define: DUK_OPT_EXTSTR_FREE
introduced: 1.1.0
requires:
  - DUK_OPT_EXTERNAL_STRINGS
tags:
  - memory
  - experimental
description: >
  Optional counterpart to DUK_OPT_EXTSTR_INTERN_CHECK.  Invoked when an
  external string is about to be freed by Duktape.

  The argument "ptr" is a void ptr and points to the external string data.
  Concretely, it is the (non-NULL) value returned by
  DUK_OPT_EXTSTR_INTERN_CHECK.  The "udata" argument is the heap userdata
  which may be ignored if not needed.

  Also enable DUK_OPT_EXTERNAL_STRINGS to use this feature.

  NOTE: Right now there is no API to push external strings; external strings
  come into being as a resul of DUK_OPT_EXTSTR_INTERN_CHECK() only.  If/when
  this is changed, this hook will get called for every string, even if pushed
  by the user using an API call; this may need to be rethought at that time.