For most users, the graphical debugging interface presented by
ddd is the best way to debug programs. However, hard-core
emacs users may prefer the gdb interface
provided by that editor. (One advantage of the emacs interface is
that it is available essentially everywhere, whereas ddd
might not be available on many machines.)
These instructions are written using the "conventional"
emacs notation: C-x means "hold down the
control key and type x", M-x means either
"hold down the meta key and type x" or "type the
ESC key followed by x" (the two are usually
equivalent). SPC is the space bar, and ESC
refers to the Escape or Esc key (usually the
upper-left-key on the keyboard), while RET is the
Return or Enter key on your main keyboard;
there is no guarantee whether the Enter key on the
numeric keypad will produce the same result. Finally,
[f1] refers to function key 1 along the top row of your
keyboard (and so forth).
Case matters: M-X is not the same as M-x.
Before you can use gdb effectively, you must become
familiar with emacs windows. Most window commands start
with C-x 4. For example, C-x 4 f will split
the screen in half and open a new file in the other window. C-x
4 b will select an already-opened file (buffer) in the other
window. These are the two that I use most often; you can also type
C-x 4 [f1] (function key 1) to get a full list of window
commands.
There are also three useful window commands that don't start with C-x 4. These are: C-x o (oh) to switch to the other window, C-x 0 (zero) to delete the current window and go back to one-window mode, and C-x 1 to delete the other window and go back to one-window mode. If you use the menu bar, C-x 1 is available under the Files menu. You can also change to a different window by simply clicking with the mouse.
Once you've mastered the window commands, the gdb mode is very simple.
Start gdb by typing M-x gdb RET. It will prompt you
Run gdb (like this): gdb. Type in the name of your
executable and hit RET. You will then find yourself at a
gdb command prompt, where you can set breakpoints, run
the program, etc. M-p invokes a command history to save
typing.
When you hit a breakpoint, emacs will automatically bring up the source code in the other window. If you step through the code, the little arrow will follow the source code.
The other really handy thing is that if you place the cursor on a source code line (e.g., you use C-x o or C-x 4 f to enter the other window and then navigate to an interesting place in your code), you can type C-x SPC (the space bar), and emacs will set a breakpoint at that line of code. That saves me a lot of time, because I never have to concern myself with line numbers.