docs/gdb_post_mortem.html

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   7 <h1 align="center">How-to debug kernel crashes with GDB</h1>
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  16   If you are hacking the Kernel and experiencing Kernel Crashes,
  17   the kernel now supports some minimal remote debugging capabilities
  18   to hook into GDB. Here are some simple indications.
  19   </td>
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  28    <center><h3>What you need</h3></center>
  29    <ul>
  30     <li> A GDB enabled machine and OS (I use FreeBSD myself, but any
  31      Linux or even Windows+CygWin will do.)</li>
  32
  33     <li> A null modem cable. If you are hacking the kernel you probably
  34      already have one for the serial debug output.</li>
  35
  36     <li> A kernel file with symbols. In the root makefile edit the lines
  37      for GLOBAL_CFLAGS and GLOBAL_LDFLAGS and add -g to them. Rebuild the kernel.<br><br>
  38      <b>*You dont need to boot off a kernel with symbols*</b><br>
  39       The kernel with symbols is only required in the
  40       host running GDB. However, it is important that
  41       both kernels (the running one and the one in the
  42       GDB host) perfectly match each other: this means
  43       they must be compiled from the same source tree
  44       with the same compile options (except for the -g).
  45
  46     <li> A kernel crash to analyse.</li>
  47    </ul>
  48   </td>
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  50 </table>
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  57    <center><h3>How to debug</h3></center>
  58    <ul>
  59     <li> once the kernel has crashed, close the terminal emulator (if you are using one)
  60      used for serial debug output.</li>
  61
  62     <li> launch GDB specifying the path to kernel file as argument.</li>
  63
  64     <li> in GDB type the following commands:
  65      <br><br>
  66      set remotebaud XXXX<br>
  67      target remote YYYY<br>
  68      <br>
  69      XXXX is the baud rate your kernel is using for
  70      serial debug output (115200 by default, unless
  71      you changed it). YYYY is the device path to
  72      the serial port used for debug output.<br><br>
  73
  74      Once you type the "target remote YYYY" command
  75      you will be debugging your kernel. The kernel
  76      debugger will remain in GDB mode until you quit
  77      from GDB, at that point it will return to the
  78      normal crash prompt.</li>
  79    </ul>
  80   </td>
  81  </tr>
  82 </table>
  83
  84 <hr>
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  86 <table border="0" width="100%" bgcolor="#E8E8E8">
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  89    <center><h3>Notes</h3></center>
  90    <ul>
  91    <li>Personally I have obtained better results by using
  92    57600 as serial port speed. Your mileage may vary
  93    depending on your serial ports, the length of your
  94    serial cable and the phase of the moon. If you feel
  95    that it is sluggish try reducing the serial port
  96    speed in your kernel (kernel/arch/i386/ arch_dbg_console.c)</li>
  97
  98    <li>Currently only inspecting the crash is supported,
  99    you cannot single step, set breakpoints etc.</li>
 100
 101    <li>For better debugging you may want to have your
 102    kernel debugged without optimizations. In the
 103    GLOBAL_CFLAGS change the -O to -O0. When compiling
 104    with optimizations the compiler may assign some
 105    of your local variables to registers, and you
 106    will not be able to inspect some values for
 107    functions other than the call stack tip.</li>
 108
 109    <li>After the "target remote" command you may get an eror message similar to:
 110    <br>
 111    &nbsp;&nbsp;&nbsp;ptrace(PT_GETDBREGS) failed: No such process
 112    <br>
 113    you can safely ignore it. It's just GDB being silly.</li>
 114    </ul>
 115   </td>
 116  </tr>
 117 </table>
 118
 119 <hr>
 120
 121 <table border="0" width="100%" bgcolor="#E8E8E8">
 122  <tr>
 123   <td width="100%" height="100%" valign="top" align="left">
 124    <center><h3>Troubleshooting</h3></center>
 125    If you have troubles, before typing the "target remote" command. Type the
 126    following command:<br>
 127    &nbsp;&nbsp;&nbsp;set remotedebug 1<br>
 128    This will enable tracing all (actually most) of the data packets exchanged
 129    between GDB and the kernel.  Now try to reproduce your troubles and send the
 130    traces to the NewOS developer list.
