Red Hat ENTERPRISE LINUX 3 - SECURITY GUIDE User manual

Red Hat Enterprise Linux 3
Debugging with gdb

Red Hat Enterprise Linux 3: Debugging with gdb
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Table of Contents
1. Debugging with gdb ........................................................................................................................ 1
2. Summary of gdb.............................................................................................................................. 3
2.1. Free software...................................................................................................................... 3
2.2. Free Software Needs Free Documentation ........................................................................ 3
2.3. Contributors to gdb ............................................................................................................ 4
3. A Sample gdb Session ..................................................................................................................... 7
4. Getting In and Out of gdb............................................................................................................ 11
4.1. Invoking gdb .................................................................................................................... 11
4.1.1. Choosing files ................................................................................................... 12
4.1.2. Choosing modes................................................................................................ 13
4.2. Quitting gdb ..................................................................................................................... 16
4.3. Shell commands ............................................................................................................... 16
4.4. Logging output................................................................................................................. 16
5. gdb Commands ............................................................................................................................. 19
5.1. Command syntax ............................................................................................................. 19
5.2. Command completion...................................................................................................... 19
5.3. Getting help...................................................................................................................... 21
6. Running Programs Under gdb .................................................................................................... 25
6.1. Compiling for debugging................................................................................................. 25
6.2. Starting your program ...................................................................................................... 25
6.3. Your program’s arguments............................................................................................... 26
6.4. Your program’s environment ........................................................................................... 27
6.5. Your program’s working directory................................................................................... 28
6.6. Your program’s input and output ..................................................................................... 28
6.7. Debugging an already-running process ........................................................................... 29
6.8. Killing the child process .................................................................................................. 29
6.9. Debugging programs with multiple threads..................................................................... 30
6.10. Debugging programs with multiple processes............................................................... 32
7. Stopping and Continuing ............................................................................................................. 35
7.1. Breakpoints, watchpoints, and catchpoints...................................................................... 35
7.1.1. Setting breakpoints............................................................................................ 35
7.1.2. Setting watchpoints........................................................................................... 38
7.1.3. Setting catchpoints ............................................................................................ 40
7.1.4. Deleting breakpoints ......................................................................................... 41
7.1.5. Disabling breakpoints ....................................................................................... 42
7.1.6. Break conditions ............................................................................................... 43
7.1.7. Breakpoint command lists................................................................................. 44
7.1.8. Breakpoint menus ............................................................................................. 45
7.1.9. "Cannot insert breakpoints" .............................................................................. 46
7.2. Continuing and stepping .................................................................................................. 46
7.3. Signals.............................................................................................................................. 49
7.4. Stopping and starting multi-thread programs .................................................................. 51
8. Examining the Stack..................................................................................................................... 53
8.1. Stack frames..................................................................................................................... 53
8.2. Backtraces ........................................................................................................................ 54
8.3. Selecting a frame.............................................................................................................. 55
8.4. Information about a frame ............................................................................................... 56

9. Examining Source Files ................................................................................................................ 59
9.1. Printing source lines......................................................................................................... 59
9.2. Editing source files........................................................................................................... 60
9.2.1. Choosing your editor......................................................................................... 61
9.3. Searching source files ...................................................................................................... 61
9.4. Specifying source directories........................................................................................... 62
9.5. Source and machine code................................................................................................. 63
10. Examining Data........................................................................................................................... 65
10.1. Expressions .................................................................................................................... 65
10.2. Program variables .......................................................................................................... 66
10.3. Artificial arrays .............................................................................................................. 67
10.4. Output formats ............................................................................................................... 68
10.5. Examining memory........................................................................................................ 69
10.6. Automatic display .......................................................................................................... 71
10.7. Print settings .................................................................................................................. 72
10.8. Value history .................................................................................................................. 77
10.9. Convenience variables ................................................................................................... 78
10.10. Registers....................................................................................................................... 79
10.11. Floating point hardware ............................................................................................... 80
10.12. Vector Unit................................................................................................................... 81
10.13. Memory region attributes............................................................................................. 81
10.13.1. Attributes....................................................................................................... 82
10.14. Copy between memory and a file................................................................................. 83
10.15. Character Sets .............................................................................................................. 84
11. C Preprocessor Macros............................................................................................................... 89
12. Tracepoints .................................................................................................................................. 93
12.1. Commands to Set Tracepoints ....................................................................................... 93
12.1.1. Create and Delete Tracepoints ........................................................................ 93
12.1.2. Enable and Disable Tracepoints...................................................................... 94
12.1.3. Tracepoint Passcounts..................................................................................... 94
12.1.4. Tracepoint Action Lists................................................................................... 95
12.1.5. Listing Tracepoints ......................................................................................... 96
12.1.6. Starting and Stopping Trace Experiment ........................................................ 96
12.2. Using the collected data ................................................................................................. 97
12.2.1. tfind n.......................................................................................................... 97
12.2.2. tdump.............................................................................................................. 99
12.2.3. save-tracepoints filename .................................................................. 100
12.3. Convenience Variables for Tracepoints........................................................................ 100
13. Debugging Programs That Use Overlays................................................................................ 103
13.1. How Overlays Work..................................................................................................... 103
13.2. Overlay Commands ..................................................................................................... 104
13.3. Automatic Overlay Debugging .................................................................................... 106
13.4. Overlay Sample Program............................................................................................. 107
14. Using gdb with Different Languages....................................................................................... 109
14.1. Switching between source languages........................................................................... 109
14.1.1. List of filename extensions and languages.................................................... 109
14.1.2. Setting the working language........................................................................ 110
14.1.3. Having gdb infer the source language .......................................................... 110
14.2. Displaying the language............................................................................................... 110
14.3. Type and range checking.............................................................................................. 111
14.3.1. An overview of type checking ...................................................................... 111
14.3.2. An overview of range checking .................................................................... 112
14.4. Supported languages .................................................................................................... 113

