Emscripten: Memory leak when a C function wants you to free its return value

[mvasilkov]

Hi all, I'm porting the cmark markdown implementation to Node.js using Emscripten, and this is what I found.

There's a C function returning a char* string. The C function states in its docs that the caller should free the return value.

But in ccall, specifically in this line, we're discarding the pointer to the return value without calling _free. This leads to a memory leak, I have tested it.

When I replace this line with something more elaborate, like this:

if (returnType === 'string') {
    let foo = Pointer_stringify(ret)
    _free(ret)
    ret = foo
}

Everything is great again. (But I assume this patch can break someone else's C function.)

What do?

[kripken]

In such a case, you need to call free yourself after doing ccall. Something like this:

var ptr = ccall('foo');
// use ptr
Module._free(ptr);

That basically does what a C program would do for that API: call it, then free it.

[mvasilkov]

@kripken Hi! Yep, thanks for the tip. The thing is, ccall() doesn't return the pointer, instead it converts it to a string and throws away. So I ended up patching the function.

[kripken]

Oh, now I understand. Yeah, since ccall converts it, the caller can't free...

We should probably add an optional parameter to ccall, or something like that. Reopening.

[lormuc]

@kripken
If the caller wants to the memory to be released, they can specify number instead of string as the return type, convert the returned pointer to a string manually with pointer_stringify(), and then free() it. Is the optional parameter to do that automatically really needed?

[kripken]

@asnasnasn yeah, good point, with some extra work this is already possible. Probably not worth a new option, then, I agree.

[stale[bot]]

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