anb (Jun 12 2023 at 22:38): anb (Jun 12 2023 at 22:38):Currently it is defined as u32 in wasmtime: https://github.com/bytecodealliance/wasmtime/blob/v9.0.0/crates/wasi-common/src/table.rs#L16, but in wasi, the raw fd is i32 https://github.com/rust-lang/rust/blob/master/library/std/src/os/wasi/io/raw.rs#L20. I wonder if there's a reason behind this mismatch, and should we lean to one instead of having both? Thanks.
Dan Gohman (Jun 12 2023 at 22:40):On the Wasm side of things, u32 makes slightly more sense because it's an index into a table, it's never negative, and in theory there's no fundamental problem with having 1<<31 handles live at a time.
Dan Gohman (Jun 12 2023 at 22:40):On the C ABI side of things, i32 makes slightly more sense because it's the type that corresponds to the C int type, which is what Unix uses for file descriptors, and thus what a lot of existing code is expecting.
Dan Gohman (Jun 12 2023 at 22:42):Rust's std::os::wasi follows the C ABI side of things, because existing Rust code may be similarly expecting it to be i32.
Dan Gohman (Jun 12 2023 at 22:43):In fact, I think on the Rust side RawFd used to be u32, but we changed it to i32 for better compatibility.
anb (Jun 12 2023 at 22:48):It doesn't look good to cast u32 to i32 from host calls, but using a negative fd in wasi should "just work" I guess, unless some lib has the assumption you mentioned. I guess the only issue is, without multiple return value(well, having std recompiled is not always an option), it's going to be tricky for host functions to return a fd or an err.
Dan Gohman (Jun 13 2023 at 00:41):WASI interfaces have never used -1 to indicate an invalid file descriptor. In Preview1 witx they use (result $error (expected $fd (error $errno))) which lowers to a __wasi_fd_t *retptr0 outparam.
Dan Gohman (Jun 13 2023 at 00:42):In Preview2, they use a result as well.
Last updated: Dec 06 2025 at 06:05 UTC