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gdb: Type Printing API

 
 23.2.2.8 Type Printing API
 ..........................
 
 GDB provides a way for Python code to customize type display.  This is
 mainly useful for substituting canonical typedef names for types.
 
    A "type printer" is just a Python object conforming to a certain
 protocol.  A simple base class implementing the protocol is provided;
 see gdb.types.  A type printer must supply at least:
 
  -- Instance Variable of type_printer: enabled
      A boolean which is True if the printer is enabled, and False
      otherwise.  This is manipulated by the 'enable type-printer' and
      'disable type-printer' commands.
 
  -- Instance Variable of type_printer: name
      The name of the type printer.  This must be a string.  This is used
      by the 'enable type-printer' and 'disable type-printer' commands.
 
  -- Method on type_printer: instantiate (self)
      This is called by GDB at the start of type-printing.  It is only
      called if the type printer is enabled.  This method must return a
      new object that supplies a 'recognize' method, as described below.
 
    When displaying a type, say via the 'ptype' command, GDB will compute
 a list of type recognizers.  This is done by iterating first over the
 per-objfile type printers (Objfiles In Python), followed by the
 per-progspace type printers (Progspaces In Python), and finally
 the global type printers.
 
    GDB will call the 'instantiate' method of each enabled type printer.
 If this method returns 'None', then the result is ignored; otherwise, it
 is appended to the list of recognizers.
 
    Then, when GDB is going to display a type name, it iterates over the
 list of recognizers.  For each one, it calls the recognition function,
 stopping if the function returns a non-'None' value.  The recognition
 function is defined as:
 
  -- Method on type_recognizer: recognize (self, type)
      If TYPE is not recognized, return 'None'.  Otherwise, return a
      string which is to be printed as the name of TYPE.  The TYPE
      argument will be an instance of 'gdb.Type' (Types In
      Python).
 
    GDB uses this two-pass approach so that type printers can efficiently
 cache information without holding on to it too long.  For example, it
 can be convenient to look up type information in a type printer and hold
 it for a recognizer's lifetime; if a single pass were done then type
 printers would have to make use of the event system in order to avoid
 holding information that could become stale as the inferior changed.
 

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