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C++ GDB pretty print

Date: 2023-02-25Last modified: 2024-11-02
C++gdbpretty-print

Table of contents

Pretty-Printer Introduction

Here is how a C++ std::string looks without a pretty-printer:

(gdb) print s
$1 = {
  static npos = 4294967295, 
  _M_dataplus = {
    <std::allocator<char>> = {
      <__gnu_cxx::new_allocator<char>> = {
        <No data fields>}, <No data fields>
      },
    members of std::basic_string<char, std::char_traits<char>,
      std::allocator<char> >::_Alloc_hider:
    _M_p = 0x804a014 "abcd"
  }
}

With a pretty-printer for std::string only the contents are printed:

(gdb) print s
$2 = "abcd"

#include <stdlib.h>

typedef struct {
  const char *first_name;
  const char *last_name;
  struct {
    int day;
    int month;
    int year;
  } dob;
  char *comments;
} student;

student students[]
    = { [0] = { .first_name = "Fred", .last_name = "Smith", .dob = { 1, 1, 1970 } },
        [1]
        = { .first_name = "Sarah",
            .last_name  = "G0T0",
            .dob        = { 1, 1, 2002 },
            .comments   = "Lorem ipsum enchanta constrata" } };

int main( void )
{
  students[1].comments     = (char *) malloc( 128 );
  students[1].comments[51] = 'X';
  return 0;
}

GDB commands

set verbose off
set pagination off

start

set logging file output/gdb_pretty_print.gdb
set logging overwrite on
set logging on

echo print students with pretty off\n
set print pretty off
print students

echo print students with pretty on\n
set print pretty on
print students

echo Printing and watching elements of arrays\n
# 10 character max width
print -elements 10 -- students[1].comments
# All character
print -elements unlimited -- students[1].comments
print students[1].comments[5]@10
next

print students[1].comments[50]@4
watch students[1].comments[50]@4
continue

set logging off
quit

#  vim: ft=gdb

GDB output

print students with pretty off
$1 = {{first_name = 0x555555556008 "Fred", last_name = 0x55555555600d "Smith", dob = {day = 1, month = 1, year = 1970}, comments = 0x0}, {first_name = 0x555555556013 "Sarah", last_name = 0x555555556019 "G0T0", dob = {day = 1, month = 1, year = 2002}, comments = 0x555555556020 "Lorem ipsum enchanta constrata"}}
print students with pretty on
$2 = {{
    first_name = 0x555555556008 "Fred",
    last_name = 0x55555555600d "Smith",
    dob = {
      day = 1,
      month = 1,
      year = 1970
    },
    comments = 0x0
  }, {
    first_name = 0x555555556013 "Sarah",
    last_name = 0x555555556019 "G0T0",
    dob = {
      day = 1,
      month = 1,
      year = 2002
    },
    comments = 0x555555556020 "Lorem ipsum enchanta constrata"
  }}
Printing and watching elements of arrays
$3 = 0x555555556020 "Lorem ipsu"...
$4 = 0x555555556020 "Lorem ipsum enchanta constrata"
$5 = " ipsum enc"
66	  students[1].comments[51] = 'X';
$6 = "\000\000\000"
Hardware watchpoint 2: students[1].comments[50]@4

Hardware watchpoint 2: students[1].comments[50]@4

Old value = "\000\000\000"
New value = "\000X\000"
main () at gdb_pretty_print.cpp:67
67	  return 0;
Warning: 'set logging off', an alias for the command 'set logging enabled', is deprecated.
Use 'set logging enabled off'.

References