Debugging with GDB: Selecting Pretty-Printers

Debugging with GDB: Selecting Pretty-Printers

23.2.2.6 Selecting Pretty-Printers

The Python list gdb.pretty_printers contains an array of functions or callable objects that have been registered via addition as a pretty-printer. Printers in this list are called global printers, they’re available when debugging all inferiors. Each gdb.Progspace contains a pretty_printers attribute. Each gdb.Objfile also contains a pretty_printers attribute.

Each function on these lists is passed a single gdb.Value argument and should return a pretty-printer object conforming to the interface definition above (see Pretty Printing API). If a function cannot create a pretty-printer for the value, it should return None.

GDB first checks the pretty_printers attribute of each gdb.Objfile in the current program space and iteratively calls each enabled lookup routine in the list for that gdb.Objfile until it receives a pretty-printer object. If no pretty-printer is found in the objfile lists, GDB then searches the pretty-printer list of the current program space, calling each enabled function until an object is returned. After these lists have been exhausted, it tries the global gdb.pretty_printers list, again calling each enabled function until an object is returned.

The order in which the objfiles are searched is not specified. For a given list, functions are always invoked from the head of the list, and iterated over sequentially until the end of the list, or a printer object is returned.

For various reasons a pretty-printer may not work. For example, the underlying data structure may have changed and the pretty-printer is out of date.

The consequences of a broken pretty-printer are severe enough that GDB provides support for enabling and disabling individual printers. For example, if print frame-arguments is on, a backtrace can become highly illegible if any argument is printed with a broken printer.

Pretty-printers are enabled and disabled by attaching an enabled attribute to the registered function or callable object. If this attribute is present and its value is False, the printer is disabled, otherwise the printer is enabled.