42. Debugging

42.1 Source Level Debugging

Maxima has a built-in source level debugger. The user can set a breakpoint at a function, and then step line by line from there. The call stack may be examined, together with the variables bound at that level.

The command :help or :h shows the list of debugger commands. (In general, commands may be abbreviated if the abbreviation is unique. If not unique, the alternatives will be listed.) Within the debugger, the user can also use any ordinary Maxima functions to examine, define, and manipulate variables and expressions.

A breakpoint is set by the :br command at the Maxima prompt. Within the debugger, the user can advance one line at a time using the :n ("next") command. The :bt ("backtrace") command shows a list of stack frames. The :r ("resume") command exits the debugger and continues with execution. These commands are demonstrated in the example below.

(%i1) load ("/tmp/foobar.mac");

(%o1)                           /tmp/foobar.mac

(%i2) :br foo
Turning on debugging debugmode(true)
Bkpt 0 for foo (in /tmp/foobar.mac line 1) 

(%i2) bar (2,3);
Bkpt 0:(foobar.mac 1)
/tmp/foobar.mac:1::

(dbm:1) :bt                        <-- :bt typed here gives a backtrace
#0: foo(y=5)(foobar.mac line 1)
#1: bar(x=2,y=3)(foobar.mac line 9)

(dbm:1) :n                         <-- Here type :n to advance line
(foobar.mac 2)
/tmp/foobar.mac:2::

(dbm:1) :n                         <-- Here type :n to advance line
(foobar.mac 3)
/tmp/foobar.mac:3::

(dbm:1) u;                         <-- Investigate value of u
28

(dbm:1) u: 33;                     <-- Change u to be 33
33

(dbm:1) :r                         <-- Type :r to resume the computation

(%o2)                                1094

The file /tmp/foobar.mac is the following:

foo(y) := block ([u:y^2],
  u: u+3,
  u: u^2,
  u);
 
bar(x,y) := (
  x: x+2,
  y: y+2,
  x: foo(y),
  x+y);

USE OF THE DEBUGGER THROUGH EMACS

If the user is running the code under GNU emacs in a shell window (dbl shell), or is running the graphical interface version, xmaxima, then if he stops at a break point, he will see his current position in the source file which will be displayed in the other half of the window, either highlighted in red, or with a little arrow pointing at the right line. He can advance single lines at a time by typing M-n (Alt-n).

Under Emacs you should run in a dbl shell, which requires the dbl.el file in the elisp directory. Make sure you install the elisp files or add the Maxima elisp directory to your path: e.g., add the following to your `.emacs' file or the site-init.el

(setq load-path (cons "/usr/share/maxima/5.9.1/emacs" load-path))
(autoload 'dbl "dbl")

then in emacs

M-x dbl

should start a shell window in which you can run programs, for example Maxima, gcl, gdb etc. This shell window also knows about source level debugging, and display of source code in the other window.

The user may set a break point at a certain line of the file by typing C-x space. This figures out which function the cursor is in, and then it sees which line of that function the cursor is on. If the cursor is on, say, line 2 of foo, then it will insert in the other window the command, ":br foo 2", to break foo at its second line. To have this enabled, the user must have maxima-mode.el turned on in the window in which the file foobar.mac is visiting. There are additional commands available in that file window, such as evaluating the function into the Maxima, by typing Alt-Control-x.

42.2 Keyword Commands

Keyword commands are special keywords which are not interpreted as Maxima expressions. A keyword command can be entered at the Maxima prompt or the debugger prompt, although not at the break prompt. Keyword commands start with a colon, ':'. For example, to evaluate a Lisp form you may type :lisp followed by the form to be evaluated.

(%i1) :lisp (+ 2 3) 
5

The number of arguments taken depends on the particular command. Also, you need not type the whole command, just enough to be unique among the break keywords. Thus :br would suffice for :break.

The keyword commands are listed below.

:break F n

Set a breakpoint in function F at line offset n from the beginning of the function. If F is given as a string, then it is assumed to be a file, and n is the offset from the beginning of the file. The offset is optional. If not given, it is assumed to be zero (first line of the function or file).

:bt

Print a backtrace of the stack frames

:continue

Continue the computation

:delete

Delete the specified breakpoints, or all if none are specified

:disable

Disable the specified breakpoints, or all if none are specified

:enable

Enable the specified breakpoints, or all if none are specified

:frame n

Print stack frame n, or the current frame if none is specified

:help

Print help on a debugger command, or all commands if none is specified

:info

Print information about item

:lisp some-form

Evaluate some-form as a Lisp form

:lisp-quiet some-form

Evaluate Lisp form some-form without any output

:next

Like :step, except :next steps over function calls

:quit

Quit the current debugger level without completing the computation

:resume

Continue the computation

:step

Continue the computation until it reaches a new source line

:top

Return to the Maxima prompt (from any debugger level) without completing the computation

42.3 Definitions for Debugging

Option variable: refcheck

Default value: false

When refcheck is true, Maxima prints a message each time a bound variable is used for the first time in a computation.

Option variable: setcheck

Default value: false

If setcheck is set to a list of variables (which can be subscripted), Maxima prints a message whenever the variables, or subscripted occurrences of them, are bound with the ordinary assignment operator :, the :: assignment operator, or function argument binding, but not the function assignment := nor the macro assignment ::= operators. The message comprises the name of the variable and the value it is bound to.

setcheck may be set to all or true thereby including all variables.

