如何为boost::multiprecision::mpfr_float_50创建可正常输出double值的GDB美化打印机？

It looks like your pretty printer isn't being applied correctly, and there are a couple of small issues in your code preventing it from outputting the double representation you want. Let's walk through the fixes step by step.

Key Issues in Your Original Code #

1. to_string returns a GDB Value instead of a string: GDB's pretty printer expects this method to return a human-readable string, not a raw gdb.Value instance.
2. Regex matching is too strict: Your regex only matches the exact type name, but GDB might present the type with modifiers like const in some cases.
3. Printer registration doesn't override defaults: Without explicitly enabling replacement, your printer might not take precedence over GDB's default struct printer.

Corrected Pretty Printer Code #

Here's the revised version of your gdb_pretty_printers.py script:

import gdb
import gdb.printing

class BoostMultiprecisionMPFRPrinter(object):
    "Pretty Printer for boost::multiprecision::mpfr_float_50"
    def __init__(self, val):
        self.val = val
        self.double_type = gdb.lookup_type('double')

    def to_string(self):
        # Directly call the convert_to<double>() method on the mpfr_float_50 object
        # This is safer and cleaner than string-based evaluation
        double_value = self.val.convert_to(self.double_type)
        # Convert the GDB Value to a string for proper display
        return str(double_value)

    def display_hint(self):
        # Tell GDB to treat this as a floating-point number for formatting
        return 'float'

def build_pretty_printer():
    pp = gdb.printing.RegexpCollectionPrettyPrinter("boost-multiprecision")
    # Adjust regex to match const-qualified versions of the type
    pp.add_printer(
        'mpfr_float_50',
        r'^boost::multiprecision::mpfr_float_50(\s+const)?$',
        BoostMultiprecisionMPFRPrinter
    )
    return pp

def register_printers():
    # Use replace=True to ensure our printer overrides default struct printing
    gdb.printing.register_pretty_printer(
        gdb.current_objfile(),
        build_pretty_printer(),
        replace=True
    )

Key Fixes Explained #

• String return type: The to_string method now converts the resulting gdb.Value to a string with str(double_value), which GDB can display directly.
• Robust regex matching: The updated regex matches both the base type and const boost::multiprecision::mpfr_float_50 to cover more use cases.
• Override default printers: Adding replace=True to register_pretty_printer ensures your custom printer takes priority over GDB's default handling of the struct.
• Cleaner method invocation: Using self.val.convert_to(self.double_type) avoids messy string-based evaluation, making the script more reliable and easier to maintain.

Verify the Setup #

1. Double-check that the path in your .gdbinit points correctly to your script directory:

   python
   import sys
   sys.path.insert(1, '/full/path/to/your/script/directory')
   import gdb_pretty_printers
   gdb_pretty_printers.register_printers()
   end
   

2. Start GDB, load your binary, and set a breakpoint where you have an mpfr_float_50 variable. When you print the variable (e.g., print my_mpfr_var), you should see the double representation instead of the internal struct details.

内容的提问来源于stack exchange，提问作者Steve Lorimer