14.4.1. C and C++ .................................................................................................... 113
14.4.2. Objective-C ................................................................................................... 119
14.4.3. Modula-2....................................................................................................... 121
14.5. Unsupported languages................................................................................................ 126
15. Examining the Symbol Table ................................................................................................... 129
16. Altering Execution .................................................................................................................... 135
16.1. Assignment to variables............................................................................................... 135
16.2. Continuing at a different address ................................................................................. 136
16.3. Giving your program a signal ...................................................................................... 137
16.4. Returning from a function............................................................................................ 137
16.5. Calling program functions ........................................................................................... 137
16.6. Patching programs ....................................................................................................... 138
17. gdb Files ..................................................................................................................................... 139
17.1. Commands to specify files ........................................................................................... 139
17.2. Debugging Information in Separate Files .................................................................... 145
17.3. Errors reading symbol files .......................................................................................... 147
18. Specifying a Debugging Target ................................................................................................ 149
18.1. Active targets ............................................................................................................... 149
18.2. Commands for managing targets ................................................................................. 149
18.3. Choosing target byte order........................................................................................... 151
18.4. Remote debugging ....................................................................................................... 151
18.5. Kernel Object Display.................................................................................................. 151
19. Debugging remote programs.................................................................................................... 153
19.1. Connecting to a remote target ...................................................................................... 153
19.2. Using the gdbserverprogram .................................................................................... 154
19.3. Using the gdbserve.nlmprogram.............................................................................. 155
19.4. Remote configuration................................................................................................... 155
19.5. Implementing a remote stub......................................................................................... 156
19.5.1. What the stub can do for you ........................................................................ 157
19.5.2. What you must do for the stub...................................................................... 157
19.5.3. Putting it all together..................................................................................... 158
20. Configuration-Specific Information ........................................................................................ 161
20.1. Native ........................................................................................................................... 161
20.1.1. HP-UX .......................................................................................................... 161
20.1.2. SVR4 process information............................................................................ 161
20.1.3. Features for Debugging djgpp Programs ...................................................... 161
20.1.4. Features for Debugging MS Windows PE executables................................. 163
20.2. Embedded Operating Systems ..................................................................................... 166
20.2.1. Using gdb with VxWorks.............................................................................. 166
20.3. Embedded Processors .................................................................................................. 168
20.3.1. ARM ............................................................................................................. 168
20.3.2. Hitachi H8/300.............................................................................................. 168
20.3.3. H8/500........................................................................................................... 171
20.3.4. Mitsubishi M32R/D ...................................................................................... 171
20.3.5. M68k ............................................................................................................. 171
20.3.6. MIPS Embedded ........................................................................................... 172
20.3.7. OpenRISC 1000 ............................................................................................ 173
20.3.8. PowerPC ....................................................................................................... 175
20.3.9. HP PA Embedded.......................................................................................... 176
20.3.10. Hitachi SH................................................................................................... 176
20.3.11. Tsqware Sparclet......................................................................................... 176
20.3.12. Fujitsu Sparclite .......................................................................................... 178
20.3.13. Tandem ST2000 .......................................................................................... 178

20.3.14. Zilog Z8000 ................................................................................................ 179
20.4. Architectures ................................................................................................................ 180
20.4.1. A29K............................................................................................................. 180
20.4.2. Alpha............................................................................................................. 180
20.4.3. MIPS ............................................................................................................. 180
21. Controlling gdb ......................................................................................................................... 181
21.1. Prompt.......................................................................................................................... 181
21.2. Command editing......................................................................................................... 181
21.3. Command history......................................................................................................... 181
21.4. Screen size ................................................................................................................... 183
21.5. Numbers....................................................................................................................... 183
21.6. Configuring the current ABI ........................................................................................ 184
21.7. Optional warnings and messages ................................................................................. 185
21.8. Optional messages about internal happenings ............................................................. 186
22. Canned Sequences of Commands............................................................................................ 189
22.1. User-defined commands............................................................................................... 189
22.2. User-defined command hooks...................................................................................... 190
22.3. Command files ............................................................................................................. 191
22.4. Commands for controlled output ................................................................................. 192
23. Command Interpreters............................................................................................................. 195
24. gdb Text User Interface ............................................................................................................ 197
24.1. TUI overview ............................................................................................................... 197
24.2. TUI Key Bindings........................................................................................................ 198
24.3. TUI Single Key Mode.................................................................................................. 200
24.4. TUI specific commands ............................................................................................... 200
24.5. TUI configuration variables ......................................................................................... 201
25. Using gdb under gnu Emacs .................................................................................................... 203
26. The gdb/mi Interface ................................................................................................................ 207
26.1. Function and Purpose................................................................................................... 207
26.2. Notation and Terminology ........................................................................................... 207
26.3. Acknowledgments........................................................................................................ 207
26.4. gdb/mi Command Syntax ............................................................................................ 207
26.4.1. gdb/mi Input Syntax...................................................................................... 207
26.4.2. gdb/mi Output Syntax ................................................................................... 208
26.4.3. Simple Examples of gdb/mi Interaction ....................................................... 210
26.5. gdb/mi Compatibility with CLI ................................................................................... 211
26.6. gdb/mi Output Records ................................................................................................ 211
26.6.1. gdb/mi Result Records.................................................................................. 211
26.6.2. gdb/mi Stream Records................................................................................. 212
26.6.3. gdb/mi Out-of-band Records ........................................................................ 212
26.7. gdb/mi Command Description Format ........................................................................ 212
26.7.1. Motivation ..................................................................................................... 212
26.7.2. Introduction................................................................................................... 213
26.7.3. Commands .................................................................................................... 213
26.8. gdb/mi Breakpoint table commands ............................................................................ 213
26.8.1. The -break-afterCommand..................................................................... 213
26.8.2. The -break-conditionCommand ............................................................ 214
26.8.3. The -break-deleteCommand................................................................... 215
26.8.4. The -break-disableCommand ................................................................ 216
26.8.5. The -break-enableCommand................................................................... 216
26.8.6. The -break-infoCommand....................................................................... 217
26.8.7. The -break-insertCommand................................................................... 217
26.8.8. The -break-listCommand....................................................................... 219

