有没有更有效的方法将数组从C ++返回到javascript? [英] Is there a more efficient way to return arrays from C++ to javascript?
问题描述
要将键入的数组从脚本化的C ++传递给javascript，我想出了这段代码
To pass typed array from emscripten'ed C++ to javascript I came up with this code
#include <emscripten/bind.h>
#include <emscripten/val.h>
auto test(const emscripten::val &input) {
const auto data = emscripten::convertJSArrayToNumberVector<float>(input); // copies data
// generate output in some form
std::vector<float> output = { 1, 2, 3 };
// make a typed array view of the output
emscripten::val view{ emscripten::typed_memory_view(output.size(), output.data()) };
// create new typed array to return
auto result = emscripten::val::global("Float32Array").new_(output.size());
// copy data from generated output to return object
result.call<void>("set", view);
return result;
}
EMSCRIPTEN_BINDINGS(KissFft) {
emscripten::function("test", &test);
}
(使用 em ++ test.cpp -o test.html --bind 构建)
在这种情况下，有两个额外的副本:
In this case there are two extra copies:
- 据我所知，从输入数组到wasm内存的拷贝是不可避免的.
- the copy from input array to wasm memory, as far as I understand it's unavoidable;
const auto data = emscripten::convertJSArrayToNumberVector<float>(input);
emscripten::val view{ emscripten::typed_memory_view(output.size(), output.data()) };
auto result = emscripten::val::global("Float32Array").new_(output.size());
result.call<void>("set", view);
return result;
是否有一种方法可以避免在第二种情况下从生成的输出到javascript对象的多余副本?
我知道有可能返回这样的内存视图:
I'm aware of the possibility to return memory view like this:
std::vector<float> output;
auto test(const emscripten::val &input) {
const auto data = emscripten::convertJSArrayToNumberVector<float>(input);
//generate output
return emscripten::val{ emscripten::typed_memory_view(output.size(), output.data()) };
}
EMSCRIPTEN_BINDINGS(KissFft) {
emscripten::function("test", &test);
}
但是在这种情况下，返回的对象引用了 output 静态对象所拥有的基础内存，从而产生了所有后果，例如在C ++端修改内存，甚至取消分配内存.
But in this case the returned object refers to the underlying memory owned by output static object with all the consequences, like modifying the memory on the C++ side, or even deallocating it.
推荐答案
我当时想降低一点.在此处(而不是注释)粘贴代码片段(摘录自emscripten的embind示例，在C中，但您可以对C ++进行相同操作.)
I was thinking to go lower a bit. Putting here (instead of comment) to paste snippets (taken from embind examples from emscripten, here in C but you may do the same with C++).
// quick_example.cpp
#include <emscripten/bind.h>
#include <stdio.h>
using namespace emscripten;
struct buffer {
unsigned int pointer;
unsigned int size;
};
buffer lerp() {
buffer myBuffer;
unsigned int size = 10;
float * myTab = (float*)malloc(size * sizeof(float));
for (int i = 0; i < size; i++) {
myTab[i] = 2.5 * i;
printf(" Native side index: %d value: %f address: %p\n", i, myTab[i], &myTab[i]);
}
myBuffer.pointer = (unsigned int) myTab;
myBuffer.size = size;
printf(" Native side pointer: %p size: %d\n", myTab, size);
return myBuffer;
}
EMSCRIPTEN_BINDINGS(my_module) {
value_array<buffer>("buffer")
.element(&buffer::pointer)
.element(&buffer::size)
;
function("lerp", &lerp);
}
index.html-您可以在js中复制，查看并最终释放内存什么(据我了解，这是上面的问题之一?)
index.html - here in js you may copy, make a view and finally free the memory what (as I understood was one of the problems from above?)
<!doctype html>
<html>
<script>
var Module = {
onRuntimeInitialized: function() {
var result = Module.lerp();
console.log(" JS side pointer: " + result[0] + " size: " + result[1]);
for (var i = 0; i < result[1]; i++) {
console.log("index: " + i + " value: " + Module.HEAPF32[(result[0] + i * 4) / 4] + " pointerInc: " + (result[0] + i * 4));
}
Module._free(result[0]);
}
};
</script>
<script src="lerp.js"></script>
</html>
构建命令
emcc --bind -o lerp.js lerp.cpp
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