Constant THREAD_CREATE_REQ_LEN 

const THREAD_CREATE_REQ_LEN: usize = 32;

Binary protocol for /thread/create writes.

#[repr(C)] struct ThreadCreateRequest {
    entry: u64,       // user trampoline address (< USER_TOP)
    stack_size: u64,  // requested size; kernel allocates and places RSP
    arg0: u64,        // passed in RDI (e.g. closure pointer)
    tls_base: u64,    // FS.base (0 = none)
}
// write(fd, &req, 32) then read(fd, &tid, 4)