26.8.9. The -break-watchCommand..................................................................... 220
26.9. gdb/mi Data Manipulation ........................................................................................... 223
26.9.1. The -data-disassembleCommand.......................................................... 223
26.9.2. The -data-evaluate-expressionCommand......................................... 225
26.9.3. The -data-list-changed-registersCommand .................................. 226
26.9.4. The -data-list-register-namesCommand......................................... 227
26.9.5. The -data-list-register-valuesCommand ...................................... 227
26.9.6. The -data-read-memoryCommand.......................................................... 229
26.9.7. The -display-deleteCommand .............................................................. 231
26.9.8. The -display-disableCommand ............................................................ 231
26.9.9. The -display-enableCommand .............................................................. 231
26.9.10. The -display-insertCommand ............................................................ 232
26.9.11. The -display-listCommand................................................................. 232
26.9.12. The -environment-cdCommand ............................................................ 233
26.9.13. The -environment-directoryCommand ............................................. 233
26.9.14. The -environment-pathCommand........................................................ 234
26.9.15. The -environment-pwdCommand .......................................................... 235
26.10. gdb/mi Program control ............................................................................................. 235
26.10.1. Program termination ................................................................................... 235
26.10.2. Examples..................................................................................................... 235
26.11. Command Descriptions.............................................................................................. 236
26.11.1. The -exec-abortCommand..................................................................... 236
26.11.2. The -exec-argumentsCommand ............................................................ 236
26.11.3. The -exec-continueCommand .............................................................. 237
26.11.4. The -exec-finishCommand................................................................... 237
26.11.5. The -exec-interruptCommand ............................................................ 238
26.11.6. The -exec-nextCommand ....................................................................... 239
26.11.7. The -exec-next-instructionCommand ............................................. 239
26.11.8. The -exec-returnCommand................................................................... 240
26.11.9. The -exec-runCommand ......................................................................... 241
26.11.10. The -exec-show-argumentsCommand ............................................... 242
26.11.11. The -exec-stepCommand ..................................................................... 242
26.11.12. The -exec-step-instructionCommand ........................................... 243
26.11.13. The -exec-untilCommand................................................................... 243
26.11.14. The -file-exec-and-symbolsCommand ........................................... 244
26.11.15. The -file-exec-fileCommand .......................................................... 245
26.11.16. The -file-list-exec-sectionsCommand....................................... 245
26.11.17. The -file-list-exec-source-fileCommand ................................ 246
26.11.18. The -file-list-exec-source-filesCommand .............................. 246
26.11.19. The -file-list-shared-librariesCommand ................................ 247
26.11.20. The -file-list-symbol-filesCommand......................................... 247
26.11.21. The -file-symbol-fileCommand...................................................... 247
26.12. Miscellaneous gdb commands in gdb/mi................................................................... 248
26.12.1. The -gdb-exitCommand ......................................................................... 248
26.12.2. The -gdb-setCommand ........................................................................... 249
26.12.3. The -gdb-showCommand ......................................................................... 249
26.12.4. The -gdb-versionCommand................................................................... 250
26.12.5. The -interpreter-execCommand........................................................ 250
26.12.6. Synopsis ...................................................................................................... 250
26.12.7. gdb Command............................................................................................. 251
26.12.8. Example ...................................................................................................... 251
26.13. gdb/mi Stack Manipulation Commands .................................................................... 251
26.13.1. The -stack-info-frameCommand........................................................ 251
26.13.2. The -stack-info-depthCommand........................................................ 251
26.13.3. The -stack-list-argumentsCommand ............................................... 252
26.13.4. The -stack-list-framesCommand...................................................... 253

