GDB pretty printer loaded but not used

Question

I am about to develop a custom GDB pretty printer. Trying to follow simple guide from the GDB documentation I made the following:

printer.py:

import gdb.printing

class element_printer:
    """Print element object"""

    def __init__(self, val) -> None:
        self.val = val

    def to_string(self):
        return ("Sample element: value=<" + str(self.val["m_value"]))
        return ("value=<" + str(self.val["m_value"]))

def build_pretty_printer():
    pp = gdb.printing.RegexpCollectionPrettyPrinter("ReVolta")
    pp.add_printer('ReVolta::element', '^ReVolta::element%', element_printer)
    return pp

gdb.printing.register_pretty_printer(gdb.current_objfile(), build_pretty_printer())

which is intended to pretty print the objects of the following class:

element.cpp:

namespace ReVolta {
    struct element {
        size_t m_value;
    };
    ...
    element elm { 42 };
    ...
}

Unless loaded into GDB once in debug session, the object is shown as:

GDB:

print elm
$1 = {m_value = 42}

which is as expected, a generic way to show the content. Just to verify which pretty printers are loaded, here is the check I made:

GDB:

info pretty-printer
global pretty-printers:
  builtin
    mpx_bound128
objfile /lib/aarch64-linux-gnu/libstdc++.so.6 pretty-printers:
  libstdc++-v6
    __gnu_cxx::_Slist_iterator
    __gnu_cxx::__8::_Slist_iterator
    __gnu_cxx::__8::__normal_iterator
  ....

Notice just the printers from libstdc++-v6 are loaded (not a complete list above). Notice no custom printers are loaded. Now I load my printer modules:

GDB:

source /home/martin/git/revolta/test/librevolta/element.py

and try to verify the successful load:

GDB:

info pretty-printer
global pretty-printers:
  ReVolta
    ReVolta::element     <----- MY CUSTOM PRETTY PRINTER HERE
  builtin
    mpx_bound128
objfile /lib/aarch64-linux-gnu/libstdc++.so.6 pretty-printers:
  libstdc++-v6
    __gnu_cxx::_Slist_iterator
    __gnu_cxx::__8::_Slist_iterator
    __gnu_cxx::__8::__normal_iterator
    __gnu_cxx::__8::slist
  ....

Notice my custom pretty printer is loaded successfully (seems so).

PROBLEM: The problem is, it seems my pretty printer is not used. When I try to print the elm instance of ReVolta::element class, it gets the same result:

GDB:

print elm
$1 = {m_value = 42}

May I kindly ask you for help to make the pretty printer running? Many thanks to anyone willing to help! Martin

Answer 1

The issue was resolved by fixing the bug in regular expression. The correct regex to use in adding the printer is:

pp.add_printer('ReVolta::element', '^ReVolta::element$', element_printer)

Notice the $ used instead of % at the end of the regex.

Thanks @ssbssa for kicking me the right direction (as in question comments)