Handle application initialisation

[tsoutsman]

This should be trivial to do (once libtheseus is sorted out).

We can define a function in std::sys::theseus as follows:

pub unsafe fn __theseus_start(args: Vec<String>) -> isize {
    let argc = args.len();
    let argv = args.as_ptr();

    extern "C" {
        // This function is exported by Rust (and all other language) binaries.
        fn main(argc: isize, argv: *const *const c_char) -> i32;
    }
    
    // Do initialisation.
    let result = unsafe { main(argc as size, argv) };
    // Do cleanup.
    result as isize
}

* This code is untested.

To run a crate we would then run __theseus_start similar to how we currently run main functions exported by applications. The process of finding the section and transmuting it to a function pointer would remain the same.

Hermit does a slightly similar thing.

I believe std::sys::theseus::init is called by the main function exported by binaries. E.g. Hermit's runtime_entry function doesn't initialise the network because it's done in init, which will be called by main.

[kevinaboos]

yep that seems generally correct. 👍