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3.1 main function and lifecycle

When native code is generated, as the entry point function of a C/C++ program, main() usually stands for the lifecycle of the program, which terminates as main() returns. Under Emscripten, the situation is different, as the example shows:

//main.cc
#include <stdio.h>

EM_PORT_API(int) show_me_the_answer() {
	return 42;
}

int main() {
	printf("你好，世界！\n");  //"你好，世界！" means "Hello, wolrd!" in chinese
	return 0;
}

HTML:

//main.html
  <body>
	<button id = btn_test onclick=Test() disabled = true>test</button>
	<script>
	function Test(){
		console.log(Module._show_me_the_answer());
	}	
	Module = {};
	Module.onRuntimeInitialized = function() {
		var btn = document.getElementById("btn_test");
		btn.disabled = false;
	}
	</script>
	<script src="main.js"></script>
  </body>

Upon loading the page, main() is executed, as you can see the "你好，世界！"(which means "Hello, wolrd!" in chinese) output to the console:

At this time, if you click the "test" button, the console outputs:

After main() has returned, Emscripten runtime core(Module) is still usable! Also in previous chapters, some examples don't even have main() functions. It is evident that to Emscripten, main() function is neither indispensable nor controlling the lifecycle.

To destruct the Emscripten runtime after main() returns, use the -s NO_EXIT_RUNTIME=0 option when compiling:

emcc main.cc -s NO_EXIT_RUNTIME=0 -o main.js

Compiled with the above command, the page outputs an error when the "test" button is clicked:

info Starting from Emscripten v1.37.26, NO_EXIT_RUNTIME defaults to 1.