26.13.5. The -stack-list-localsCommand...................................................... 255
26.13.6. The -stack-select-frameCommand.................................................... 256
26.14. gdb/mi Symbol Query Commands ............................................................................ 256
26.14.1. The -symbol-info-addressCommand ................................................. 256
26.14.2. The -symbol-info-fileCommand........................................................ 257
26.14.3. The -symbol-info-functionCommand ............................................... 257
26.14.4. The -symbol-info-lineCommand........................................................ 258
26.14.5. The -symbol-info-symbolCommand.................................................... 258
26.14.6. The -symbol-list-functionsCommand ............................................. 258
26.14.7. The -symbol-list-linesCommand...................................................... 259
26.14.8. The -symbol-list-typesCommand...................................................... 259
26.14.9. The -symbol-list-variablesCommand ............................................. 260
26.14.10. The -symbol-locateCommand ............................................................ 260
26.14.11. The -symbol-typeCommand................................................................. 261
26.15. gdb/mi Target Manipulation Commands ................................................................... 261
26.15.1. The -target-attachCommand .............................................................. 261
26.15.2. The -target-compare-sectionsCommand......................................... 262
26.15.3. The -target-detachCommand .............................................................. 262
26.15.4. The -target-disconnectCommand...................................................... 262
26.15.5. The -target-downloadCommand .......................................................... 263
26.15.6. The -target-exec-statusCommand.................................................... 265
26.15.7. The -target-list-available-targetsCommand............................ 265
26.15.8. The -target-list-current-targetsCommand ................................ 266
26.15.9. The -target-list-parametersCommand........................................... 266
26.15.10. The -target-selectCommand ............................................................ 266
26.16. gdb/mi Thread Commands......................................................................................... 267
26.16.1. The -thread-infoCommand................................................................... 267
26.16.2. The -thread-list-all-threadsCommand......................................... 268
26.16.3. The -thread-list-idsCommand .......................................................... 268
26.16.4. The -thread-selectCommand .............................................................. 269
26.17. gdb/mi Tracepoint Commands................................................................................... 270
26.18. gdb/mi Variable Objects............................................................................................. 270
26.18.1. Motivation for Variable Objects in gdb/mi ................................................. 270
26.18.2. Introduction to Variable Objects in gdb/mi................................................. 270
26.18.3. Description And Use of Operations on Variable Objects ........................... 271
26.18.4. The -var-createCommand..................................................................... 271
26.18.5. The -var-deleteCommand..................................................................... 272
26.18.6. The -var-set-formatCommand ............................................................ 272
26.18.7. The -var-show-formatCommand .......................................................... 272
26.18.8. The -var-info-num-childrenCommand ............................................. 273
26.18.9. The -var-list-childrenCommand...................................................... 273
26.18.10. The -var-info-typeCommand ............................................................ 273
26.18.11. The -var-info-expressionCommand ............................................... 274
26.18.12. The -var-show-attributes Command .............................................. 274
26.18.13. The -var-evaluate-expression Command...................................... 274
26.18.14. The -var-assignCommand................................................................... 275
26.18.15. The -var-updateCommand................................................................... 275
27. gdb Annotations ........................................................................................................................ 277
27.1. What is an Annotation?................................................................................................ 277
27.2. The Server Prefix ......................................................................................................... 277
27.3. Values........................................................................................................................... 277
27.4. Frames.......................................................................................................................... 279
27.5. Displays........................................................................................................................ 281
27.6. Annotation for gdb Input ............................................................................................. 281
27.7. Errors............................................................................................................................ 282

27.8. Information on Breakpoints ......................................................................................... 282
27.9. Invalidation Notices ..................................................................................................... 283
27.10. Running the Program ................................................................................................. 283
27.11. Displaying Source...................................................................................................... 284
27.12. Annotations We Might Want in the Future ................................................................ 285
28. Reporting Bugs in gdb .............................................................................................................. 287
28.1. Have you found a bug? ................................................................................................ 287
28.2. How to report bugs....................................................................................................... 287
29. Command Line Editing ............................................................................................................ 291
29.1. Introduction to Line Editing......................................................................................... 291
29.2. Readline Interaction ..................................................................................................... 291
29.2.1. Readline Bare Essentials............................................................................... 291
29.2.2. Readline Movement Commands................................................................... 292
29.2.3. Readline Killing Commands......................................................................... 292
29.2.4. Readline Arguments...................................................................................... 293
29.2.5. Searching for Commands in the History....................................................... 293
29.3. Readline Init File.......................................................................................................... 294
29.3.1. Readline Init File Syntax .............................................................................. 294
29.3.2. Conditional Init Constructs ........................................................................... 298
29.3.3. Sample Init File............................................................................................. 299
29.4. Bindable Readline Commands..................................................................................... 301
29.4.1. Commands For Moving ................................................................................ 301
29.4.2. Commands For Manipulating The History ................................................... 302
29.4.3. Commands For Changing Text ..................................................................... 303
29.4.4. Killing And Yanking ..................................................................................... 304
29.4.5. Specifying Numeric Arguments ................................................................... 305
29.4.6. Letting Readline Type For You..................................................................... 305
29.4.7. Keyboard Macros.......................................................................................... 306
29.4.8. Some Miscellaneous Commands .................................................................. 306
29.5. Readline vi Mode......................................................................................................... 308
30. Using History Interactively ...................................................................................................... 309
30.1. History Expansion........................................................................................................ 309
30.1.1. Event Designators ......................................................................................... 309
30.1.2. Word Designators.......................................................................................... 310
30.1.3. Modifiers ....................................................................................................... 311
A. Formatting Documentation ....................................................................................................... 313
B. Installing gdb .............................................................................................................................. 315
B.1. Compiling gdb in another directory .............................................................................. 316
B.2. Specifying names for hosts and targets ......................................................................... 317
B.3. configure options....................................................................................................... 318
C. Maintenance Commands........................................................................................................... 319
D. gdb Remote Serial Protocol....................................................................................................... 321
D.1. Overview ....................................................................................................................... 321
D.2. Packets .......................................................................................................................... 322
D.3. Stop Reply Packets........................................................................................................ 330
D.4. General Query Packets.................................................................................................. 331
D.5. Register Packet Format ................................................................................................. 334
D.6. Examples....................................................................................................................... 334
D.7. File-I/O remote protocol extension ............................................................................... 335
D.7.1. File-I/O Overview .......................................................................................... 335
D.7.2. Protocol basics ............................................................................................... 335
D.7.3. The Frequest packet ....................................................................................... 336
D.7.4. The Freply packet........................................................................................... 336

