Platform: Deallocate roc data · beginners

I have no experience with system level programming at all. I am trying to get a wasm platform by try an error.

I have something that works, but I have a question about it. Here is the exported zig function:

export fn allocUint8(length: u32) [*]u8 {
    const slice = std.heap.page_allocator.alloc(u8, length) catch
        @panic("failed to allocate memory");

    return slice.ptr;
}

const RocList = struct { pointer: [*]u8, length: usize, capacity: usize };
extern fn roc__handlerForHost_1_exposed(*RocList, *RocList) void;

export fn run_roc(input: [*]u8, input_len: usize) [*]const u8 {
    defer std.heap.page_allocator.free(input[0..input_len]);

    var arg = RocList{ .pointer = input, .length = input_len, .capacity = input_len };

    // TODO: What should the pointer be for the empty callresult?
    // Is this on the stack or the heap? Do I have to deallocate?
    var callresult = RocList{ .pointer = input, .length = 0, .capacity = 0 };

    roc__handlerForHost_1_exposed(&callresult, &arg);

    return callresult.pointer;
}

First the wasm runtime calls allocUint8 to allocate some memory. Then it calls run_roc to run the roc code. The defer statement at the beginning makes sure, that the allocated memory gets freed (I hope).

But what about callresult? Roc writes a list of [*]u8 somewhere in the memory.I think, that it is on the heap. So do I have to free it? Can I do this in this function or has the wasm runtime call a free function afterwards?

Also, what should callback.pointer be initialized with? Currently, I use the input-pointer. But this seems like an hack.

Brendan Hansknecht (Jun 24 2023 at 16:15):

roc__handlerForHost_1_exposed just needs some stack space to write output data to

Brendan Hansknecht (Jun 24 2023 at 16:18):

yeah, call result should get freed, but isn't currently. Obviously it would have to be freed by the caller of run_roc, or it would need to be converted into a data structure with a different memory management plan such that it can be freed automatically.

Brendan Hansknecht (Jun 24 2023 at 16:19):

Do you have a link to the full code, I just want some more info about use and surrounding code.

Stream: beginners

Topic: Platform: Deallocate roc data

Oskar Hahn (Jun 24 2023 at 08:38):

Brendan Hansknecht (Jun 24 2023 at 16:15):

Brendan Hansknecht (Jun 24 2023 at 16:15):

Brendan Hansknecht (Jun 24 2023 at 16:18):

Brendan Hansknecht (Jun 24 2023 at 16:19):

Oskar Hahn (Jun 24 2023 at 21:35):