exists. The desired variable is specified by using C syntax. For example...
print x.y[3]
will print the value of the fourth element of the array field named y of a structure variable
named x. The variables that are accessible are those of the currently selected function's
activation frame, plus all those whose scope is global or static to the current target file.
Both the print and display functions can be used to evaluate arbitrarily complicated
expressions, even those containing, function calls, but be warned that if a function has
side-effects a variety of unpleasant and unexpected situations can arise.
Shortcuts
Finally, there are some things that make using gdb a bit simpler. All of the commands
have short-cuts so that you don’t have to type the whole command name every time you
want to do something simple. A command short-cut is specified by typing just enough of
the command name so that it unambiguously refers to a command, or for the special
commands break, delete, run, continue, step, next and print you need only
use the first letter. Additionally, the last command you entered can be repeated by just
hitting the return key again. This is really useful for single stepping for a range while
watching variables change.
Miscellaneous
editmode mode Set editmode for gdb command line. Supported
values for mode are emacs, vi, dumb.
shell command Execute the rest of the line as a shell command.
history Print command history.
Debugging Strategies
Some people avoid using debuggers because they don't want to learn another tool. This is
a mistake. Invest the time to learn to use a debugger and all its features — it will make
you much more productive in tracking down problems.
Sometimes bugs result in program crashes (a.k.a. “core dumps”, “register dumps”, etc.)
that bring your program to a halt with a message like “Segmentation Violation” or the
like. If your program has such a crash, the debugger will intercept the signal sent by the
processor that indicates the error it found, and allow you to examine the state program.
Thus with almost no extra effort, the debugger can show you the state of the program at
the moment of the crash.
Often, a bug does not crash explicitly, but instead produces symptoms of internal
problems. In such a case, one technique is to put a breakpoint where the program is
misbehaving, and then look up the call stack to get some insight about the data and
control flow path that led to the bad state. Another technique is to set a breakpoint at
some point before the problems start and step forward towards the problems, examining
the state of the program along the way.
13
Section 4 — emacs
The following is a quick introduction to the “emacs” text editor which is a free program
produced by GNU (www.gnu.org). It's a fine editor, and it happens to integrate with
many other Unix tools nicely. There's a fabulous history of various editor adherents
having long and entertaining arguments about why their editor is best, but we're just
going to avoid that subject entirely.
To start editing a new or existing file using emacs, simply type the following to the UNIX
prompt...
emacs filename
where filename is the file to be edited. The X-Windows version of emacs is called
xemacs, and if you're using it... well just look in the menus. The commands are all the
same, but you don't have to remember the funny key-combinations.
All the fancy editing commands, such as find-and-replace, are invoked through typing
special key sequences. Two important key sequences to remember are: ^x (holding down
the “ctrl” key while typing “x”) and [esc]-x (simply pressing the “esc” key followed
by typing “x”), both of which are used to start many command sequences. Note that for
historical reasons in most user manuals for emacs, the “esc” key is actually referred to as
the “Meta” or “M-” key. Therefore, you may see the [esc]-x written as equivalently
as M-x.
To save the file being edited the sequence is ^x^s. To exit (and be prompted to save)
emacs, the sequence is ^x^c. To open another file within emacs, the sequence is ^x^f.
This sequence can be used to open an existing file as well as a new file. If you have
multiple files open, emacs stores them in different “buffers”. To switch from one buffer
to another (very handy when you are editing a .c source file and need to refer to the
prototypes and definitions in the .h header file), you use the key sequence ^x-b (note
the “b” is typed plain). You can then enter the name of the file to switch to the
corresponding buffer (a default name is provided for fast switching). The arrow keys
usually work as the cursor movement keys, but there are other nagivation key
combinations listed below.
Running emacs
emacs <filename> Run emacs (on a particular file). Make sure you don't already have
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