D.7.5. Memory transfer............................................................................................. 337
D.7.6. The Ctrl-C message........................................................................................ 337
D.7.7. Console I/O .................................................................................................... 337
D.7.8. The isatty(3) call ............................................................................................ 338
D.7.9. The system(3) call.......................................................................................... 338
D.7.10. List of supported calls .................................................................................. 338
D.7.11. Protocol specific representation of datatypes............................................... 347
D.7.12. Constants...................................................................................................... 349
D.7.13. File-I/O Examples ........................................................................................ 350
E. The GDB Agent Expression Mechanism.................................................................................. 353
E.1. General Bytecode Design .............................................................................................. 353
E.2. Bytecode Descriptions................................................................................................... 355
E.3. Using Agent Expressions .............................................................................................. 359
E.4. Varying Target Capabilities ........................................................................................... 359
E.5. Tracing on Symmetrix ................................................................................................... 360
E.6. Rationale........................................................................................................................ 361
F. GNU GENERAL PUBLIC LICENSE...................................................................................... 365
F.1. Preamble ........................................................................................................................ 365
F.2. How to Apply These Terms to Your New Programs...................................................... 368
G. GNU Free Documentation License........................................................................................... 371
G.1. ADDENDUM: How to use this License for your documents....................................... 375
Index................................................................................................................................................. 377

Chapter 1.
Debugging with gdb
This file describes gdb, the gnu symbolic debugger.
This is the Ninth Edition, for gdb Version 2003-07-22-cvs.
Copyright (C) 1988-2003 Free Software Foundation, Inc.

2 Chapter 1. Debugging with gdb

Chapter 2.
Summary of gdb
The purpose of a debugger such as gdb is to allow you to see what is going on "inside" another
program while it executes--or what another program was doing at the moment it crashed.
gdb can do four main kinds of things (plus other things in support of these) to help you catch bugs in
the act:
•Start your program, specifying anything that might affect its behavior.
•Make your program stop on specified conditions.
•Examine what has happened, when your program has stopped.
•Change things in your program, so you can experiment with correcting the effects of one bug and
go on to learn about another.
You can use gdb to debug programs written in C and C++. For more information, (refer to Section
14.4 Supported languages. For more information, (refer to Section 14.4.1 C and C++.
Support for Modula-2 is partial. For information on Modula-2, refer to (refer to Section 14.4.3
Modula-2.
Debugging Pascal programs which use sets, subranges, file variables, or nested functions does not
currently work. gdb does not support entering expressions, printing values, or similar features using
Pascal syntax.
gdb can be used to debug programs written in Fortran, although it may be necessary to refer to some
variables with a trailing underscore.
gdb can be used to debug programs written in Objective-C, using either the Apple/NeXT or the GNU
Objective-C runtime.
2.1. Free software
gdb is free software, protected by the gnu General Public License (GPL). The GPL gives you the free-
dom to copy or adapt a licensed program--but every person getting a copy also gets with it the freedom
to modify that copy (which means that they must get access to the source code), and the freedom to
distribute further copies. Typical software companies use copyrights to limit your freedoms; the Free
Software Foundation uses the GPL to preserve these freedoms.
Fundamentally, the General Public License is a license which says that you have these freedoms and
that you cannot take these freedoms away from anyone else.
2.2. Free Software Needs Free Documentation
The biggest deficiency in the free software community today is not in the software--it is the lack of
good free documentation that we can include with the free software. Many of our most important
programs do not come with free reference manuals and free introductory texts. Documentation is an
essential part of any software package; when an important free software package does not come with
a free manual and a free tutorial, that is a major gap. We have many such gaps today.
Consider Perl, for instance. The tutorial manuals that people normally use are non-free. How did this
come about? Because the authors of those manuals published them with restrictive terms--no copying,
no modification, source files not available--which exclude them from the free software world.

