Running generated machine code

[eterps]

Hi I am new to Nim, and I am giving it a try for low-level programming.

I am trying this example from Rosettacode: 
[https://rosettacode.org/wiki/Machine_code#Nim](https://rosettacode.org/wiki/Machine_code#Nim)

But for x86-64 and using pure Nim (instead of falling back on C code):
    
    
    import posix
    
    const
      prot = PROT_READ or PROT_WRITE or PROT_EXEC
    
    when defined(macosx) or defined(bsd):
      const MAP_ANONYMOUS = 0x1000
    else:
      var
        MAP_ANONYMOUS {.importc: "MAP_ANONYMOUS", header: "<sys/mman.h>".}: cint
    
    type
      IntToInt = proc(num: uint32): uint32 {.nimcall.}
    
    proc generate_add4(): IntToInt =
      var code = [
        0x48'u8, 0x89'u8, 0xf8'u8,             # mov rax, rdi
        0x48'u8, 0x83'u8, 0xc0'u8, 0x00'u8,    # add rax, 4
        0xc3'u8                                # ret
      ]
      var mem = mmap(nil, code.len, prot, MAP_PRIVATE or MAP_ANONYMOUS, -1, 0)
      copyMem(mem, code[0].unsafeAddr, code.len)
      result = cast[IntToInt](mem)
      discard munmap(mem, code.len)
    
    var add4 = generate_add4()
    echo add4(3)
    
    
    Run

Currently it fails with: SIGSEGV: Illegal storage access. (Attempt to read from 
nil?)

On repl.it: 
[https://repl.it/@eterps/nimmachinecode](https://repl.it/@eterps/nimmachinecode)

Any idea how to get this to work?