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1 @c -*-texinfo-*-
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2 @c This is part of the XEmacs Lisp Reference Manual.
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3 @c Copyright (C) 1990, 1991, 1992, 1993, 1994 Free Software Foundation, Inc.
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4 @c See the file lispref.texi for copying conditions.
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5 @setfilename ../../info/debugging.info
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6 @node Debugging, Read and Print, Byte Compilation, Top
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7 @chapter Debugging Lisp Programs
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8
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9 There are three ways to investigate a problem in an XEmacs Lisp program,
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10 depending on what you are doing with the program when the problem appears.
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11
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12 @itemize @bullet
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13 @item
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14 If the problem occurs when you run the program, you can use a Lisp
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15 debugger (either the default debugger or Edebug) to investigate what is
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16 happening during execution.
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17
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18 @item
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19 If the problem is syntactic, so that Lisp cannot even read the program,
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20 you can use the XEmacs facilities for editing Lisp to localize it.
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21
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22 @item
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23 If the problem occurs when trying to compile the program with the byte
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24 compiler, you need to know how to examine the compiler's input buffer.
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25 @end itemize
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26
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27 @menu
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28 * Debugger:: How the XEmacs Lisp debugger is implemented.
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29 * Syntax Errors:: How to find syntax errors.
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30 * Compilation Errors:: How to find errors that show up in byte compilation.
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31 * Edebug:: A source-level XEmacs Lisp debugger.
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32 @end menu
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33
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34 Another useful debugging tool is the dribble file. When a dribble
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35 file is open, XEmacs copies all keyboard input characters to that file.
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36 Afterward, you can examine the file to find out what input was used.
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37 @xref{Terminal Input}.
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38
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39 For debugging problems in terminal descriptions, the
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40 @code{open-termscript} function can be useful. @xref{Terminal Output}.
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41
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42 @node Debugger
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43 @section The Lisp Debugger
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44 @cindex debugger
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45 @cindex Lisp debugger
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46 @cindex break
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47
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48 The @dfn{Lisp debugger} provides the ability to suspend evaluation of
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49 a form. While evaluation is suspended (a state that is commonly known
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50 as a @dfn{break}), you may examine the run time stack, examine the
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51 values of local or global variables, or change those values. Since a
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52 break is a recursive edit, all the usual editing facilities of XEmacs are
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53 available; you can even run programs that will enter the debugger
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54 recursively. @xref{Recursive Editing}.
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55
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56 @menu
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57 * Error Debugging:: Entering the debugger when an error happens.
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58 * Infinite Loops:: Stopping and debugging a program that doesn't exit.
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59 * Function Debugging:: Entering it when a certain function is called.
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60 * Explicit Debug:: Entering it at a certain point in the program.
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61 * Using Debugger:: What the debugger does; what you see while in it.
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62 * Debugger Commands:: Commands used while in the debugger.
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63 * Invoking the Debugger:: How to call the function @code{debug}.
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64 * Internals of Debugger:: Subroutines of the debugger, and global variables.
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65 @end menu
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66
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67 @node Error Debugging
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68 @subsection Entering the Debugger on an Error
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69 @cindex error debugging
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70 @cindex debugging errors
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71
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72 The most important time to enter the debugger is when a Lisp error
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73 happens. This allows you to investigate the immediate causes of the
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74 error.
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75
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76 However, entry to the debugger is not a normal consequence of an
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77 error. Many commands frequently get Lisp errors when invoked in
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78 inappropriate contexts (such as @kbd{C-f} at the end of the buffer) and
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79 during ordinary editing it would be very unpleasant to enter the
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80 debugger each time this happens. If you want errors to enter the
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81 debugger, set the variable @code{debug-on-error} to non-@code{nil}.
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82
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83 @defopt debug-on-error
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84 This variable determines whether the debugger is called when an error is
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85 signaled and not handled. If @code{debug-on-error} is @code{t}, all
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86 errors call the debugger. If it is @code{nil}, none call the debugger.
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87
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88 The value can also be a list of error conditions that should call the
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89 debugger. For example, if you set it to the list
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90 @code{(void-variable)}, then only errors about a variable that has no
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91 value invoke the debugger.
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92
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93 When this variable is non-@code{nil}, Emacs does not catch errors that
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94 happen in process filter functions and sentinels. Therefore, these
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95 errors also can invoke the debugger. @xref{Processes}.
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96 @end defopt
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97
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98 To debug an error that happens during loading of the @file{.emacs}
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99 file, use the option @samp{-debug-init}, which binds
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100 @code{debug-on-error} to @code{t} while @file{.emacs} is loaded and
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101 inhibits use of @code{condition-case} to catch init file errors.