4 Chapter 2. Summary of gdb
That wasn’t the first time this sort of thing happened, and it was far from the last. Many times we
have heard a GNU user eagerly describe a manual that he is writing, his intended contribution to the
community, only to learn that he had ruined everything by signing a publication contract to make it
non-free.
Free documentation, like free software, is a matter of freedom, not price. The problem with the non-
free manual is not that publishers charge a price for printed copies--that in itself is fine. (The Free
Software Foundation sells printed copies of manuals, too.) The problem is the restrictions on the use
of the manual. Free manuals are available in source code form, and give you permission to copy and
modify. Non-free manuals do not allow this.
The criteria of freedom for a free manual are roughly the same as for free software. Redistribution
(including the normal kinds of commercial redistribution) must be permitted, so that the manual can
accompany every copy of the program, both on-line and on paper.
Permission for modification of the technical content is crucial too. When people modify the software,
adding or changing features, if they are conscientious they will change the manual too--so they can
provide accurate and clear documentation for the modified program. A manual that leaves you no
choice but to write a new manual to document a changed version of the program is not really available
to our community.
Some kinds of limits on the way modification is handled are acceptable. For example, requirements
to preserve the original author’s copyright notice, the distribution terms, or the list of authors, are ok.
It is also no problem to require modified versions to include notice that they were modified. Even
entire sections that may not be deleted or changed are acceptable, as long as they deal with nontech-
nical topics (like this one). These kinds of restrictions are acceptable because they don’t obstruct the
community’s normal use of the manual.
However, it must be possible to modify all the technical content of the manual, and then distribute the
result in all the usual media, through all the usual channels. Otherwise, the restrictions obstruct the
use of the manual, it is not free, and we need another manual to replace it.
Please spread the word about this issue. Our community continues to lose manuals to proprietary
publishing. If we spread the word that free software needs free reference manuals and free tutorials,
perhaps the next person who wants to contribute by writing documentation will realize, before it is
too late, that only free manuals contribute to the free software community.
If you are writing documentation, please insist on publishing it under the GNU Free Documentation
License or another free documentation license. Remember that this decision requires your approval--
you don’t have to let the publisher decide. Some commercial publishers will use a free license if you
insist, but they will not propose the option; it is up to you to raise the issue and say firmly that this is
what you want. If the publisher you are dealing with refuses, please try other publishers. If you’re not
sure whether a proposed license is free, write to mailto:licensing@@gnu.org.
You can encourage commercial publishers to sell more free, copylefted manuals and tutorials by
buying them, and particularly by buying copies from the publishers that paid for their writing or
for major improvements. Meanwhile, try to avoid buying non-free documentation at all. Check the
distribution terms of a manual before you buy it, and insist that whoever seeks your business must
respect your freedom. Check the history of the book, and try to reward the publishers that have paid
or pay the authors to work on it.
The Free Software Foundation maintains a list of free documentation published by other publishers,
at http://www.fsf.org/doc/other-free-books.html.
2.3. Contributors to gdb
Richard Stallman was the original author of gdb, and of many other gnu programs. Many others have
contributed to its development. This section attempts to credit major contributors. One of the virtues
of free software is that everyone is free to contribute to it; with regret, we cannot actually acknowledge
everyone here. The file ChangeLog in the gdb distribution approximates a blow-by-blow account.

Chapter 2. Summary of gdb 5
Changes much prior to version 2.0 are lost in the mists of time.
Plea: Additions to this section are particularly welcome. If you or your friends (or enemies, to be even-
handed) have been unfairly omitted from this list, we would like to add your names!
So that they may not regard their many labors as thankless, we particularly thank those who shep-
herded gdb through major releases: Andrew Cagney (releases 6.0, 5.3, 5.2, 5.1 and 5.0); Jim Blandy
(release 4.18); Jason Molenda (release 4.17); Stan Shebs (release 4.14); Fred Fish (releases 4.16, 4.15,
4.13, 4.12, 4.11, 4.10, and 4.9); Stu Grossman and John Gilmore (releases 4.8, 4.7, 4.6, 4.5, and 4.4);
John Gilmore (releases 4.3, 4.2, 4.1, 4.0, and 3.9); Jim Kingdon (releases 3.5, 3.4, and 3.3); and Randy
Smith (releases 3.2, 3.1, and 3.0).
Richard Stallman, assisted at various times by Peter TerMaat, Chris Hanson, and Richard Mlynarik,
handled releases through 2.8.
Michael Tiemann is the author of most of the gnu C++ support in gdb, with significant additional
contributions from Per Bothner and Daniel Berlin. James Clark wrote the gnu C++ demangler. Early
work on C++ was by Peter TerMaat (who also did much general update work leading to release 3.0).
gdb uses the BFD subroutine library to examine multiple object-file formats; BFD was a joint project
of David V. Henkel-Wallace, Rich Pixley, Steve Chamberlain, and John Gilmore.
David Johnson wrote the original COFF support; Pace Willison did the original support for encapsu-
lated COFF.
Brent Benson of Harris Computer Systems contributed DWARF 2 support.
Adam de Boor and Bradley Davis contributed the ISI Optimum V support. Per Bothner, Noboyuki
Hikichi, and Alessandro Forin contributed MIPS support. Jean-Daniel Fekete contributed Sun 386i
support. Chris Hanson improved the HP9000 support. Noboyuki Hikichi and Tomoyuki Hasei con-
tributed Sony/News OS 3 support. David Johnson contributed Encore Umax support. Jyrki Kuop-
pala contributed Altos 3068 support. Jeff Law contributed HP PA and SOM support. Keith Packard
contributed NS32K support. Doug Rabson contributed Acorn Risc Machine support. Bob Rusk con-
tributed Harris Nighthawk CX-UX support. Chris Smith contributed Convex support (and Fortran
debugging). Jonathan Stone contributed Pyramid support. Michael Tiemann contributed SPARC sup-
port. Tim Tucker contributed support for the Gould NP1 and Gould Powernode. Pace Willison con-
tributed Intel 386 support. Jay Vosburgh contributed Symmetry support. Marko Mlinar contributed
OpenRISC 1000 support.
Andreas Schwab contributed M68K gnu/Linux support.
Rich Schaefer and Peter Schauer helped with support of SunOS shared libraries.
Jay Fenlason and Roland McGrath ensured that gdb and GAS agree about several machine instruction
sets.
Patrick Duval, Ted Goldstein, Vikram Koka and Glenn Engel helped develop remote debugging. Intel
Corporation, Wind River Systems, AMD, and ARM contributed remote debugging modules for the
i960, VxWorks, A29K UDI, and RDI targets, respectively.
Brian Fox is the author of the readline libraries providing command-line editing and command history.
Andrew Beers of SUNY Buffalo wrote the language-switching code, the Modula-2 support, and con-
tributed the Languages chapter of this manual.
Fred Fish wrote most of the support for Unix System Vr4. He also enhanced the command-completion
support to cover C++ overloaded symbols.
Hitachi America, Ltd. sponsored the support for H8/300, H8/500, and Super-H processors.
NEC sponsored the support for the v850, Vr4xxx, and Vr5xxx processors.
Mitsubishi sponsored the support for D10V, D30V, and M32R/D processors.
Toshiba sponsored the support for the TX39 Mips processor.