Each new assignment of setcheck establishes a new list of variables to check, and any variables previously assigned to setcheck are forgotten.

The names assigned to setcheck must be quoted if they would otherwise evaluate to something other than themselves. For example, if x, y, and z are already bound, then enter

setcheck: ['x, 'y, 'z]$

to put them on the list of variables to check.

No printout is generated when a variable on the setcheck list is assigned to itself, e.g., X: 'X.

Option variable: setcheckbreak

Default value: false

When setcheckbreak is true, Maxima will present a break prompt whenever a variable on the setcheck list is assigned a new value. The break occurs before the assignment is carried out. At this point, setval holds the value to which the variable is about to be assigned. Hence, one may assign a different value by assigning to setval.

See also setcheck and setval.

System variable: setval

Holds the value to which a variable is about to be set when a setcheckbreak occurs. Hence, one may assign a different value by assigning to setval.

See also setcheck and setcheckbreak.

Function: timer (f_1, ..., f_n)

Function: timer ()

Given functions f_1, ..., f_n, timer puts each one on the list of functions for which timing statistics are collected. timer(f)$ timer(g)$ puts f and then g onto the list; the list accumulates from one call to the next.

With no arguments, timer returns the list of timed functions.

Maxima records how much time is spent executing each function on the list of timed functions. timer_info returns the timing statistics, including the average time elapsed per function call, the number of calls, and the total time elapsed. untimer removes functions from the list of timed functions.

timer quotes its arguments. f(x) := x^2$ g:f$ timer(g)$ does not put f on the timer list.

If trace(f) is in effect, then timer(f) has no effect; trace and timer cannot both be in effect at the same time.

See also timer_devalue.

Function: untimer (f_1, ..., f_n)

Function: untimer ()

Given functions f_1, ..., f_n, untimer removes each function from the timer list.

With no arguments, untimer removes all functions currently on the timer list.

After untimer (f) is executed, timer_info (f) still returns previously collected timing statistics, although timer_info() (with no arguments) does not return information about any function not currently on the timer list. timer (f) resets all timing statistics to zero and puts f on the timer list again.

Option variable: timer_devalue

Default value: false

When timer_devalue is true, Maxima subtracts from each timed function the time spent in other timed functions. Otherwise, the time reported for each function includes the time spent in other functions. Note that time spent in untimed functions is not subtracted from the total time.

See also timer and timer_info.

Function: timer_info (f_1, ..., f_n)

Function: timer_info ()

Given functions f_1, ..., f_n, timer_info returns a matrix containing timing information for each function. With no arguments, timer_info returns timing information for all functions currently on the timer list.

The matrix returned by timer_info contains the function name, time per function call, number of function calls, total time, and gctime, which meant "garbage collection time" in the original Macsyma but is now always zero.

The data from which timer_info constructs its return value can also be obtained by the get function:

get(f, 'calls); get(f, 'runtime); get(f, 'gctime);

See also timer.

Function: trace (f_1, ..., f_n)

Function: trace ()

Given functions f_1, ..., f_n, trace instructs Maxima to print out debugging information whenever those functions are called. trace(f)$ trace(g)$ puts f and then g onto the list of functions to be traced; the list accumulates from one call to the next.

With no arguments, trace returns a list of all the functions currently being traced.

The untrace function disables tracing. See also trace_options.

trace quotes its arguments. Thus, f(x) := x^2$ g:f$ trace(g)$ does not put f on the trace list.

When a function is redefined, it is removed from the timer list. Thus after timer(f)$ f(x) := x^2$, function f is no longer on the timer list.

If timer (f) is in effect, then trace (f) has no effect; trace and timer can't both be in effect for the same function.

Function: trace_options (f, option_1, ..., option_n)

Function: trace_options (f)

Sets the trace options for function f. Any previous options are superseded. trace_options (f, ...) has no effect unless trace (f) is also called (either before or after trace_options).

trace_options (f) resets all options to their default values.

The option keywords are:

• noprint Do not print a message at function entry and exit.
• break Put a breakpoint before the function is entered, and after the function is exited. See break.
• lisp_print Display arguments and return values as Lisp objects.
• info Print -> true at function entry and exit.
• errorcatch Catch errors, giving the option to signal an error, retry the function call, or specify a return value.

Trace options are specified in two forms. The presence of the option keyword alone puts the option into effect unconditionally. (Note that option foo is not put into effect by specifying foo: true or a similar form; note also that keywords need not be quoted.) Specifying the option keyword with a predicate function makes the option conditional on the predicate.

The argument list to the predicate function is always [level, direction, function, item] where level is the recursion level for the function, direction is either enter or exit, function is the name of the function, and item is the argument list (on entering) or the return value (on exiting).

Here is an example of unconditional trace options:

(%i1) ff(n) := if equal(n, 0) then 1 else n * ff(n - 1)$ (%i2) trace (ff)$ (%i3) trace_options (ff, lisp_print, break)$ (%i4) ff(3);

Here is the same function, with the break option conditional on a predicate:

(%i5) trace_options (ff, break(pp))$ (%i6) pp (level, direction, function, item) := block (print (item), return (function = 'ff and level = 3 and direction = exit))$ (%i7) ff(6);

Function: untrace (f_1, ..., f_n)

Function: untrace ()

Given functions f_1, ..., f_n, untrace disables tracing enabled by the trace function. With no arguments, untrace disables tracing for all functions.

untrace returns a list of the functions for which it disabled tracing.

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