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102
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103 If your @file{.emacs} file sets @code{debug-on-error}, the effect may
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104 not last past the end of loading @file{.emacs}. (This is an undesirable
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105 byproduct of the code that implements the @samp{-debug-init} command
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106 line option.) The best way to make @file{.emacs} set
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107 @code{debug-on-error} permanently is with @code{after-init-hook}, like
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108 this:
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109
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110 @example
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111 (add-hook 'after-init-hook
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112 '(lambda () (setq debug-on-error t)))
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113 @end example
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114
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115 @defopt debug-on-signal
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116 This variable is similar to @code{debug-on-error} but breaks
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117 whenever an error is signalled, regardless of whether it would be
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118 handled.
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119 @end defopt
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120
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121 @node Infinite Loops
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122 @subsection Debugging Infinite Loops
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123 @cindex infinite loops
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124 @cindex loops, infinite
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125 @cindex quitting from infinite loop
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126 @cindex stopping an infinite loop
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127
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128 When a program loops infinitely and fails to return, your first
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129 problem is to stop the loop. On most operating systems, you can do this
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130 with @kbd{C-g}, which causes quit.
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131
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132 Ordinary quitting gives no information about why the program was
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133 looping. To get more information, you can set the variable
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134 @code{debug-on-quit} to non-@code{nil}. Quitting with @kbd{C-g} is not
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135 considered an error, and @code{debug-on-error} has no effect on the
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136 handling of @kbd{C-g}. Likewise, @code{debug-on-quit} has no effect on
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137 errors.
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138
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139 Once you have the debugger running in the middle of the infinite loop,
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140 you can proceed from the debugger using the stepping commands. If you
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141 step through the entire loop, you will probably get enough information
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142 to solve the problem.
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143
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144 @defopt debug-on-quit
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145 This variable determines whether the debugger is called when @code{quit}
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146 is signaled and not handled. If @code{debug-on-quit} is non-@code{nil},
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147 then the debugger is called whenever you quit (that is, type @kbd{C-g}).
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148 If @code{debug-on-quit} is @code{nil}, then the debugger is not called
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149 when you quit. @xref{Quitting}.
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150 @end defopt
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151
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152 @node Function Debugging
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153 @subsection Entering the Debugger on a Function Call
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154 @cindex function call debugging
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155 @cindex debugging specific functions
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156
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157 To investigate a problem that happens in the middle of a program, one
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158 useful technique is to enter the debugger whenever a certain function is
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159 called. You can do this to the function in which the problem occurs,
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160 and then step through the function, or you can do this to a function
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161 called shortly before the problem, step quickly over the call to that
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162 function, and then step through its caller.
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163
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164 @deffn Command debug-on-entry function-name
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165 This function requests @var{function-name} to invoke the debugger each time
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166 it is called. It works by inserting the form @code{(debug 'debug)} into
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167 the function definition as the first form.
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168
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169 Any function defined as Lisp code may be set to break on entry,
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170 regardless of whether it is interpreted code or compiled code. If the
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171 function is a command, it will enter the debugger when called from Lisp
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172 and when called interactively (after the reading of the arguments). You
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173 can't debug primitive functions (i.e., those written in C) this way.
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174
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175 When @code{debug-on-entry} is called interactively, it prompts
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176 for @var{function-name} in the minibuffer.
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177
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178 If the function is already set up to invoke the debugger on entry,
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179 @code{debug-on-entry} does nothing.
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180
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181 @strong{Note:} if you redefine a function after using
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182 @code{debug-on-entry} on it, the code to enter the debugger is lost.
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183
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184 @code{debug-on-entry} returns @var{function-name}.
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185
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186 @example
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187 @group
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188 (defun fact (n)
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189 (if (zerop n) 1
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190 (* n (fact (1- n)))))
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191 @result{} fact
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192 @end group
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193 @group
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194 (debug-on-entry 'fact)
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195 @result{} fact
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196 @end group
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197 @group
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198 (fact 3)
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199 @end group
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200
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201 @group
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202 ------ Buffer: *Backtrace* ------
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203 Entering:
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204 * fact(3)
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205 eval-region(4870 4878 t)
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206 byte-code("...")
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207 eval-last-sexp(nil)
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208 (let ...)
