@c Copyright (C) 1996, 1997, 2007 John W. Eaton
@c This is part of the Octave manual.
@c For copying conditions, see the file gpl.texi.

@node Debugging
@chapter Debugging

Octave includes a built-in debugger to aid in the development of
scripts. This can be used to interrupt the execution of an Octave script
at a certain point, or when certain conditions are met. Once execution
has stopped, and debug mode is entered, the symbol table at the point
where execution has stopped can be examined and modified to check for
errors.

The normal commandline editing and history functions are available in
debug mode. However, one limitation on the debug mode is that
commands entered at the debug prompt are evaluated as strings, rather
than being handled by the Octave parser. This means that all commands in
debug mode must be contained on a single line. That is, it is alright to
write

@example
debug> for i = 1:n, foo(i); endfor
@end example

@noindent
in debug mode. However, writing the above in three lines will not be
correctly evaluated. To leave the debug mode, you should simply type
either @code{quit}, @code{exit}, @code{return} or @code{dbcont}.

@menu
* Entering Debug Mode::
* Breakpoints::
* Debug Mode::
@end menu

@node Entering Debug Mode
@section Entering Debug Mode

There are two basic means of interrupting the execution of an Octave
script. These are breakpoints @ref{Breakpoints}, discussed in the next
section and interruption based on some condition.

Octave supports three means to stop execution based on the values set in
the functions @code{debug_on_interrupt}, @code{debug_on_warning} and
@code{debug_on_error}.

@DOCSTRING(debug_on_interrupt)

@DOCSTRING(debug_on_warning)

@DOCSTRING(debug_on_error)

@node Breakpoints
@section Breakpoints

Breakpoints can be set in any Octave function, using the @code{dbstop}
function.

@DOCSTRING(dbstop)

@noindent
Note that breakpoints can not be set in built-in functions
(eg. @code{sin}, etc) or dynamically loaded function (ie. oct-files). To
set a breakpoint immediately on entering a function, the breakpoint
should be set to line 1. The leading comment block will be ignored and
the breakpoint will be set to the first executable statement in the
function. For example

@example
@group
dbstop ("asind", 1)
@result{} 27
@end group
@end example

@noindent
Note that the return value of @code{27} means that the breakpoint was
effectively set to line 27. The status of breakpoints in a function can
be queried with the @code{dbstatus} function.

@DOCSTRING(dbstatus)

@noindent
Taking the above as an example, @code{dbstatus ("asind")} should return
27. The breakpoints can then be cleared with the @code{dbclear} function

@DOCSTRING(dbclear)

@noindent
To clear all of the breakpoints in a function the recommended means,
following the above example, is then

@example
dbclear ("asind", dbstatus ("asind"));
@end example

Another simple means of setting a breakpoint in an Octave script is the
use of the @code{keyboard} function.

@DOCSTRING(keyboard)

@noindent
The @code{keyboard} function is typically placed in a script at the
point where the user desires that the execution is stopped. It
automatically sets the running script into the debug mode.

@node Debug Mode
@section Debug Mode

There are two additional support functions that allow the user to
interrogate where in the execution of a script Octave entered the debug
mode and to print the code in the script surrounding the point where
Octave entered debug mode.

@DOCSTRING(dbwhere)

@DOCSTRING(dbtype)

Debug mode equally allows single line stepping through a function using
the commands @code{dbstep} and @code{dbnext}.  These differ slightly in
the way they treat the next executable line if the next line itself is a
function defined in an m-file.  The @code{dbnext} command will execute
the next line, while staying in the existing function being debugged.
The @code{dbstep} command will step in to the new function.