 131   </td>
 132  </tr>
 133 </table>
 134
 135 <hr>
 136
 137 <table border="0" width="100%" bgcolor="#E8E8E8">
 138  <tr>
 139   <td width="100%" height="100%" valign="top" align="left">
 140    <center><h3>Sample debug session</h3></center>
 141 &nbsp;&nbsp;&nbsp;[freston@cairo&nbsp;~/NewOS/newos]<br>
 142 &nbsp;&nbsp;&nbsp;[freston@cairo&nbsp;~/NewOS/newos]&nbsp;gdb&nbsp;kernel/obj.i386/system<br>
 143 &nbsp;&nbsp;&nbsp;GNU&nbsp;gdb&nbsp;4.18<br>
 144 &nbsp;&nbsp;&nbsp;Copyright&nbsp;1998&nbsp;Free&nbsp;Software&nbsp;Foundation,&nbsp;Inc.<br>
 145 &nbsp;&nbsp;&nbsp;GDB&nbsp;is&nbsp;free&nbsp;software,&nbsp;covered&nbsp;by&nbsp;the&nbsp;GNU&nbsp;General&nbsp;Public&nbsp;License,&nbsp;and&nbsp;you&nbsp;are<br>
 146 &nbsp;&nbsp;&nbsp;welcome&nbsp;to&nbsp;change&nbsp;it&nbsp;and/or&nbsp;distribute&nbsp;copies&nbsp;of&nbsp;it&nbsp;under&nbsp;certain&nbsp;conditions.<br>
 147 &nbsp;&nbsp;&nbsp;Type&nbsp;"show&nbsp;copying"&nbsp;to&nbsp;see&nbsp;the&nbsp;conditions.<br>
 148 &nbsp;&nbsp;&nbsp;There&nbsp;is&nbsp;absolutely&nbsp;no&nbsp;warranty&nbsp;for&nbsp;GDB.&nbsp;&nbsp;Type&nbsp;"show&nbsp;warranty"&nbsp;for&nbsp;details.<br>
 149 &nbsp;&nbsp;&nbsp;This&nbsp;GDB&nbsp;was&nbsp;configured&nbsp;as&nbsp;"i386-unknown-freebsd"...<br>
 150 &nbsp;&nbsp;&nbsp;(gdb)&nbsp;set&nbsp;remotebaud&nbsp;57600<br>
 151 &nbsp;&nbsp;&nbsp;(gdb)&nbsp;target&nbsp;remote&nbsp;/dev/cuaa0<br>
 152 &nbsp;&nbsp;&nbsp;Remote&nbsp;debugging&nbsp;using&nbsp;/dev/cuaa0<br>
 153 &nbsp;&nbsp;&nbsp;ptrace(PT_GETDBREGS)&nbsp;failed:&nbsp;No&nbsp;such&nbsp;process<br>
 154 &nbsp;&nbsp;&nbsp;0x80009132&nbsp;in&nbsp;kernel_debugger&nbsp;()&nbsp;at&nbsp;kernel/debug.c:227<br>
 155 &nbsp;&nbsp;&nbsp;227&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;dbg_save_registers(&(dbg_register_file[smp_get_current_cpu()][0]));<br>
 156 &nbsp;&nbsp;&nbsp;(gdb)&nbsp;where<br>
 157 &nbsp;&nbsp;&nbsp;#0&nbsp;&nbsp;0x80009132&nbsp;in&nbsp;kernel_debugger&nbsp;()&nbsp;at&nbsp;kernel/debug.c:227<br>
 158 &nbsp;&nbsp;&nbsp;#1&nbsp;&nbsp;0x800091d5&nbsp;in&nbsp;panic&nbsp;(fmt=0x80036de0&nbsp;"Keyboard&nbsp;Requested&nbsp;Halt\n")<br>
 159 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;at&nbsp;kernel/debug.c:257<br>
 160 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;#2&nbsp;&nbsp;0x8002bf85&nbsp;in&nbsp;handle_keyboard_interrupt&nbsp;(data=0x0)<br>
 161 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;at&nbsp;kernel/dev/arch/i386/keyboard/keyboard.c:185<br>
 162 &nbsp;&nbsp;&nbsp;#3&nbsp;&nbsp;0x80008aca&nbsp;in&nbsp;int_io_interrupt_handler&nbsp;(vector=33)&nbsp;at&nbsp;kernel/int.c:135<br>
 163 &nbsp;&nbsp;&nbsp;#4&nbsp;&nbsp;0x80025981&nbsp;in&nbsp;i386_handle_trap&nbsp;(frame={gs&nbsp;=&nbsp;16,&nbsp;fs&nbsp;=&nbsp;16,&nbsp;es&nbsp;=&nbsp;16,<br>
 164 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;ds&nbsp;=&nbsp;2147745808,&nbsp;edi&nbsp;=&nbsp;0,&nbsp;esi&nbsp;=&nbsp;0,&nbsp;ebp&nbsp;=&nbsp;2147786692,&nbsp;esp&nbsp;=&nbsp;2147786648,<br>