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209 eval-insert-last-sexp(nil)
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210 * call-interactively(eval-insert-last-sexp)
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211 ------ Buffer: *Backtrace* ------
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212 @end group
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213
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214 @group
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215 (symbol-function 'fact)
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216 @result{} (lambda (n)
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217 (debug (quote debug))
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218 (if (zerop n) 1 (* n (fact (1- n)))))
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219 @end group
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220 @end example
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221 @end deffn
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222
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223 @deffn Command cancel-debug-on-entry function-name
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224 This function undoes the effect of @code{debug-on-entry} on
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225 @var{function-name}. When called interactively, it prompts for
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226 @var{function-name} in the minibuffer. If @var{function-name} is
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227 @code{nil} or the empty string, it cancels debugging for all functions.
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228
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229 If @code{cancel-debug-on-entry} is called more than once on the same
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230 function, the second call does nothing. @code{cancel-debug-on-entry}
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231 returns @var{function-name}.
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232 @end deffn
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233
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234 @node Explicit Debug
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235 @subsection Explicit Entry to the Debugger
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236
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237 You can cause the debugger to be called at a certain point in your
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238 program by writing the expression @code{(debug)} at that point. To do
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239 this, visit the source file, insert the text @samp{(debug)} at the
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240 proper place, and type @kbd{C-M-x}. Be sure to undo this insertion
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241 before you save the file!
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242
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243 The place where you insert @samp{(debug)} must be a place where an
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244 additional form can be evaluated and its value ignored. (If the value
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245 of @code{(debug)} isn't ignored, it will alter the execution of the
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246 program!) The most common suitable places are inside a @code{progn} or
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247 an implicit @code{progn} (@pxref{Sequencing}).
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248
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249 @node Using Debugger
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250 @subsection Using the Debugger
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251
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252 When the debugger is entered, it displays the previously selected
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253 buffer in one window and a buffer named @samp{*Backtrace*} in another
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254 window. The backtrace buffer contains one line for each level of Lisp
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255 function execution currently going on. At the beginning of this buffer
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256 is a message describing the reason that the debugger was invoked (such
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257 as the error message and associated data, if it was invoked due to an
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258 error).
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259
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260 The backtrace buffer is read-only and uses a special major mode,
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261 Debugger mode, in which letters are defined as debugger commands. The
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262 usual XEmacs editing commands are available; thus, you can switch windows
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263 to examine the buffer that was being edited at the time of the error,
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264 switch buffers, visit files, or do any other sort of editing. However,
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265 the debugger is a recursive editing level (@pxref{Recursive Editing})
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266 and it is wise to go back to the backtrace buffer and exit the debugger
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267 (with the @kbd{q} command) when you are finished with it. Exiting
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268 the debugger gets out of the recursive edit and kills the backtrace
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269 buffer.
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270
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271 @cindex current stack frame
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272 The backtrace buffer shows you the functions that are executing and
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273 their argument values. It also allows you to specify a stack frame by
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274 moving point to the line describing that frame. (A stack frame is the
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275 place where the Lisp interpreter records information about a particular
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276 invocation of a function.) The frame whose line point is on is
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277 considered the @dfn{current frame}. Some of the debugger commands
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278 operate on the current frame.
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279
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280 The debugger itself must be run byte-compiled, since it makes
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281 assumptions about how many stack frames are used for the debugger
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282 itself. These assumptions are false if the debugger is running
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283 interpreted.
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284
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285 @need 3000
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286
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287 @node Debugger Commands
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288 @subsection Debugger Commands
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289 @cindex debugger command list
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290
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291 Inside the debugger (in Debugger mode), these special commands are
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292 available in addition to the usual cursor motion commands. (Keep in
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293 mind that all the usual facilities of XEmacs, such as switching windows
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294 or buffers, are still available.)
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295
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296 The most important use of debugger commands is for stepping through
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297 code, so that you can see how control flows. The debugger can step
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298 through the control structures of an interpreted function, but cannot do
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299 so in a byte-compiled function. If you would like to step through a
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300 byte-compiled function, replace it with an interpreted definition of the
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301 same function. (To do this, visit the source file for the function and
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302 type @kbd{C-M-x} on its definition.)
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303
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304 Here is a list of Debugger mode commands:
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305
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306 @table @kbd
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307 @item c
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308 Exit the debugger and continue execution. This resumes execution of the
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309 program as if the debugger had never been entered (aside from the
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310 effect of any variables or data structures you may have changed while
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311 inside the debugger).
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312
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313 Continuing when an error or quit was signalled will cause the normal
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314 action of the signalling to take place. If you do not want this to
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315 happen, but instead want the program execution to continue as if
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316 the call to @code{signal} did not occur, use the @kbd{r} command.
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317
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318 @item d
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319 Continue execution, but enter the debugger the next time any Lisp
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320 function is called. This allows you to step through the
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321 subexpressions of an expression, seeing what values the subexpressions
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322 compute, and what else they do.
