There are three sets of pretty-printers that gdb searches:
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 For example, the underlying data structure may have changed and the pretty-printer is out of date.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.