 165 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;ebx&nbsp;=&nbsp;0,&nbsp;edx&nbsp;=&nbsp;39,&nbsp;ecx&nbsp;=&nbsp;1,&nbsp;eax&nbsp;=&nbsp;39,&nbsp;vector&nbsp;=&nbsp;33,&nbsp;error_code&nbsp;=&nbsp;0,<br>
 166 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;eip&nbsp;=&nbsp;2147506764,&nbsp;cs&nbsp;=&nbsp;8,&nbsp;flags&nbsp;=&nbsp;646,&nbsp;user_esp&nbsp;=&nbsp;0,&nbsp;user_ss&nbsp;=&nbsp;0})<br>
 167 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;at&nbsp;kernel/arch/i386/arch_int.c:232<br>
 168 &nbsp;#5&nbsp;&nbsp;0x8002555c&nbsp;in&nbsp;int_bottom&nbsp;()<br>
 169 &nbsp;#6&nbsp;&nbsp;0x0&nbsp;in&nbsp;??&nbsp;()<br>
 170 &nbsp;(gdb)&nbsp;list<br>
 171 &nbsp;222&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;}<br>
 172 &nbsp;223&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;}<br>
 173 &nbsp;224<br>
 174 &nbsp;225&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;void&nbsp;kernel_debugger()<br>
 175 &nbsp;226&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;{<br>
 176 &nbsp;227&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;dbg_save_registers(&(dbg_register_file[smp_get_current_cpu()][0]));<br>
 177 &nbsp;228<br>
 178 &nbsp;229&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;kernel_debugger_loop();<br>
 179 &nbsp;230&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;}<br>
 180 &nbsp;231<br>
 181 &nbsp;(gdb)&nbsp;up<br>
 182 &nbsp;#1&nbsp;&nbsp;0x800091d5&nbsp;in&nbsp;panic&nbsp;(fmt=0x80036de0&nbsp;"Keyboard&nbsp;Requested&nbsp;Halt\n")<br>
 183 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;at&nbsp;kernel/debug.c:257<br>
 184 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;257&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;kernel_debugger();<br>
 185 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;(gdb)&nbsp;up<br>
 186 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;#2&nbsp;&nbsp;0x8002bf85&nbsp;in&nbsp;handle_keyboard_interrupt&nbsp;(data=0x0)<br>
 187 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;at&nbsp;kernel/dev/arch/i386/keyboard/keyboard.c:185<br>
 188 &nbsp;185&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;panic("Keyboard&nbsp;Requested&nbsp;Halt\n");<br>
 189 &nbsp;(gdb)&nbsp;list<br>
 190 &nbsp;180&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;else<br>
 191 &nbsp;181&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;leds&nbsp;|=&nbsp;LED_NUM;<br>
 192 &nbsp;182&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;set_leds();<br>
 193 &nbsp;183&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;break;<br>
 194 &nbsp;184&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;case&nbsp;SYSREQ:<br>
 195 &nbsp;185&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;panic("Keyboard&nbsp;Requested&nbsp;Halt\n");<br>
 196 &nbsp;186&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;break;<br>