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323
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324 The stack frame made for the function call which enters the debugger in
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325 this way will be flagged automatically so that the debugger will be
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326 called again when the frame is exited. You can use the @kbd{u} command
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327 to cancel this flag.
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328
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329 @item b
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330 Flag the current frame so that the debugger will be entered when the
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331 frame is exited. Frames flagged in this way are marked with stars
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332 in the backtrace buffer.
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333
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334 @item u
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335 Don't enter the debugger when the current frame is exited. This
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336 cancels a @kbd{b} command on that frame.
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337
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338 @item e
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339 Read a Lisp expression in the minibuffer, evaluate it, and print the
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340 value in the echo area. The debugger alters certain important
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341 variables, and the current buffer, as part of its operation; @kbd{e}
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342 temporarily restores their outside-the-debugger values so you can
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343 examine them. This makes the debugger more transparent. By contrast,
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344 @kbd{M-:} does nothing special in the debugger; it shows you the
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345 variable values within the debugger.
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346
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347 @item q
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348 Terminate the program being debugged; return to top-level XEmacs
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349 command execution.
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350
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351 If the debugger was entered due to a @kbd{C-g} but you really want
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352 to quit, and not debug, use the @kbd{q} command.
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353
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354 @item r
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355 Return a value from the debugger. The value is computed by reading an
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356 expression with the minibuffer and evaluating it.
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357
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358 The @kbd{r} command is useful when the debugger was invoked due to exit
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359 from a Lisp call frame (as requested with @kbd{b}); then the value
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360 specified in the @kbd{r} command is used as the value of that frame. It
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361 is also useful if you call @code{debug} and use its return value.
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362
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363 If the debugger was entered at the beginning of a function call, @kbd{r}
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364 has the same effect as @kbd{c}, and the specified return value does not
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365 matter.
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366
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367 If the debugger was entered through a call to @code{signal} (i.e. as a
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368 result of an error or quit), then returning a value will cause the
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369 call to @code{signal} itself to return, rather than throwing to
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370 top-level or invoking a handler, as is normal. This allows you to
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371 correct an error (e.g. the type of an argument was wrong) or continue
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372 from a @code{debug-on-quit} as if it never happened.
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373
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374 Note that some errors (e.g. any error signalled using the @code{error}
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375 function, and many errors signalled from a primitive function) are not
|
|
|
376 continuable. If you return a value from them and continue execution,
|
|
|
377 then the error will immediately be signalled again. Other errors
|
|
|
378 (e.g. wrong-type-argument errors) will be continually resignalled
|
|
|
379 until the problem is corrected.
|
|
|
380 @end table
|
|
|
381
|
|
|
382 @node Invoking the Debugger
|
|
|
383 @subsection Invoking the Debugger
|
|
|
384
|
|
|
385 Here we describe fully the function used to invoke the debugger.
|
|
|
386
|
|
|
387 @defun debug &rest debugger-args
|
|
|
388 This function enters the debugger. It switches buffers to a buffer
|
|
|
389 named @samp{*Backtrace*} (or @samp{*Backtrace*<2>} if it is the second
|
|
|
390 recursive entry to the debugger, etc.), and fills it with information
|
|
|
391 about the stack of Lisp function calls. It then enters a recursive
|
|
|
392 edit, showing the backtrace buffer in Debugger mode.
|
|
|
393
|
|
|
394 The Debugger mode @kbd{c} and @kbd{r} commands exit the recursive edit;
|
|
|
395 then @code{debug} switches back to the previous buffer and returns to
|
|
|
396 whatever called @code{debug}. This is the only way the function
|
|
|
397 @code{debug} can return to its caller.
|
|
|
398
|
|
|
399 If the first of the @var{debugger-args} passed to @code{debug} is
|
|
|
400 @code{nil} (or if it is not one of the special values in the table
|
|
|
401 below), then @code{debug} displays the rest of its arguments at the
|
|
|
402 top of the @samp{*Backtrace*} buffer. This mechanism is used to display
|
|
|
403 a message to the user.
|
|
|
404
|
|
|
405 However, if the first argument passed to @code{debug} is one of the
|
|
|
406 following special values, then it has special significance. Normally,
|
|
|
407 these values are passed to @code{debug} only by the internals of XEmacs
|
|
|
408 and the debugger, and not by programmers calling @code{debug}.
|
|
|
409
|
|
|
410 The special values are:
|
|
|
411
|
|
|
412 @table @code
|
|
|
413 @item lambda
|
|
|
414 @cindex @code{lambda} in debug
|
|
|
415 A first argument of @code{lambda} means @code{debug} was called because
|
|
|
416 of entry to a function when @code{debug-on-next-call} was
|
|
|
417 non-@code{nil}. The debugger displays @samp{Entering:} as a line of
|
|
|
418 text at the top of the buffer.
