dlopen + pthreads with FS not working

[ravisumit33]

Please include the following in your bug report:

Version of emscripten/emsdk:

emcc (Emscripten gcc/clang-like replacement + linker emulating GNU ld) 3.1.35 (a90c5abb489c516f59c0d16864d953d4bedd37da)
clang version 17.0.0 (https://github.com/llvm/llvm-project 6865cff8ea8b07d9f2385fd92cecb422404f0f35)
Target: wasm32-unknown-emscripten
Thread model: posix
InstalledDir: ~/emsdk/upstream/bin

We currently do runtime dynamic linking (dlopen) in our app which is single-threaded. For dlopen to work, we have used FS.createLazyFile for all the side modules in Module.preInit. When dlopen is called side-modules are downloaded and used. Now, we are trying to enable pthreads in our app. When dlopen is called on a thread other than main application thread (on which we have mounted the side modules), FS is unable to find these mounted side modules and hence our whole logic fails. Does FS even work with pthreads? Or is there any alternate way of using dlopen with pthreads?

[ravisumit33]

@sbc100 Thoughts on this?

[ravisumit33]

@kripken @sbc100 Please confirm if FS does supoort multithreading. If not, what are other options to try for the scenario described above?

[sbc100]

The FS object only exists (or rather is only usable) on the main thread.

However, all filesystem operations in C/C++ are proxied back to the main thread. In particular with dlopen the operation is always proxied back to the main thread. So we have several test for dlopen on multiple threads so we know it works in the general case and there should no no FS operations that occur on the side module.

Can you try with the latest version of emscripten (3.1.35 is a year old now)? It could be that some of the fixes needed for side modules and threading happened in more recent versions.