elisp: Breakpoints
17.2.6.1 Edebug Breakpoints
...........................
While using Edebug, you can specify “breakpoints” in the program you are
testing: these are places where execution should stop. You can set a
breakpoint at any stop point, as defined in 
Using Edebug. For
setting and unsetting breakpoints, the stop point that is affected is
the first one at or after point in the source code buffer. Here are the
Edebug commands for breakpoints:
‘b’
Set a breakpoint at the stop point at or after point
(‘edebug-set-breakpoint’). If you use a prefix argument, the
breakpoint is temporary—it turns off the first time it stops the
program.
‘u’
Unset the breakpoint (if any) at the stop point at or after point
(‘edebug-unset-breakpoint’).
‘x CONDITION <RET>’
Set a conditional breakpoint which stops the program only if
evaluating CONDITION produces a non-‘nil’ value
(‘edebug-set-conditional-breakpoint’). With a prefix argument, the
breakpoint is temporary.
‘B’
Move point to the next breakpoint in the current definition
(‘edebug-next-breakpoint’).
While in Edebug, you can set a breakpoint with ‘b’ and unset one with
‘u’. First move point to the Edebug stop point of your choice, then
type ‘b’ or ‘u’ to set or unset a breakpoint there. Unsetting a
breakpoint where none has been set has no effect.
Re-evaluating or reinstrumenting a definition removes all of its
previous breakpoints.
A “conditional breakpoint” tests a condition each time the program
gets there. Any errors that occur as a result of evaluating the
condition are ignored, as if the result were ‘nil’. To set a
conditional breakpoint, use ‘x’, and specify the condition expression in
the minibuffer. Setting a conditional breakpoint at a stop point that
has a previously established conditional breakpoint puts the previous
condition expression in the minibuffer so you can edit it.
You can make a conditional or unconditional breakpoint “temporary” by
using a prefix argument with the command to set the breakpoint. When a
temporary breakpoint stops the program, it is automatically unset.
Edebug always stops or pauses at a breakpoint, except when the Edebug
mode is Go-nonstop. In that mode, it ignores breakpoints entirely.
To find out where your breakpoints are, use the ‘B’ command, which
moves point to the next breakpoint following point, within the same
function, or to the first breakpoint if there are no following
breakpoints. This command does not continue execution—it just moves
point in the buffer.
Info Catalog
elisp: Breaks
elisp: Global Break Condition