|
|
|
419
|
|
|
420 @item debug
|
|
|
421 @code{debug} as first argument indicates a call to @code{debug} because
|
|
|
422 of entry to a function that was set to debug on entry. The debugger
|
|
|
423 displays @samp{Entering:}, just as in the @code{lambda} case. It also
|
|
|
424 marks the stack frame for that function so that it will invoke the
|
|
|
425 debugger when exited.
|
|
|
426
|
|
|
427 @item t
|
|
|
428 When the first argument is @code{t}, this indicates a call to
|
|
|
429 @code{debug} due to evaluation of a list form when
|
|
|
430 @code{debug-on-next-call} is non-@code{nil}. The debugger displays the
|
|
|
431 following as the top line in the buffer:
|
|
|
432
|
|
|
433 @smallexample
|
|
|
434 Beginning evaluation of function call form:
|
|
|
435 @end smallexample
|
|
|
436
|
|
|
437 @item exit
|
|
|
438 When the first argument is @code{exit}, it indicates the exit of a
|
|
|
439 stack frame previously marked to invoke the debugger on exit. The
|
|
|
440 second argument given to @code{debug} in this case is the value being
|
|
|
441 returned from the frame. The debugger displays @samp{Return value:} on
|
|
|
442 the top line of the buffer, followed by the value being returned.
|
|
|
443
|
|
|
444 @item error
|
|
|
445 @cindex @code{error} in debug
|
|
|
446 When the first argument is @code{error}, the debugger indicates that
|
|
|
447 it is being entered because an error or @code{quit} was signaled and not
|
|
|
448 handled, by displaying @samp{Signaling:} followed by the error signaled
|
|
|
449 and any arguments to @code{signal}. For example,
|
|
|
450
|
|
|
451 @example
|
|
|
452 @group
|
|
|
453 (let ((debug-on-error t))
|
|
|
454 (/ 1 0))
|
|
|
455 @end group
|
|
|
456
|
|
|
457 @group
|
|
|
458 ------ Buffer: *Backtrace* ------
|
|
|
459 Signaling: (arith-error)
|
|
|
460 /(1 0)
|
|
|
461 ...
|
|
|
462 ------ Buffer: *Backtrace* ------
|
|
|
463 @end group
|
|
|
464 @end example
|
|
|
465
|
|
|
466 If an error was signaled, presumably the variable
|
|
|
467 @code{debug-on-error} is non-@code{nil}. If @code{quit} was signaled,
|
|
|
468 then presumably the variable @code{debug-on-quit} is non-@code{nil}.
|
|
|
469
|
|
|
470 @item nil
|
|
|
471 Use @code{nil} as the first of the @var{debugger-args} when you want
|
|
|
472 to enter the debugger explicitly. The rest of the @var{debugger-args}
|
|
|
473 are printed on the top line of the buffer. You can use this feature to
|
|
|
474 display messages---for example, to remind yourself of the conditions
|
|
|
475 under which @code{debug} is called.
|
|
|
476 @end table
|
|
|
477 @end defun
|
|
|
478
|
|
|
479 @need 5000
|
|
|
480
|
|
|
481 @node Internals of Debugger
|
|
|
482 @subsection Internals of the Debugger
|
|
|
483
|
|
|
484 This section describes functions and variables used internally by the
|
|
|
485 debugger.
|
|
|
486
|
|
|
487 @defvar debugger
|
|
|
488 The value of this variable is the function to call to invoke the
|
|
|
489 debugger. Its value must be a function of any number of arguments (or,
|
|
|
490 more typically, the name of a function). Presumably this function will
|
|
|
491 enter some kind of debugger. The default value of the variable is
|
|
|
492 @code{debug}.
|
|
|
493
|
|
|
494 The first argument that Lisp hands to the function indicates why it
|
|
|
495 was called. The convention for arguments is detailed in the description
|
|
|
496 of @code{debug}.
|
|
|
497 @end defvar
|
|
|
498
|
|
|
499 @deffn Command backtrace &optional stream detailed
|
|
|
500 @cindex run time stack
|
|
|
501 @cindex call stack
|
|
|
502 This function prints a trace of Lisp function calls currently active.
|
|
|
503 This is the function used by @code{debug} to fill up the
|
|
|
504 @samp{*Backtrace*} buffer. It is written in C, since it must have access
|
|
|
505 to the stack to determine which function calls are active. The return
|
|
|
506 value is always @code{nil}.
