Scott Waye (May 04 2023 at 23:31): Scott Waye (May 04 2023 at 23:31):mmap seems to be in both the top half and bottom half of wasi-sdk's muslc https://github.com/WebAssembly/wasi-libc/blob/1dfe5c302d1c5ab621f7abf04620fae92700fd22/libc-top-half/musl/src/mman/mmap.c and https://github.com/WebAssembly/wasi-libc/blob/1dfe5c302d1c5ab621f7abf04620fae92700fd22/libc-bottom-half/mman/mman.c . The top half is a weak reference so does that means it's not used, in which case why does it exist?
Dan Gohman (May 04 2023 at 23:33):The top half code is there because we didn't go through musl and delete everything that we're not using. It's disabled either via #ifdef or via the Makefile.
Scott Waye (May 04 2023 at 23:34):Thank you!
Notification Bot (May 04 2023 at 23:34):Scott Waye has marked this topic as resolved.
Dan Gohman (May 04 2023 at 23:34):WASI and Wasm don't have a real mmap, so musl's mmap code which just expects to invoke a system call isn't usable. In the bottom half code, we have code that emulates some amount of mmap functionality using malloc + read
Scott Waye (May 04 2023 at 23:41):Yes, I'm looking at the bottom half and the fact that it injects a struct at the start, which presumably is why it doesn't attempt to do the page aligning
Scott Waye (May 04 2023 at 23:41):It could align internally, but the value returned would not be aligned as the struct would offset the usable space.
Scott Waye (May 04 2023 at 23:42):I guess it could, wastefully, allocate an extra page which just contained the struct.
Jesús González (May 10 2023 at 08:39):@Scott Waye , yes, that's it. Getting non-aligned memory can be problematic or not depending on your needs. Also, the page size in Wasm is 64KiB (vs. 4KiB in x86_64), so the amount of wasted memory would be quite big.
Last updated: Dec 23 2024 at 12:05 UTC