ def test_exceptions_rethrow_missing(self):
     create_file('main.cpp', 'int main() { throw; }')
     self.do_runf('main.cpp', None, assert_returncode=NON_ZERO)
 
+  def test_format_exception(self):
+    # needs to flush stdio streams
+    self.set_setting('EXIT_RUNTIME')
+    self.set_setting('EXPORTED_FUNCTIONS', ['_main', '_throw_exc'])
+    self.set_setting('FORMAT_EXCEPTION_SUPPORT')
+    self.set_setting('DISABLE_EXCEPTION_CATCHING', 0)
+    self.maybe_closure()
+    self.do_run(
+      """
+      #include <emscripten.h>
+      #include <exception>
+      #include <stdexcept>
+      using namespace std;
+
+      class myexception : public exception
+      {
+        virtual const char* what() const throw() { return "My exception happened"; }
+      } myex;
+
+      extern "C" void
+      throw_exc(int x)
+      {
+        if (x == 1) {
+          throw 1000;
+        }
+        if (x == 2) {
+          throw 'c';
+        }
+        if (x == 3) {
+          throw runtime_error("abc");
+        }
+        if (x == 4) {
+          throw myex;
+        }
+        if (x == 5) {
+          throw "abc";
+        }
+      }
+
+      int
+      main(){
+          EM_ASM({
+            for(let i = 1; i < 6; i++){
+              try {
+                  Module["_throw_exc"](i);
+              } catch(p){
+                  console.log(Module["formatException"](p).replace(/0x[0-9a-f]*/, "xxx"));
+              }
+            }
+          });
+      }
+      """,
+      "Cpp Exception: The exception is an object of type 'int' at address xxx which does not inherit from std::exception\n"
+      "Cpp Exception: The exception is an object of type 'char' at address xxx which does not inherit from std::exception\n"
+      "Cpp Exception std::runtime_error: abc\n"
+      "Cpp Exception myexception: My exception happened\n"
+      "Cpp Exception: The exception is an object of type 'char const*' at address xxx which does not inherit from std::exception\n"
+    )
+
   @with_both_eh_sjlj
   def test_bad_typeid(self):
     self.do_run(r'''

tools/system_libs.py

@@ -1127,7 +1127,8 @@ def get_files(self):
       'stdlib_exception.cpp',
       'stdlib_stdexcept.cpp',
       'stdlib_typeinfo.cpp',
-      'private_typeinfo.cpp'
+      'private_typeinfo.cpp',
+      'format_exception.cpp'
     ]
     if self.eh_mode == Exceptions.NONE:
       filenames += ['cxa_noexception.cpp']