Zig/WASM - std.Io.Reader.fixed.takeDelimiter issue

Help
1

Hi guys,

I stumbled upon a strange issue in zig/wasm and I need big brain support.

When I send a string from javascript to zig, the new std.Io.Reader.fixed is not able to find anything using takeDelimiter.

Previously, instead of transferring the string via javascript to zig/wasm I had the string embedded and std.Io.Reader.fixed takeDelimiter worked fine, so I knew should work.

So I thought that it’s the transferred data, but after validating the string - as far as I can tell it’s fine - I tried the old std.io.reader and I was able to break up the input into lines. I didn’t want to rewrite it with the old interface though.

On a whim, I tried using std.heap.wasm_allocator.dupe to copy the string before passing it to the new reader interface and takeDelimiter worked again, so my javascript->wasm works now, but I have no idea why I need to copy the data for the new reader.

Has anybody an idea why that might be the case?
Do I do something wrong?

source: aoc2025/src/wasm.zig at master · 8/aoc2025 · GitHub
the hosted wasm: https://8.github.io/aoc2025/

Thank you!

Take care,
Martin

2

Not sure if this is the issue, but it might be related to where you place the string in memory. It seems like you are placing it at offset 0, which may cause issue. When you dupe it, it places it elsewhere in memory. You could try figureing out an offset where it works vs where it doesn’t.

3

Muchos gracias, I will try that out!