##### 23.3.3.9 Selecting Guile Pretty-Printers

There are three sets of pretty-printers that gdb searches:

- Per-objfile list of pretty-printers (see Objfiles In Guile).
- Per-progspace list of pretty-printers (see Progspaces In Guile).
- The global list of pretty-printers (see Guile Pretty Printing API). These printers are available when debugging any inferior.

Pretty-printer lookup is done by passing the value to be printed to the lookup function of each enabled object in turn. Lookup stops when a lookup function returns a non-`#f` value or when the list is exhausted. Lookup functions must return either a `<gdb:pretty-printer-worker>` object or `#f`. Otherwise an exception is thrown.

gdb first checks the result of `objfile-pretty-printers` of each `<gdb:objfile>` in the current program space and iteratively calls each enabled lookup function in the list for that `<gdb:objfile>` until a non-`#f` object is returned. If no pretty-printer is found in the objfile lists, gdb then searches the result of `progspace-pretty-printers` of the current program space, calling each enabled function until a non-`#f` object is returned. After these lists have been exhausted, it tries the global pretty-printers list, obtained with `pretty-printers`, again calling each enabled function until a non-`#f` 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 `<gdb:pretty-printer-worker>` 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 For example, the underlying data structure may have changed and the pretty-printer is out of date.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 from Scheme by calling `set-pretty-printer-enabled!`. See Guile Pretty Printing API.