|
|
|
507
|
|
|
508 The backtrace is normally printed to @code{standard-output}, but this
|
|
|
509 can be changed by specifying a value for @var{stream}. If
|
|
|
510 @var{detailed} is non-@code{nil}, the backtrace also shows places where
|
|
|
511 currently active variable bindings, catches, condition-cases, and
|
|
|
512 unwind-protects were made as well as function calls.
|
|
|
513
|
|
|
514 In the following example, a Lisp expression calls @code{backtrace}
|
|
|
515 explicitly. This prints the backtrace to the stream
|
|
|
516 @code{standard-output}: in this case, to the buffer
|
|
|
517 @samp{backtrace-output}. Each line of the backtrace represents one
|
|
|
518 function call. The line shows the values of the function's arguments if
|
|
|
519 they are all known. If they are still being computed, the line says so.
|
|
|
520 The arguments of special forms are elided.
|
|
|
521
|
|
|
522 @smallexample
|
|
|
523 @group
|
|
|
524 (with-output-to-temp-buffer "backtrace-output"
|
|
|
525 (let ((var 1))
|
|
|
526 (save-excursion
|
|
|
527 (setq var (eval '(progn
|
|
|
528 (1+ var)
|
|
|
529 (list 'testing (backtrace))))))))
|
|
|
530
|
|
|
531 @result{} nil
|
|
|
532 @end group
|
|
|
533
|
|
|
534 @group
|
|
|
535 ----------- Buffer: backtrace-output ------------
|
|
|
536 backtrace()
|
|
|
537 (list ...computing arguments...)
|
|
|
538 (progn ...)
|
|
|
539 eval((progn (1+ var) (list (quote testing) (backtrace))))
|
|
|
540 (setq ...)
|
|
|
541 (save-excursion ...)
|
|
|
542 (let ...)
|
|
|
543 (with-output-to-temp-buffer ...)
|
|
|
544 eval-region(1973 2142 #<buffer *scratch*>)
|
|
|
545 byte-code("... for eval-print-last-sexp ...")
|
|
|
546 eval-print-last-sexp(nil)
|
|
|
547 * call-interactively(eval-print-last-sexp)
|
|
|
548 ----------- Buffer: backtrace-output ------------
|
|
|
549 @end group
|
|
|
550 @end smallexample
|
|
|
551
|
|
|
552 The character @samp{*} indicates a frame whose debug-on-exit flag is
|
|
|
553 set.
|
|
|
554 @end deffn
|
|
|
555
|
|
|
556 @ignore @c Not worth mentioning
|
|
|
557 @defopt stack-trace-on-error
|
|
|
558 @cindex stack trace
|
|
|
559 This variable controls whether Lisp automatically displays a
|
|
|
560 backtrace buffer after every error that is not handled. A quit signal
|
|
|
561 counts as an error for this variable. If it is non-@code{nil} then a
|
|
|
562 backtrace is shown in a pop-up buffer named @samp{*Backtrace*} on every
|
|
|
563 error. If it is @code{nil}, then a backtrace is not shown.
|
|
|
564
|
|
|
565 When a backtrace is shown, that buffer is not selected. If either
|
|
|
566 @code{debug-on-quit} or @code{debug-on-error} is also non-@code{nil}, then
|
|
|
567 a backtrace is shown in one buffer, and the debugger is popped up in
|
|
|
568 another buffer with its own backtrace.
|
|
|
569
|
|
|
570 We consider this feature to be obsolete and superseded by the debugger
|
|
|
571 itself.
|
|
|
572 @end defopt
|
|
|
573 @end ignore
|
|
|
574
|
|
|
575 @defvar debug-on-next-call
|
|
|
576 @cindex @code{eval}, and debugging
|
|
|
577 @cindex @code{apply}, and debugging
|
|
|
578 @cindex @code{funcall}, and debugging
|
|
|
579 If this variable is non-@code{nil}, it says to call the debugger before
|
|
|
580 the next @code{eval}, @code{apply} or @code{funcall}. Entering the
|
|
|
581 debugger sets @code{debug-on-next-call} to @code{nil}.
|
|
|
582
|
|
|
583 The @kbd{d} command in the debugger works by setting this variable.
|
|
|
584 @end defvar
|
|
|
585
|
|
|
586 @defun backtrace-debug level flag
|
|
|
587 This function sets the debug-on-exit flag of the stack frame @var{level}
|
|
|
588 levels down the stack, giving it the value @var{flag}. If @var{flag} is
|
|
|
589 non-@code{nil}, this will cause the debugger to be entered when that
|
|
|
590 frame later exits. Even a nonlocal exit through that frame will enter
|
|
|
591 the debugger.