6 Chapter 2. Summary of gdb
Matsushita sponsored the support for the MN10200 and MN10300 processors.
Fujitsu sponsored the support for SPARClite and FR30 processors.
Kung Hsu, Jeff Law, and Rick Sladkey added support for hardware watchpoints.
Michael Snyder added support for tracepoints.
Stu Grossman wrote gdbserver.
Jim Kingdon, Peter Schauer, Ian Taylor, and Stu Grossman made nearly innumerable bug fixes and
cleanups throughout gdb.
The following people at the Hewlett-Packard Company contributed support for the PA-RISC 2.0 ar-
chitecture, HP-UX 10.20, 10.30, and 11.0 (narrow mode), HP’s implementation of kernel threads,
HP’s aC++ compiler, and the terminal user interface: Ben Krepp, Richard Title, John Bishop, Su-
san Macchia, Kathy Mann, Satish Pai, India Paul, Steve Rehrauer, and Elena Zannoni. Kim Haase
provided HP-specific information in this manual.
DJ Delorie ported gdb to MS-DOS, for the DJGPP project. Robert Hoehne made significant contribu-
tions to the DJGPP port.
Cygnus Solutions has sponsored gdb maintenance and much of its development since 1991. Cygnus
engineers who have worked on gdb fulltime include Mark Alexander, Jim Blandy, Per Bothner, Kevin
Buettner, Edith Epstein, Chris Faylor, Fred Fish, Martin Hunt, Jim Ingham, John Gilmore, Stu Gross-
man, Kung Hsu, Jim Kingdon, John Metzler, Fernando Nasser, Geoffrey Noer, Dawn Perchik, Rich
Pixley, Zdenek Radouch, Keith Seitz, Stan Shebs, David Taylor, and Elena Zannoni. In addition, Dave
Brolley, Ian Carmichael, Steve Chamberlain, Nick Clifton, JT Conklin, Stan Cox, DJ Delorie, Ulrich
Drepper, Frank Eigler, Doug Evans, Sean Fagan, David Henkel-Wallace, Richard Henderson, Jeff
Holcomb, Jeff Law, Jim Lemke, Tom Lord, Bob Manson, Michael Meissner, Jason Merrill, Catherine
Moore, Drew Moseley, Ken Raeburn, Gavin Romig-Koch, Rob Savoye, Jamie Smith, Mike Stump,
Ian Taylor, Angela Thomas, Michael Tiemann, Tom Tromey, Ron Unrau, Jim Wilson, and David Zuhn
have made contributions both large and small.
Jim Blandy added support for preprocessor macros, while working for Red Hat.

Chapter 3.
A Sample gdb Session
You can use this manual at your leisure to read all about gdb. However, a handful of commands are
enough to get started using the debugger. This chapter illustrates those commands.
One of the preliminary versions of gnu m4 (a generic macro processor) exhibits the following bug:
sometimes, when we change its quote strings from the default, the commands used to capture one
macro definition within another stop working. In the following short m4 session, we define a macro
foo which expands to 0000; we then use the m4 built-in defn to define bar as the same thing. How-
ever, when we change the open quote string to

QUOTE

and the close quote string to

UNQUOTE

,
the same procedure fails to define a new synonym baz:
$cd gnu/m4
$./m4
define(foo,0000)
foo
0000
define(bar,defn(‘foo’))
bar
0000
changequote(

QUOTE

,

UNQUOTE

)
define(baz,defn(

QUOTE

foo

UNQUOTE

))
baz
C-d
m4: End of input: 0: fatal error: EOF in string
Let us use gdb to try to see what is going on.
$gdb m4
gdb is free software and you are welcome to distribute copies
of it under certain conditions; type "show copying" to see
the conditions.
There is absolutely no warranty for gdb; type "show warranty"
for details.
gdb 2003-07-22-cvs, Copyright 1999 Free Software Foundation, Inc...
(gdb)
gdb reads only enough symbol data to know where to find the rest when needed; as a result, the first
prompt comes up very quickly. We now tell gdb to use a narrower display width than usual, so that
examples fit in this manual.
(gdb) set width 70
We need to see how the m4 built-in changequote works. Having looked at the source, we know the
relevant subroutine is m4_changequote, so we set a breakpoint there with the gdb break command.

