How to pass strings between C++ and javascript via emscripten

问题

I am learning emscripten, and I can't even get the most basic string manipulation working, when passing strings between C++ and JS.

For example, I would like to write a string length function. In C++:

extern "C" int stringLen(std::string p)
{
    return p.length();
}

Called from javascript as:

var len = _stringLen("hi.");

This yields 0 for me. How do I make this work as expected? Which string type should I use here? char const*? std::wstring? std::string? None seem to work; I always get pretty random values.

This is only the beginning... How do I then return a string from C++ like this?

extern "C" char *stringTest()
{
    return "...";
}

And in JS:

var str = _stringTest();

Again, I cannot find a way to make this work; I always get garbage in JS.

So my question is clearly: How do I marshal string types between JS and C++ via Emscripten?

回答1:

extern "C" doesn't recognize std::string.

You may want to try this:
Test.cpp

#include <emscripten.h>
#include <string.h>

extern "C" int stringLen(char* p)
        {
            return strlen(p);
        }

Use the following command to compile the cpp code :

emcc Test.cpp -s EXPORTED_FUNCTIONS="['_stringLen']

Sample test code :
Test.html

<!DOCTYPE html>
<html lang="en">
    <head>
        <meta charset="utf-8">
        <title>Hello World !</title>
        <script src="a.out.js"></script>
        <script>
             var strLenFunction =  Module.cwrap('stringLen', 'number', ['string']);
             var len1 = strLenFunction("hi.");  // alerts 3
             alert(len1);
             var len2 = strLenFunction("Hello World"); // alerts 11
             alert(len2);
        </script>
    </head>
</html>

回答2:

If you use extern "C" with a function, you cannot use C++ types in it's signature.

So if you want to use std::string, then you can use "Embind" or "WebIDL Binder". Refer here

I prefer embind, so this is a sample code for your problem.

P.S I am not sure how to pass variables by reference here, so doing it by value.

// This is your routine C++ code
size_t MyStrLen(std::string inStr) {
    return inStr.length();
}

// This is the extra code you need to write to expose your function to JS
EMSCRIPTEN_BINDINGS(my_module) {
    function("MyStrLen", &MyStrLen);
}

Now in JS all you need to do is:

var myStr = "TestString";
Module.MyStrLen(myStr);

Make sure you pass the flag

--bind

when calling emcc.

There is another approach where you can do a Malloc on the C++ heap from JS and then do manipulations, but the above approach should be easier.

回答3:

A few thoughts:

The only way I have called a method is by using crwap or ccall?
var length = Module.ccall('stringLen', ['string'], 'number');
Are you including stringLen and stringTest in your EXPORTED_FUNCTIONS parameter?
emcc hello_world.cpp ... -s EXPORTED_FUNCTIONS=['_stringLen','_stringTest']

Take a look here for more details:
http://kripken.github.io/emscripten-site/docs/porting/connecting_cpp_and_javascript/Interacting-with-code.html

Or my hello_world tutorial:
http://www.brightdigit.com/hello-emscripten/

Hopefully that helps.

来源：https://stackoverflow.com/questions/21816960/how-to-pass-strings-between-c-and-javascript-via-emscripten

标签

javascript

c++

emscripten