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13553 :: immutability of passed data within a thread

• pegra4 (orginal) [2025-12-08T17:27:29+01:00] view original

  When passing a 'seq' to a 'proc' that's been used for 'spawn'-ing a thread,
  the 'seq' inside the new thread is immutable.
  I found out that the 'seq' inside the new thread is already a physical copy
  of the 'seq' inside the 'spawn'-ing thread.
  
  But then I don't understand why the 'seq' inside the new thread needs to be immutable.
  Could someone tell me please?
  
  My intension is to modify such a 'seq' and 'spawn' new treads again
  with the modified 'seq'.
  Actually I assign the passed 'seq' to an additional local 'seq'.
  That makes two copy actions per 'spawn'.
  One done by the runtime system because of the 'spawn' and one done by me.
  

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• sonicsmooth (orginal) [2025-12-08T21:15:39+01:00] view original

  The variable passed to a proc is only mutable if you pass it with var like this: myproc(i: var int) = .... I'm not sure if you can create a proc with var signature if you intend to spawn it. I was recently trying this with createThread and the compiler didn't let me use a var argument. I wound up using a ref type inside a tuple instead.

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• pegra4 (orginal) [2025-12-09T10:04:07+01:00] view original

  Thank you,
  but the compiler denies.
  Error: 'toTask'ed function cannot have a 'var' parameter
  

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• Araq (orginal) [2025-12-09T10:25:15+01:00] view original

  Use ptr and addr for this.

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• darkestpigeon (orginal) [2025-12-09T12:10:17+01:00] view original

  It's not as much about immutability. A var parameter is secretly a pointer, and this is problematic as the argument can go out of scope before the thread exits.

  If the thread proc argument is passed by value, it should be safe to bypass the immutability via e.g.

  
  
  proc doStuff(x: var seq[int]) =
    x.add 3
  
  # in this case, {.bycopy.} is redundant
  proc myThread(x {.bycopy.}: seq[int]) {.thread.} =
    var y = move (addr x)[]
    doStuff(y)
  Run

  But maybe the signature of createThread should really be

  
  proc(t: var Thread[TArg], tp: proc(arg: sink TArg) {.nimcall, gcsafe.}, param: sink TArg)
  Run

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