8 Chapter 3. A Sample gdb Session
(gdb) break m4_changequote
Breakpoint 1 at 0x62f4: file builtin.c, line 879.
Using the run command, we start m4 running under gdb control; as long as control does not reach the
m4_changequote subroutine, the program runs as usual:
(gdb) run
Starting program: /work/Editorial/gdb/gnu/m4/m4
define(foo,0000)
foo
0000
To trigger the breakpoint, we call changequote. gdb suspends execution of m4, displaying informa-
tion about the context where it stops.
changequote(

QUOTE

,

UNQUOTE

)
Breakpoint 1, m4_changequote (argc=3, argv=0x33c70)
at builtin.c:879
879 if (bad_argc(TOKEN_DATA_TEXT(argv[0]),argc,1,3))
Now we use the command n(next) to advance execution to the next line of the current function.
(gdb) n
882 set_quotes((argc

= 2) ? TOKEN_DATA_TEXT(argv[1])\
: nil,
set_quotes looks like a promising subroutine. We can go into it by using the command s(step)
instead of next.step goes to the next line to be executed in any subroutine, so it steps into
set_quotes.
(gdb) s
set_quotes (lq=0x34c78 "

QUOTE

", rq=0x34c88 "

UNQUOTE

")
at input.c:530
530 if (lquote != def_lquote)
The display that shows the subroutine where m4 is now suspended (and its arguments) is called a stack
frame display. It shows a summary of the stack. We can use the backtrace command (which can
also be spelled bt), to see where we are in the stack as a whole: the backtrace command displays a
stack frame for each active subroutine.
(gdb) bt
#0 set_quotes (lq=0x34c78 "

QUOTE

", rq=0x34c88 "

UNQUOTE

")
at input.c:530
#1 0x6344 in m4_changequote (argc=3, argv=0x33c70)
at builtin.c:882
#2 0x8174 in expand_macro (sym=0x33320) at macro.c:242
#3 0x7a88 in expand_token (obs=0x0, t=209696, td=0xf7fffa30)
at macro.c:71

Chapter 3. A Sample gdb Session 9
#4 0x79dc in expand_input () at macro.c:40
#5 0x2930 in main (argc=0, argv=0xf7fffb20) at m4.c:195
We step through a few more lines to see what happens. The first two times, we can use s; the next two
times we use nto avoid falling into the xstrdup subroutine.
(gdb) s
0x3b5c 532 if (rquote != def_rquote)
(gdb) s
0x3b80 535 lquote = (lq == nil || *lq == ’\0’) ? \
def_lquote : xstrdup(lq);
(gdb) n
536 rquote = (rq == nil || *rq == ’\0’) ? def_rquote\
: xstrdup(rq);
(gdb) n
538 len_lquote = strlen(rquote);
The last line displayed looks a little odd; we can examine the variables lquote and rquote to see
if they are in fact the new left and right quotes we specified. We use the command p(print) to see
their values.
(gdb) p lquote
$1 = 0x35d40 "

QUOTE

"
(gdb) p rquote
$2 = 0x35d50 "

UNQUOTE

"
lquote and rquote are indeed the new left and right quotes. To look at some context, we can display
ten lines of source surrounding the current line with the l(list) command.
(gdb) l
533 xfree(rquote);
534
535 lquote = (lq == nil || *lq == ’\0’) ? def_lquote\
: xstrdup (lq);
536 rquote = (rq == nil || *rq == ’\0’) ? def_rquote\
: xstrdup (rq);
537
538 len_lquote = strlen(rquote);
539 len_rquote = strlen(lquote);
540 }
541
542 void
Let us step past the two lines that set len_lquote and len_rquote, and then examine the values of
those variables.
(gdb) n
539 len_rquote = strlen(lquote);
(gdb) n
540 }
(gdb) p len_lquote
$3 = 9

10 Chapter 3. A Sample gdb Session
(gdb) p len_rquote
$4 = 7
That certainly looks wrong, assuming len_lquote and len_rquote are meant to be the lengths of
lquote and rquote respectively. We can set them to better values using the pcommand, since it can
print the value of any expression--and that expression can include subroutine calls and assignments.
(gdb) p len_lquote=strlen(lquote)
$5 = 7
(gdb) p len_rquote=strlen(rquote)
$6 = 9
Is that enough to fix the problem of using the new quotes with the m4 built-in defn? We can allow m4
to continue executing with the c(continue) command, and then try the example that caused trouble
initially:
(gdb) c
Continuing.
define(baz,defn(

QUOTE

foo

UNQUOTE

))
baz
0000
Success! The new quotes now work just as well as the default ones. The problem seems to have been
just the two typos defining the wrong lengths. We allow m4 exit by giving it an EOF as input:
C-d
Program exited normally.
The message Program exited normally. is from gdb; it indicates m4 has finished executing. We
can end our gdb session with the gdb quit command.
(gdb) quit