in reply to WHY copying does happen (fork)

Perl uses reference counting for variables, and hence even reading a variable means increasing the reference count, reading the value, and then decreasing the reference count again. So even a read can cause a memory write in Perl. You don't show much code, so I can't tell whether that's the case with your code, but I think that's a propable explanation.

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Re^2: WHY copying does happen (fork)
by moritz (Cardinal) on May 05, 2008 at 17:13 UTC
    Could you explain why reading should increase the refcount? 
    use Devel::Peek;
my $x = 3;
Dump $x;
fork;
Dump $x;
__END__
SV = IV(0x90e54bc) at 0x90ca648
  REFCNT = 1
  FLAGS = (PADBUSY,PADMY,IOK,pIOK)
  IV = 3
SV = IV(0x90e54bc) at 0x90ca648
  REFCNT = 1
  FLAGS = (PADBUSY,PADMY,IOK,pIOK)
  IV = 3
SV = IV(0x90e54bc) at 0x90ca648
  REFCNT = 1
  FLAGS = (PADBUSY,PADMY,IOK,pIOK)
  IV = 3

    [download]
    The refcount isn't changed, even though a variable is read from a forked process. And why should it? I see no need for that.

    The matter is different when variables go out of scope: then their refcount is reduced, thus possibly modifying it before deallocating.

    Update It's probably this you meant: 

    use Devel::Peek;
my $x = 3;
Dump $x;
my $y = \$x;
Dump $x;
__END__
SV = IV(0x97634bc) at 0x9748648
  REFCNT = 1
  FLAGS = (PADBUSY,PADMY,IOK,pIOK)
  IV = 3
SV = IV(0x97634bc) at 0x9748648
  REFCNT = 2
  FLAGS = (PADBUSY,PADMY,IOK,pIOK)
  IV = 3

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      If you have a complex data structure and iterate over it, I expect refcounting to increase and then decrease the refcount:

      C:\>perl -MDevel::Peek -e "$x=[{foo=>1}]; Dump $x; $a=$x->[0]; Dump ($
+a); "
SV = RV(0x1aa4ffc) at 0x1a93898
  REFCNT = 1
  FLAGS = (ROK)
  RV = 0x1a44bac
  SV = PVAV(0x1a45f9c) at 0x1a44bac
    REFCNT = 1
    FLAGS = ()
    IV = 0
    NV = 0
    ARRAY = 0x1a4baac
    FILL = 0
    MAX = 0
    ARYLEN = 0x0
    FLAGS = (REAL)
    Elt No. 0
    SV = RV(0x1aa4ff4) at 0x1a44bb8
      REFCNT = 1
      FLAGS = (ROK)
      RV = 0x1a44a98
      SV = PVHV(0x1a92fec) at 0x1a44a98
        REFCNT = 1                       # <- only one reference
        FLAGS = (SHAREKEYS)
        IV = 1
        NV = 0
        ARRAY = 0x1a4b9b4  (0:7, 1:1)
        hash quality = 100.0%
        KEYS = 1
        FILL = 1
        MAX = 7
        RITER = -1
        EITER = 0x0
        Elt "foo" HASH = 0x238678dd
        SV = IV(0x1a989f0) at 0x1a44b88
          REFCNT = 1
          FLAGS = (IOK,pIOK)
          IV = 1

SV = RV(0x1aa4fd4) at 0x1a938e0
  REFCNT = 1
  FLAGS = (ROK)
  RV = 0x1a44a98
  SV = PVHV(0x1a92fec) at 0x1a44a98
    REFCNT = 2                         # <- now two references
    FLAGS = (SHAREKEYS)
    IV = 1
    NV = 0
    ARRAY = 0x1a4b9b4  (0:7, 1:1)
    hash quality = 100.0%
    KEYS = 1
    FILL = 1
    MAX = 7
    RITER = -1
    EITER = 0x0
    Elt "foo" HASH = 0x238678dd
    SV = IV(0x1a989f0) at 0x1a44b88
      REFCNT = 1
      FLAGS = (IOK,pIOK)
      IV = 1

      [download]

      I guess you can do clever things with aliases to avoid refcounting (as the stack currently is not refcounted), but it's easy to miss just one of these.

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