0x22a : cmpl %eax,0x9530c

0x230 : jne 0x24c

0x232 : jmp 0x29c

0x234 : nop

0x235 : nop

...

0x2a8 : addb %al,(%eax)

0x2aa : addb %al,(%eax)

(gdb)

This is equivalent to using the command disass 0x21c (where 0x21c is the literal address of the beginning of play_timeout).

You can specify an optional second argument to disass, which will be used as the address where disassembly should stop. Using disass 0x21c 0x232 will display only the first seven lines of the assembly listing in the previous example (the instruction starting with 0x232 itself will not be displayed).

If you use nexti and stepi often, you may wish to use the command:

display/i $pc

This causes the current instruction to be displayed after every nexti or stepi command. display specifies variables to watch or commands to execute after every stepping command. $pc is a gdb internal register that corresponds to the processor's program counter, pointing to the current instruction.

Using Emacs with gdb

(X)Emacs (described in Chapter 19) provides a debugging mode that lets you run gdb--or another debugger—within the integrated program-tracing environment provided by Emacs. This so-called Grand Unified Debugger library is very powerful and allows you to debug and edit your programs entirely within Emacs.

To start gdb under Emacs, use the Emacs command M-x gdb and give the name of the executable to debug as the argument. A buffer will be created for gdb, which is similar to using gdb alone. You can then use core-file to load a core file or attach to attach to a running process, if you wish.

Whenever you step to a new frame (when you first trigger a breakpoint), gdb opens a separate window that displays the source corresponding to the current stack frame. You may use this buffer to edit the source text just as you normally would with Emacs, but the current source line is highlighted with an arrow (the characters =>). This allows you to watch the source in one window and execute gdb commands in the other.

Within the debugging window, you can use several special key sequences. They are fairly long, though, so it's not clear that you'll find them more convenient than just entering gdb commands directly. Some of the more common commands include the following:

C-x C-a C-s

The equivalent of a gdb step command, updating the source window appropriately

C-x C-a C-i

The equivalent of a stepi command

C-x C-a C-n

The equivalent of a next command

C-x C-a C-r

The equivalent of a continue command

C-x C-a <

The equivalent of an up command

C-x C-a >

The equivalent of a down command

If you do enter commands in the traditional manner, you can use M-p to move backward to previously issued commands and M-n to move forward. You can also move around in the buffer using Emacs commands for searching, cursor movement, and so on. All in all, using gdb within Emacs is more convenient than using it from the shell.

In addition, you may edit the source text in the gdb source buffer; the prefix arrow will not be present in the source when it is saved.

Emacs is very easy to customize, and you can write many extensions to this gdb interface yourself. You can define Emacs keys for other commonly used gdb commands or change the behavior of the source window. (For example, you can highlight all breakpoints in some fashion or provide keys to disable or clear breakpoints.)

* * *

[*] The sample programs in this section are not programs you're likely to run into anywhere; they were thrown together by the authors for the purpose of demonstration.

Useful Utilities for C Programmers

Along with languages and compilers, there is a plethora of programming tools out there, including libraries, interface builders, debuggers , and other utilities to aid the programming process. In this section, we talk about some of the most interesting bells and whistles