Read/write access to global variable in multithreaded application

[mratsim]

    var data = cast[seq[string]](allocShared0(sizeof(seq[string])))
    data.add("Test")
    
    
    Run

You cannot do that, you are casting unmanaged memory, to GC-ed memory, this 
will end with a crash in the GC. Though here you have a global variable so the 
memory is never reclaimed.

Instead compile with `--gc:arc` or `--gc:orc` so that all threads can access 
that sequence.
    
    
    proc myThread(test : ptr seq[string]) {.thread.} =
        echo "Start of new thread"
        echo test[][0]
        test[].add("I want to access this from the main thread!")
        echo "End of new thread"
    
    
    Run

Use locks to access shared memory