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1 Debugging GNU Emacs
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2 Copyright (c) 1985 Richard M. Stallman.
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3
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4 Permission is granted to anyone to make or distribute verbatim copies
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5 of this document as received, in any medium, provided that the
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6 copyright notice and permission notice are preserved,
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7 and that the distributor grants the recipient permission
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8 for further redistribution as permitted by this notice.
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9
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10 Permission is granted to distribute modified versions
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11 of this document, or of portions of it,
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12 under the above conditions, provided also that they
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13 carry prominent notices stating who last changed them.
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14
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15 On 4.2 you will probably find that dbx does not work for
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16 debugging GNU Emacs. For one thing, dbx does not keep the
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17 inferior process's terminal modes separate from its own.
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18 For another, dbx does not put the inferior in a separate
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19 process group, which makes trouble when an inferior uses
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20 interrupt input, which GNU Emacs must do on 4.2.
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21
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22 dbx has also been observed to have other problems,
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23 such as getting incorrect values for register variables
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24 in stack frames other than the innermost one.
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25
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26 The Emacs distribution now contains GDB, the new source-level
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27 debugger for the GNU system. GDB works for debugging Emacs.
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28 GDB currently runs on vaxes under 4.2 and on Sun 2 and Sun 3
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29 systems.
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30
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31
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32 ** Some useful techniques
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33
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34 `Fsignal' is a very useful place to stop in.
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35 All Lisp errors go through there.
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36
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37 It is useful, when debugging, to have a guaranteed way
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38 to return to the debugger at any time. If you are using
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39 interrupt-driven input, which is the default, then Emacs is using
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40 RAW mode and the only way you can do it is to store
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41 the code for some character into the variable stop_character:
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42
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43 set stop_character = 29
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44
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45 makes Control-] (decimal code 29) the stop character.
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46 Typing Control-] will cause immediate stop. You cannot
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47 use the set command until the inferior process has been started.
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48 Put a breakpoint early in `main', or suspend the Emacs,
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49 to get an opportunity to do the set command.
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50
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51 If you are using cbreak input (see the Lisp function set-input-mode),
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52 then typing Control-g will cause a SIGINT, which will return control
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53 to the debugger immediately unless you have done
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54
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55 ignore 3 (in dbx)
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56 or handle 3 nostop noprint (in gdb)
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57
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58 You will note that most of GNU Emacs is written to avoid
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59 declaring a local variable in an inner block, even in
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60 cases where using one would be the cleanest thing to do.
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61 This is because dbx cannot access any of the variables
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62 in a function which has even one variable defined in an
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63 inner block. A few functions in GNU Emacs do have variables
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64 in inner blocks, only because I wrote them before realizing
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65 that dbx had this problem and never rewrote them to avoid it.
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66
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67 I believe that GDB does not have such a problem.
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68
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69
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70 ** Examining Lisp object values.
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71
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72 When you have a live process to debug, and it has not encountered a
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73 fatal error, you can use the GDB command `pr'. First print the value
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74 in the ordinary way, with the `p' command. Then type `pr' with no
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75 arguments. This calls a subroutine which uses the Lisp printer.
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76
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77 If you can't use this command, either because the process can't run
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78 a subroutine or because the data is invalid, you can fall back on
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79 lower-level commands.
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80
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81 Use the `xtype' command to print out the data type of the last data
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82 value. Once you know the data type, use the command that corresponds
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83 to that type. Here are these commands:
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84
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85 xint xptr xwindow xmarker xoverlay xmiscfree xintfwd xboolfwd xobjfwd
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86 xbufobjfwd xkbobjfwd xbuflocal xbuffer xsymbol xstring xvector xframe
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87 xwinconfig xcompiled xcons xcar xcdr xsubr xprocess xfloat xscrollbar
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88
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89 Each one of them applies to a certain type or class of types.
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90 (Some of these types are not visible in Lisp, because they exist only
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91 internally.)
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92
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93 Each x... command prints some information about the value, and
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94 produces a GDB value (subsequently available in $) through which you
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95 can get at the rest of the contents.
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96
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97 In general, most of the rest of the contents will be addition Lisp
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98 objects which you can examine in turn with the x... commands.
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99
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100 ** If GDB does not run and your debuggers can't load Emacs.
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101
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102 On some systems, no debugger can load Emacs with a symbol table,
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103 perhaps because they all have fixed limits on the number of symbols
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104 and Emacs exceeds the limits. Here is a method that can be used
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105 in such an extremity. Do
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106
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107 nm -n temacs > nmout
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108 strip temacs
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109 adb temacs
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110 0xd:i
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111 0xe:i
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112 14:i
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113 17:i
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114 :r -l loadup (or whatever)
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115
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116 It is necessary to refer to the file `nmout' to convert
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117 numeric addresses into symbols and vice versa.
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118
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119 It is useful to be running under a window system.
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120 Then, if Emacs becomes hopelessly wedged, you can create
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121 another window to do kill -9 in. kill -ILL is often
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122 useful too, since that may make Emacs dump core or return
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123 to adb.
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124
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125
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126 ** Debugging incorrect screen updating.
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127
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128 To debug Emacs problems that update the screen wrong, it is useful
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129 to have a record of what input you typed and what Emacs sent to the
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130 screen. To make these records, do
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131
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132 (open-dribble-file "~/.dribble")
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133 (open-termscript "~/.termscript")
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134
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135 The dribble file contains all characters read by Emacs from the
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136 terminal, and the termscript file contains all characters it sent to
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137 the terminal. The use of the directory `~/' prevents interference
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138 with any other user.
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139
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140 If you have irreproducible display problems, put those two expressions
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141 in your ~/.emacs file. When the problem happens, exit the Emacs that
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142 you were running, kill it, and rename the two files. Then you can start
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143 another Emacs without clobbering those files, and use it to examine them.
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