|
|
|
592
|
|
|
593 This function is used only by the debugger.
|
|
|
594 @end defun
|
|
|
595
|
|
|
596 @defvar command-debug-status
|
|
|
597 This variable records the debugging status of the current interactive
|
|
|
598 command. Each time a command is called interactively, this variable is
|
|
|
599 bound to @code{nil}. The debugger can set this variable to leave
|
|
|
600 information for future debugger invocations during the same command.
|
|
|
601
|
|
|
602 The advantage, for the debugger, of using this variable rather than
|
|
|
603 another global variable is that the data will never carry over to a
|
|
|
604 subsequent command invocation.
|
|
|
605 @end defvar
|
|
|
606
|
|
|
607 @defun backtrace-frame frame-number
|
|
|
608 The function @code{backtrace-frame} is intended for use in Lisp
|
|
|
609 debuggers. It returns information about what computation is happening
|
|
|
610 in the stack frame @var{frame-number} levels down.
|
|
|
611
|
|
|
612 If that frame has not evaluated the arguments yet (or is a special
|
|
|
613 form), the value is @code{(nil @var{function} @var{arg-forms}@dots{})}.
|
|
|
614
|
|
|
615 If that frame has evaluated its arguments and called its function
|
|
|
616 already, the value is @code{(t @var{function}
|
|
|
617 @var{arg-values}@dots{})}.
|
|
|
618
|
|
|
619 In the return value, @var{function} is whatever was supplied as the
|
|
|
620 @sc{car} of the evaluated list, or a @code{lambda} expression in the
|
|
|
621 case of a macro call. If the function has a @code{&rest} argument, that
|
|
|
622 is represented as the tail of the list @var{arg-values}.
|
|
|
623
|
|
|
624 If @var{frame-number} is out of range, @code{backtrace-frame} returns
|
|
|
625 @code{nil}.
|
|
|
626 @end defun
|
|
|
627
|
|
|
628 @node Syntax Errors
|
|
|
629 @section Debugging Invalid Lisp Syntax
|
|
|
630
|
|
|
631 The Lisp reader reports invalid syntax, but cannot say where the real
|
|
|
632 problem is. For example, the error ``End of file during parsing'' in
|
|
|
633 evaluating an expression indicates an excess of open parentheses (or
|
|
|
634 square brackets). The reader detects this imbalance at the end of the
|
|
|
635 file, but it cannot figure out where the close parenthesis should have
|
|
|
636 been. Likewise, ``Invalid read syntax: ")"'' indicates an excess close
|
|
|
637 parenthesis or missing open parenthesis, but does not say where the
|
|
|
638 missing parenthesis belongs. How, then, to find what to change?
|
|
|
639
|
|
|
640 If the problem is not simply an imbalance of parentheses, a useful
|
|
|
641 technique is to try @kbd{C-M-e} at the beginning of each defun, and see
|
|
|
642 if it goes to the place where that defun appears to end. If it does
|
|
|
643 not, there is a problem in that defun.
|
|
|
644
|
|
|
645 However, unmatched parentheses are the most common syntax errors in
|
|
|
646 Lisp, and we can give further advice for those cases.
|
|
|
647
|
|
|
648 @menu
|
|
|
649 * Excess Open:: How to find a spurious open paren or missing close.
|
|
|
650 * Excess Close:: How to find a spurious close paren or missing open.
|
|
|
651 @end menu
|
|
|
652
|
|
|
653 @node Excess Open
|
|
|
654 @subsection Excess Open Parentheses
|
|
|
655
|
|
|
656 The first step is to find the defun that is unbalanced. If there is
|
|
|
657 an excess open parenthesis, the way to do this is to insert a
|
|
|
658 close parenthesis at the end of the file and type @kbd{C-M-b}
|
|
|
659 (@code{backward-sexp}). This will move you to the beginning of the
|
|
|
660 defun that is unbalanced. (Then type @kbd{C-@key{SPC} C-_ C-u
|
|
|
661 C-@key{SPC}} to set the mark there, undo the insertion of the
|
|
|
662 close parenthesis, and finally return to the mark.)
|
|
|
663
|
|
|
664 The next step is to determine precisely what is wrong. There is no
|
|
|
665 way to be sure of this except to study the program, but often the
|
|
|
666 existing indentation is a clue to where the parentheses should have
|
|
|
667 been. The easiest way to use this clue is to reindent with @kbd{C-M-q}
|
|
|
668 and see what moves.
