Randy Reddig (Aug 28 2024 at 17:30): Randy Reddig (Aug 28 2024 at 17:30):We’re working on wasip2 for Go (and TinyGo), and generate a component from a module using wasm-tools component new. When the same program is compiled and run in wasmtime under wasip1, the host calls _initialize. Under wasip2, when the generated component is executed, the _initialize export is not called.
What is the best/right way to have the runtime initialized when running as a component? What function can the module export that is guaranteed to be called before any other exports?
Also, how would this interact with the *-module compilation target? https://github.com/WebAssembly/component-model/pull/378
Thanks!
Joel Dice (Aug 28 2024 at 17:49):A start function would certainly work, as long as your init function doesn't need to call any imports. If it does need to call imports, I don't know that there's currently a component-level equivalent to _initialize. I know there's been talk about such a thing, but I don't know of anything official, yet.
Joel Dice (Aug 28 2024 at 17:52):The component tooling I'm familiar with for e.g. C, Rust, Python, and .NET either uses lazy initialization or build-time pre-initialization and snapshotting (via e.g. Wizer).
Randy Reddig (Aug 28 2024 at 17:57):_start?
Joel Dice (Aug 28 2024 at 17:58):https://webassembly.github.io/spec/core/valid/modules.html?highlight=start#start-function
Pat Hickey (Aug 28 2024 at 18:00):_start is the (unfortunately named, because of frequent confusion) entry point for wasi p1 commands
Pat Hickey (Aug 28 2024 at 18:00):whereas start is the wasm core spec construct
Randy Reddig (Aug 28 2024 at 18:43):Hm, tried start and _start and neither are called in wasip2 context
Luke Wagner (Aug 28 2024 at 19:10):I could totally be misremembering but @Alex Crichton I vaguely recall that the <prefix>_initialize function in the BuildTargets.md PR was a variation on something already implemented by wasm-tools component new... is that the case and, if so, is it not _initialize?
Alex Crichton (Aug 28 2024 at 19:15):ah no this is just in BuildTargets.md for now, wasm-tools component new doesn't have support for _initialize right now
Alex Crichton (Aug 28 2024 at 19:16):that being said it would be pretty easy to add support for it, basically just instantiate the "main module", setup tables, then have a core wasm start sectoin pointing to _initialize get instantiated
Joel Dice (Aug 28 2024 at 19:25):Randy Reddig said:
Hm, tried
startand_startand neither are called in wasip2 context
Just to be clear: I was referring to a core Wasm start function, not an export named start or _start. I'm having trouble finding a link in the spec the describes the semantics, but a module's start function, if present, is guaranteed to be run before any export is called.
Randy Reddig (Aug 28 2024 at 19:55):Alex Crichton said:
that being said it would be pretty easy to add support for it, basically just instantiate the "main module", setup tables, then have a core wasm start sectoin pointing to
_initializeget instantiated
That’d be great. It’d map to the WASI 0.1 behavior we see now. Are there any restrictions on what _initialize can do?
Randy Reddig (Aug 28 2024 at 21:16):I’m happy to try to make this work in wasm-tools if I know roughly where to start. Where would the call to _initialize be inserted?
Alex Crichton (Aug 28 2024 at 21:19):Oh that'd be awesome, thanks! I actually had just started to take a look at this and whipped up https://github.com/alexcrichton/wasm-tools/commit/a111b057d7c4b45ee2c1f501903cbcf66dd6382f so far, but that could use some double-checking and some real-world testing too (e.g. making sure it's working for Go modules). Would you be up for starting from that diff and finishing it up?
Randy Reddig (Aug 28 2024 at 22:46):Can confirm this works with TinyGo + wasip2!
image.png
Alex Crichton (Aug 29 2024 at 14:44):ok great! I'll look to clean up and send a PR today
Alex Crichton (Aug 29 2024 at 15:04):https://github.com/bytecodealliance/wasm-tools/pull/1747
Randy Reddig (Aug 29 2024 at 21:05):Thanks!
Last updated: Dec 06 2025 at 06:05 UTC