Setup-GDB-with-Qt-pretty-printers.md

Introduction

When debugging qBittorrent with GDB, values of variables with Qt types such as QByteArray and QString are not easily printable.

Fortunately, GDB has a pretty printer functionality that enables it to easily print such values in human readable form.

This guide assumes you are relatively inexperienced with GDB, and either don't know what a .gdbinit file is or don't use one. If you are an experienced user with an existing .gdbinit setup, you should be able follow along and customize your existing setup as needed.

Setup

1. Clone the qBittorrent repository if you haven't done so already. In this example, it is assumed it was cloned to /home/user/Documents/qBittorrent

2. Create a folder called .gdb inside the cloned repository folder (/home/user/Documents/qBittorrent). Inside .gdb create another folder called qt5prettyprinters.

3. Download the qt.py and helper.py files from here (backup link) and place them inside the .gdb/qt5prettyprinters folder.

4. Create a file called .gdbinit inside the cloned repository folder with the following contents:

   python
   
   import sys, os
   
   print(f".gdbinit Python: current working directory is {os.getcwd()}")
   print(f".gdbinit Python: adding custom pretty-printers directory to the GDB path: {os.getcwd() + '/.gdb/qt5prettyprinters'}")
   
   sys.path.insert(0, "./.gdb/qt5prettyprinters")
   
   from qt import register_qt_printers
   register_qt_printers (None)
   
   end

Usage

After starting a debugging session with qBittorrent (gdb qbittorrent, for example), execute source .gdbinit in the GDB console. You should see some output informing you that the pretty printers have been loaded.

(gdb) source .gdbinit
.gdbinit Python: current working directory is /home/user/Documents/qBittorrent
.gdbinit Python: adding custom pretty-printers directory to the GDB path: /home/user/Documents/qBittorrent/.gdb/qt5prettyprinters

Now, you can use GDB as you usually would. Whenever you print a variable with a Qt type, the output is immediately human-readable without additional effort.

For example, suppose you are at a breakpoint and print the value of a variable named data, which is a QByteArray, containing an HTTP request.

• output without pretty printers:

  (gdb) print data
  $1 = (const QByteArray &) @0x555555e2ecd0: {d = 0x555555e46610}
  

• output with pretty printers:

  (gdb) print data
  $1 = "POST /api/v2/auth/login HTTP/1.1\r\nHost: localhost:8080\r\nUser-Agent: python-requests/2.22.0\r\nAccept-Encoding: gzip, deflate\r\nAccept: */*\r\nConnection: keep-alive\r\nContent-Length: 34\r\nContent-Type: appli"... = {[0] = 80 'P', [1] = 79 'O',
  [2] = 83 'S', [3] = 84 'T', [4] = 32 ' ', [5] = 47 '/', [6] = 97 'a', [7] = 112 'p', [8] = 105 'i', [9] = 47 '/', [10] = 118 'v', [11] = 50 '2', [12] = 47 '/', [13] = 97 'a', [14] = 117 'u', [15] = 116 't', [16] = 104 'h', [17] = 47 '/', [18] = 108 'l',
  [19] = 111 'o', [20] = 103 'g', [21] = 105 'i', [22] = 110 'n', [23] = 32 ' ', [24] = 72 'H', [25] = 84 'T', [26] = 84 'T', [27] = 80 'P', [28] = 47 '/', [29] = 49 '1', [30] = 46 '.', [31] = 49 '1', [32] = 13 '\r', [33] = 10 '\n', [34] = 72 'H',
  [35] = 111 'o', [36] = 115 's', [37] = 116 't', [38] = 58 ':', [39] = 32 ' ', [40] = 108 'l', [41] = 111 'o', [42] = 99 'c', [43] = 97 'a', [44] = 108 'l', [45] = 104 'h', [46] = 111 'o', [47] = 115 's', [48] = 116 't', [49] = 58 ':', [50] = 56 '8',
  [51] = 48 '0', [52] = 56 '8', [53] = 48 '0', [54] = 13 '\r', [55] = 10 '\n', [56] = 85 'U', [57] = 115 's', [58] = 101 'e', [59] = 114 'r', [60] = 45 '-', [61] = 65 'A', [62] = 103 'g', [63] = 101 'e', [64] = 110 'n', [65] = 116 't', [66] = 58 ':',
  [67] = 32 ' ', [68] = 112 'p', [69] = 121 'y', [70] = 116 't', [71] = 104 'h', [72] = 111 'o', [73] = 110 'n', [74] = 45 '-', [75] = 114 'r', [76] = 101 'e', [77] = 113 'q', [78] = 117 'u', [79] = 101 'e', [80] = 115 's', [81] = 116 't', [82] = 115 's',
  [83] = 47 '/', [84] = 50 '2', [85] = 46 '.', [86] = 50 '2', [87] = 50 '2', [88] = 46 '.', [89] = 48 '0', [90] = 13 '\r', [91] = 10 '\n', [92] = 65 'A', [93] = 99 'c', [94] = 99 'c', [95] = 101 'e', [96] = 112 'p', [97] = 116 't', [98] = 45 '-', [99] = 69 'E',
  [100] = 110 'n', [101] = 99 'c', [102] = 111 'o', [103] = 100 'd', [104] = 105 'i', [105] = 110 'n', [106] = 103 'g', [107] = 58 ':', [108] = 32 ' ', [109] = 103 'g', [110] = 122 'z', [111] = 105 'i', [112] = 112 'p', [113] = 44 ',', [114] = 32 ' ',
  [115] = 100 'd', [116] = 101 'e', [117] = 102 'f', [118] = 108 'l', [119] = 97 'a', [120] = 116 't', [121] = 101 'e', [122] = 13 '\r', [123] = 10 '\n', [124] = 65 'A', [125] = 99 'c', [126] = 99 'c', [127] = 101 'e', [128] = 112 'p', [129] = 116 't',
  [130] = 58 ':', [131] = 32 ' ', [132] = 42 '*', [133] = 47 '/', [134] = 42 '*', [135] = 13 '\r', [136] = 10 '\n', [137] = 67 'C', [138] = 111 'o', [139] = 110 'n', [140] = 110 'n', [141] = 101 'e', [142] = 99 'c', [143] = 116 't', [144] = 105 'i',
  [145] = 111 'o', [146] = 110 'n', [147] = 58 ':', [148] = 32 ' ', [149] = 107 'k', [150] = 101 'e', [151] = 101 'e', [152] = 112 'p', [153] = 45 '-', [154] = 97 'a', [155] = 108 'l', [156] = 105 'i', [157] = 118 'v', [158] = 101 'e', [159] = 13 '\r',
  [160] = 10 '\n', [161] = 67 'C', [162] = 111 'o', [163] = 110 'n', [164] = 116 't', [165] = 101 'e', [166] = 110 'n', [167] = 116 't', [168] = 45 '-', [169] = 76 'L', [170] = 101 'e', [171] = 110 'n', [172] = 103 'g', [173] = 116 't', [174] = 104 'h',
  [175] = 58 ':', [176] = 32 ' ', [177] = 51 '3', [178] = 52 '4', [179] = 13 '\r', [180] = 10 '\n', [181] = 67 'C', [182] = 111 'o', [183] = 110 'n', [184] = 116 't', [185] = 101 'e', [186] = 110 'n', [187] = 116 't', [188] = 45 '-', [189] = 84 'T',
  [190] = 121 'y', [191] = 112 'p', [192] = 101 'e', [193] = 58 ':', [194] = 32 ' ', [195] = 97 'a', [196] = 112 'p', [197] = 112 'p', [198] = 108 'l', [199] = 105 'i'...}
  

The pretty printer output is further customizable via the usual GDB options for output customization. For example, you can suppress output of individual array elements, or reduce the number of array elements printed.