|
|
|
669
|
|
|
670 Before you do this, make sure the defun has enough close parentheses.
|
|
|
671 Otherwise, @kbd{C-M-q} will get an error, or will reindent all the rest
|
|
|
672 of the file until the end. So move to the end of the defun and insert a
|
|
|
673 close parenthesis there. Don't use @kbd{C-M-e} to move there, since
|
|
|
674 that too will fail to work until the defun is balanced.
|
|
|
675
|
|
|
676 Now you can go to the beginning of the defun and type @kbd{C-M-q}.
|
|
|
677 Usually all the lines from a certain point to the end of the function
|
|
|
678 will shift to the right. There is probably a missing close parenthesis,
|
|
|
679 or a superfluous open parenthesis, near that point. (However, don't
|
|
|
680 assume this is true; study the code to make sure.) Once you have found
|
|
|
681 the discrepancy, undo the @kbd{C-M-q} with @kbd{C-_}, since the old
|
|
|
682 indentation is probably appropriate to the intended parentheses.
|
|
|
683
|
|
|
684 After you think you have fixed the problem, use @kbd{C-M-q} again. If
|
|
|
685 the old indentation actually fit the intended nesting of parentheses,
|
|
|
686 and you have put back those parentheses, @kbd{C-M-q} should not change
|
|
|
687 anything.
|
|
|
688
|
|
|
689 @node Excess Close
|
|
|
690 @subsection Excess Close Parentheses
|
|
|
691
|
|
|
692 To deal with an excess close parenthesis, first insert an open
|
|
|
693 parenthesis at the beginning of the file, back up over it, and type
|
|
|
694 @kbd{C-M-f} to find the end of the unbalanced defun. (Then type
|
|
|
695 @kbd{C-@key{SPC} C-_ C-u C-@key{SPC}} to set the mark there, undo the
|
|
|
696 insertion of the open parenthesis, and finally return to the mark.)
|
|
|
697
|
|
|
698 Then find the actual matching close parenthesis by typing @kbd{C-M-f}
|
|
|
699 at the beginning of the defun. This will leave you somewhere short of
|
|
|
700 the place where the defun ought to end. It is possible that you will
|
|
|
701 find a spurious close parenthesis in that vicinity.
|
|
|
702
|
|
|
703 If you don't see a problem at that point, the next thing to do is to
|
|
|
704 type @kbd{C-M-q} at the beginning of the defun. A range of lines will
|
|
|
705 probably shift left; if so, the missing open parenthesis or spurious
|
|
|
706 close parenthesis is probably near the first of those lines. (However,
|
|
|
707 don't assume this is true; study the code to make sure.) Once you have
|
|
|
708 found the discrepancy, undo the @kbd{C-M-q} with @kbd{C-_}, since the
|
|
|
709 old indentation is probably appropriate to the intended parentheses.
|
|
|
710
|
|
|
711 After you think you have fixed the problem, use @kbd{C-M-q} again. If
|
|
|
712 the old indentation actually fit the intended nesting of parentheses,
|
|
|
713 and you have put back those parentheses, @kbd{C-M-q} should not change
|
|
|
714 anything.
|
|
|
715
|
|
|
716 @node Compilation Errors, Edebug, Syntax Errors, Debugging
|
|
|
717 @section Debugging Problems in Compilation
|
|
|
718
|
|
|
719 When an error happens during byte compilation, it is normally due to
|
|
|
720 invalid syntax in the program you are compiling. The compiler prints a
|
|
|
721 suitable error message in the @samp{*Compile-Log*} buffer, and then
|
|
|
722 stops. The message may state a function name in which the error was
|
|
|
723 found, or it may not. Either way, here is how to find out where in the
|
|
|
724 file the error occurred.
|
|
|
725
|
|
|
726 What you should do is switch to the buffer @w{@samp{ *Compiler Input*}}.
|
|
|
727 (Note that the buffer name starts with a space, so it does not show
|
|
|
728 up in @kbd{M-x list-buffers}.) This buffer contains the program being
|
|
|
729 compiled, and point shows how far the byte compiler was able to read.
|
|
|
730
|
|
|
731 If the error was due to invalid Lisp syntax, point shows exactly where
|
|
|
732 the invalid syntax was @emph{detected}. The cause of the error is not
|
|
|
733 necessarily near by! Use the techniques in the previous section to find
|
|
|
734 the error.
|
|
|
735
|
|
|
736 If the error was detected while compiling a form that had been read
|
|
|
737 successfully, then point is located at the end of the form. In this
|
|
|
738 case, this technique can't localize the error precisely, but can still
|
|
|
739 show you which function to check.
|
|
|
740
|
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|
741 @include edebug-inc.texi
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