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#### 17.2.1 Using Edebug

To debug a Lisp program with Edebug, you must first instrument the Lisp code that you want to debug. A simple way to do this is to first move point into the definition of a function or macro and then do `C-u C-M-x` (`eval-defun` with a prefix argument). See Instrumenting, for alternative ways to instrument code.

Once a function is instrumented, any call to the function activates Edebug. Depending on which Edebug execution mode you have selected, activating Edebug may stop execution and let you step through the function, or it may update the display and continue execution while checking for debugging commands. The default execution mode is step, which stops execution. See Edebug Execution Modes.

Within Edebug, you normally view an Emacs buffer showing the source of the Lisp code you are debugging. This is referred to as the source code buffer, and it is temporarily read-only.

An arrow in the left fringe indicates the line where the function is executing. Point initially shows where within the line the function is executing, but this ceases to be true if you move point yourself.

If you instrument the definition of `fac` (shown below) and then execute `(fac 3)`, here is what you would normally see. Point is at the open-parenthesis before `if`.

    (defun fac (n)
    =>∗(if (< 0 n)
          (* n (fac (1- n)))
        1))

The places within a function where Edebug can stop execution are called stop points. These occur both before and after each subexpression that is a list, and also after each variable reference. Here we use periods to show the stop points in the function `fac`:

    (defun fac (n)
      .(if .(< 0 n.).
          .(* n. .(fac .(1- n.).).).
        1).)

The special commands of Edebug are available in the source code buffer in addition to the commands of Emacs Lisp mode. For example, you can type the Edebug command `SPC` to execute until the next stop point. If you type `SPC` once after entry to `fac`, here is the display you will see:

    (defun fac (n)
    =>(if ∗(< 0 n)
          (* n (fac (1- n)))
        1))

When Edebug stops execution after an expression, it displays the expression’s value in the echo area.

Other frequently used commands are `b` to set a breakpoint at a stop point, `g` to execute until a breakpoint is reached, and `q` to exit Edebug and return to the top-level command loop. Type `?` to display a list of all Edebug commands.

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