These will only occur when writing.

The problem is, that many apparently read accesses in Perl, actually do modify the SV. Some innocent looking operations (in all example, ro is apparently read-only): 

  print $ro + 3;         #  Might upgrade a PV to a PVIV;
  $ref = \$ro;           #  Increments the refcount on $ro.
  $ro =~ /./;            #  Sets the pos associated with $ro.
  $var = $ro{foo}{bar};  #  May autovivify $ro{foo}

[download]
The fact that there are hidden data modifications is the reason that in Perl variables are by default not shared between threads.

Note also that the SV* just contain some metadata of the values - the real data is some hops away. Even something as simple as a string, with no magic attached will have its data scattered in three places (the SV* itself, containing the refcount, some flags, and a pointer to a structure (svpv) that contains, among other pieces of data, the length of the string, and a pointer to the actual string itself).

In reply to Re: A faster?, safer, user transparent, shared variable "locking" mechanism. by Anonymous Monk
in thread A faster?, safer, user transparent, shared variable "locking" mechanism. by BrowserUk

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