 197 &nbsp;187&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;case&nbsp;F12:<br>
 198 &nbsp;188&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;reboot();<br>
 199 &nbsp;189&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;break;<br>
 200 &nbsp;(gdb)&nbsp;print&nbsp;leds<br>
 201 &nbsp;$1&nbsp;=&nbsp;1<br>
 202 &nbsp;(gdb)&nbsp;up<br>
 203 &nbsp;#3&nbsp;&nbsp;0x80008aca&nbsp;in&nbsp;int_io_interrupt_handler&nbsp;(vector=33)&nbsp;at&nbsp;kernel/int.c:135<br>
 204 &nbsp;135&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;temp_ret&nbsp;=&nbsp;io->func(io->data);<br>
 205 &nbsp;(gdb)&nbsp;list<br>
 206 &nbsp;130&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;struct&nbsp;io_handler&nbsp;*io;<br>
 207 &nbsp;131&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;int&nbsp;temp_ret;<br>
 208 &nbsp;132<br>
 209 &nbsp;133&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;io&nbsp;=&nbsp;io_handlers[vector];<br>
 210 &nbsp;134&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;while(io&nbsp;!=&nbsp;NULL)&nbsp;{<br>
 211 &nbsp;135&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;temp_ret&nbsp;=&nbsp;io->func(io->data);<br>
 212 &nbsp;136&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;if(temp_ret&nbsp;==&nbsp;INT_RESCHEDULE)<br>
 213 &nbsp;137&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;ret&nbsp;=&nbsp;INT_RESCHEDULE;<br>
 214 &nbsp;138&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;io&nbsp;=&nbsp;io->next;<br>
 215 &nbsp;139&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;}<br>
 216 &nbsp;(gdb)&nbsp;print&nbsp;vector<br>
 217 &nbsp;$2&nbsp;=&nbsp;33<br>
 218 &nbsp;(gdb)&nbsp;print&nbsp;io_handlers[vector]<br>
 219 &nbsp;$3&nbsp;=&nbsp;(struct&nbsp;io_handler&nbsp;*)&nbsp;0x8008fb20<br>
 220 &nbsp;(gdb)&nbsp;print&nbsp;io<br>
 221 &nbsp;$4&nbsp;=&nbsp;(struct&nbsp;io_handler&nbsp;*)&nbsp;0x8008fb20<br>
 222 &nbsp;(gdb)&nbsp;print&nbsp;*io<br>
 223 &nbsp;$5&nbsp;=&nbsp;{next&nbsp;=&nbsp;0x0,&nbsp;func&nbsp;=&nbsp;0x8002be6c&nbsp;<handle_keyboard_interrupt>,&nbsp;data&nbsp;=&nbsp;0x0}<br>
 224 &nbsp;(gdb)&nbsp;print&nbsp;*io->next<br>
 225 &nbsp;Cannot&nbsp;access&nbsp;memory&nbsp;at&nbsp;address&nbsp;0x0.<br>
 226 &nbsp;(gdb)&nbsp;quit<br>
 227 &nbsp;The&nbsp;program&nbsp;is&nbsp;running.&nbsp;&nbsp;Exit&nbsp;anyway?&nbsp;(y&nbsp;or&nbsp;n)&nbsp;y<br>
 228 &nbsp;[freston@cairo&nbsp;~/NewOS/newos]<br>
 229   </td>
 230  </tr>
 231 </table>
 232
 233 <hr>
 234
 235 <table border="1" width="100%" bgcolor="#D9D5FF">
 236  <tr>
 237   <td width="100%" height="49" valign="top" align="center"><small><font color="#000000">This
 238    page copyright 2002 </font></small><font size="2">Manuel J. Petit
 239 <!--   <br>
 240    Any questions, comments, gripes, whatever:
 241    <a href="mailto:geist@newos.org">geist@newos.org</a></font>
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