I'm following the instructions from the Emscripten documentation here, and I want to practice with minimal examples. My final goal is to create a C++ library, compile it to a single wasm binary file and then use these compiled methods in pure-frontend web application.
I do not understand what is the purpose of calling C++ methods with "ccall" when you can just instatiate them directly from "Module". For example, I can use Module._doubleNumber(9);, which is shorter than Module.ccall('doubleNumber','number', ['number'], [9]);.
Whats the difference between calling one or another?
Here is the complete code:
extern "C" {
int doubleNumber( int x ) {
int res = 2*x;
return res;
}
}
Compiled with
emcc main.cpp -o main.js -sEXPORTED_FUNCTIONS=_doubleNumber -sEXPORTED_RUNTIME_METHODS=ccall
I'm using a simple html document to test results:
<html>
<head>
<meta charset="utf-8">
<title>WASM Demo</title>
</head>
<body>
<script type="text/javascript" src="main.js"></script>
<script type="text/javascript">
Module.ccall('doubleNumber','number', ['number'], [9]); // 18
Module._doubleNumber(9); // 18
</script>
</body>
</html>
There is no difference in such the tiny example. There might be issues in big projects.
Module.ccall can perform async call with the parameter async=true, whereas call to the exported function Module._doubleNumber is always sync._doubleNumber will be changed to another name, usually unpredictable one, and Module._doubleNumber will be undefined.-sEXPORTED_FUNCTIONS=_doubleNumber prevents name minifying and can be used for small C libraries. A big C library would require a ton of items listed in EXPORTED_FUNCTIONS, and you shall take into account the limited command line length.