GDB调试指南：Linux上的C/C++代码调试利器

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"Debugging with GDB - GNU调试器GDB的使用指南，适用于Linux环境下的C和C++代码调试。" 在编程领域，调试是解决问题的关键步骤，而GDB（GNU调试器）是一个强大的工具，它允许程序员在Linux系统上对C和C++程序进行源代码级别的调试。GDB自1988年以来不断发展和更新，版本7.5.1是本文档所涵盖的版本，体现了其长期的维护和改进历史。 GDB提供了多种功能，包括设置断点、单步执行代码、查看和修改变量值、检查内存状态以及跟踪调用堆栈等。这些功能帮助开发者深入理解程序运行时的行为，从而定位和修复bug。 1. **设置断点**：在代码的特定位置设置断点，可以让程序在该点暂停执行，以便于检查当前的状态。这在查找导致错误的代码行或观察特定代码块的执行流程时非常有用。 2. **单步执行**：GDB允许你逐行执行代码，这样可以详细观察每一步的效果，了解程序是如何从一个状态过渡到另一个状态的。 3. **查看和修改变量**：在程序暂停后，你可以查看全局和局部变量的值，并在必要时修改它们。这对于理解变量在不同阶段的值变化及其对程序的影响至关重要。 4. **内存检查**：GDB让你能够查看和操作程序的内存空间，检查内存泄漏或数据溢出等问题。 5. **调用堆栈**：通过查看调用堆栈，可以追踪函数调用的层次，了解程序执行的上下文，找到问题的源头。 6. **条件断点和watchpoints**：GDB支持基于条件的断点，只有当特定条件满足时才会触发。此外，watchpoints允许你监视某个变量的变化，当其值发生变化时暂停程序。 7. **远程调试**：除了本地调试，GDB还可以连接到远程设备进行调试，这对于嵌入式系统和分布式系统开发尤其有用。 8. **多线程和多进程支持**：GDB可以处理多线程和多进程的程序，帮助你在复杂的并发环境中调试。 9. **脚本支持**：GDB支持使用Python编写扩展脚本，可以自定义命令和增强调试体验。 10. **调试核心转储**：如果程序崩溃并生成了核心转储文件，GDB可以帮助分析这个文件，重现程序崩溃时的状态。《Debugging with GDB》提供了全面的指南，指导用户如何有效地利用GDB来诊断和修复程序中的问题。通过熟练掌握GDB的使用，开发者能够提高调试效率，从而更快地找到并解决问题，确保软件的质量和可靠性。

threads, HP's aC++ compiler, and the Text User Interface (nee Terminal User Interface): Ben

Krepp, Richard Title, John Bishop, Susan 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

contributions 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 Grossman, 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.

Andrew Cagney, Fernando Nasser, and Elena Zannoni, while working for Cygnus Solutions,

implemented the original GDB/MI interface.

Jim Blandy added support for preprocessor macros, while working for Red Hat.

Andrew Cagney designed GDB's architecture vector. Many people including Andrew Cagney,

Stephane Carrez, Randolph Chung, Nick Duffek, Richard Henderson, Mark Kettenis, Grace

Sainsbury, Kei Sakamoto, Yoshinori Sato, Michael Snyder, Andreas Schwab, Jason Thorpe,

Corinna Vinschen, Ulrich Weigand, and Elena Zannoni, helped with the migration of old

architectures to this new framework.

Andrew Cagney completely re-designed and re-implemented GDB's unwinder framework, this

consisting of a fresh new design featuring frame IDs, independent frame sniffers, and the

sentinel frame. Mark Kettenis implemented the DWARF 2 unwinder, Jeff Johnston the libunwind

unwinder, and Andrew Cagney the dummy, sentinel, tramp, and trad unwinders. The

architecture-specific changes, each involving a complete rewrite of the architecture's frame code,

were carried out by Jim Blandy, Joel Brobecker, Kevin Buettner, Andrew Cagney, Stephane

Carrez, Randolph Chung, Orjan Friberg, Richard Henderson, Daniel Jacobowitz, Jeff Johnston,

Mark Kettenis, Theodore A. Roth, Kei Sakamoto, Yoshinori Sato, Michael Snyder, Corinna

Vinschen, and Ulrich Weigand.

Christian Zankel, Ross Morley, Bob Wilson, and Maxim Grigoriev from Tensilica, Inc. contributed

support for Xtensa processors. Others who have worked on the Xtensa port of GDB in the past

include Steve Tjiang, John Newlin, and Scott Foehner.

Michael Eager and staff of Xilinx, Inc., contributed support for the Xilinx MicroBlaze architecture.

Next: Invocation, Previous: Summary, Up: Top

1 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.

Debugging with GDB http://sourceware.org/gdb/download/onlinedocs/gdb.html

第16页共621页 2013-3-13 16:58

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. However, 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

Ctrl-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)

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.

(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

information 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.

Debugging with GDB http://sourceware.org/gdb/download/onlinedocs/gdb.html

第17页共621页 2013-3-13 16:58

(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

#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 n to 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

Debugging with GDB http://sourceware.org/gdb/download/onlinedocs/gdb.html

第18页共621页 2013-3-13 16:58

until you tell it to exit.

You can also run gdb with a variety of arguments and options, to specify more of your debugging

environment at the outset.

The command-line options described here are designed to cover a variety of situations; in some

environments, some of these options may effectively be unavailable.

The most usual way to start GDB is with one argument, specifying an executable program:

gdb program

You can also start with both an executable program and a core file specified:

gdb program core

You can, instead, specify a process ID as a second argument, if you want to debug a running

process:

gdb program 1234

would attach GDB to process 1234 (unless you also have a file named 1234; GDB does check for a

core file first).

Taking advantage of the second command-line argument requires a fairly complete operating

system; when you use GDB as a remote debugger attached to a bare board, there may not be

any notion of “process”, and there is often no way to get a core dump. GDB will warn you if it is

unable to attach or to read core dumps.

You can optionally have gdb pass any arguments after the executable file to the inferior using

--args. This option stops option processing.

gdb --args gcc -O2 -c foo.c

This will cause gdb to debug gcc, and to set gcc's command-line arguments (see Arguments) to

`-O2 -c foo.c'.

You can run gdb without printing the front material, which describes GDB's non-warranty, by

specifying -silent:

gdb -silent

You can further control how GDB starts up by using command-line options. GDB itself can remind

you of the options available.

Type

gdb -help

to display all available options and briefly describe their use (`gdb -h' is a shorter equivalent).

All options and command line arguments you give are processed in sequential order. The order

makes a difference when the `-x' option is used.

File Options: Choosing files

Mode Options: Choosing modes

Startup: What GDB does during startup

Debugging with GDB http://sourceware.org/gdb/download/onlinedocs/gdb.html

第20页共621页 2013-3-13 16:58

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