Debugger

A Julia debugger.

Build Status

Note: If you are looking for the docs for the Julia VSCode debugger, see this link instead

Installation

Install Debugger using Pkg:

julia> import Pkg; Pkg.add("Debugger")

Usage

Starting the debugger interface

The debug interface is entered using the @enter macro:

using Debugger

function foo(n)
    x = n+1
    ((BigInt[1 1; 1 0])^x)[2,1]
end

@enter foo(20)

This interface allows for manipulating program execution, such as stepping in and out of functions, line stepping, showing local variables, setting breakpoints and evaluating code in the context of functions.

The debug> REPL mode

Loading Debugger in an interactive session installs a debug> REPL mode, entered by pressing ) at the beginning of an empty julia> prompt (and left with backspace, like the Pkg and shell modes). In this mode:

• any expression you enter is debugged as if run through @enter,
• breakpoints can be managed with the usual bp commands (see below) — also outside of a debug session; breakpoints persist between sessions.

julia> using Debugger

debug> bp add sin
[ Info: added breakpoint for function sin

debug> cos(1.0)  # equivalent to @enter cos(1.0)

Debugger commands

Below, square brackets denote optional arguments.

All of the following commands work when the prompt is 1|debug>:

Misc:

• o: open the current line in an editor
• q: quit the debugger, returning nothing
• C: toggle compiled mode
• L: toggle showing lowered code instead of source code
• T: cycle how variable types are shown: compact (long type parameters elided), no types, types only, full types
• S: toggle "sticky" (full-screen) mode, on by default: the debugger runs on the terminal's alternate screen (restored when quitting, like less or vim) and redraws the status in place instead of scrolling; command output taller than the screen (e.g. the ? help) is shown in a scrollable pager
• +/-: increase / decrease the number of lines of source code shown

Stepping (basic):

• n: step to the next line
• u [i::Int]: step until line i or the next line past the current line
• s: step into the next call
• so: step out of the current call
• sl: step into the last call on the current line (e.g. steps into f if the line is f(g(h(x)))).
• sr: step until next return.
• c: continue execution until a breakpoint is hit
• f [i::Int]: go to the i-th function in the call stack; without an argument, pick the frame interactively (stepping is only possible in the function at the top of the call stack)
• up/down [i::Int] go up or down one or i functions in the call stack

Stepping (advanced):

• nc: step to the next call
• se: step one expression step
• si: same as se but step into a call if a call is the next expression
• sg: step into a generated function

Querying:

• st: show the "status" (current function, source code and current expression to run)
• bt [v]: show a compact backtrace; bt v also shows the variables of every frame
• fr [i::Int]: show all variables in the current or ith frame
• p
  • p: print all variables in the current frame (same as fr)
  • p x [y ...]: print the full value of the variable(s) x (and y ...), like the REPL would show it

Evaluation:

• w
  • w add expr: add an expression to the watch list; watch expressions are evaluated and shown as part of the status
  • w: interactively manage the watch list (delete entries with d)
  • w rm [i::Int]: remove all or the i-th watch expression

Breakpoints:

• bp
  • bp: interactively manage breakpoints — move with the arrow keys, toggle with space/enter, add with a (prompts for a location using the bp add syntax below), delete with d, open the breakpoint location in an editor with o, quit with q. Break-on-error and break-on-throw can be toggled from the same menu.
  • bp add
    • bp add "file.jl":line [cond]: add a breakpoint at file file.jl on line line with condition cond
    • bp add func [:line] [cond]: add a breakpoint to function func at line line (defaulting to first line) with condition cond
    • bp add func(::Float64, Int)[:line] [cond]: add a breakpoint to methods matching the signature at line line (defaulting to first line) with condition cond
    • bp add func(x, y)[:line] [cond]: add a breakpoint to the method matching the types of the local variable x, y etc with condition cond
    • bp add line [cond]: add a breakpoint to line of the file of the current function with condition cond
  • bp rm [i::Int]: remove all or the i-th breakpoint
  • bp rm "file.jl":line: remove the breakpoint at the given file and line
  • bp rm func [:line]: remove breakpoints for the function func (at line line)
  • bp toggle [i::Int]: toggle all or the i-th breakpoint
  • bp disable [i::Int]: disable all or the i-th breakpoint
  • bp enable [i::Int]: enable all or the i-th breakpoint
  • bp on/off
    • bp on/off error - turn on or off break on error
    • bp on/off throw - turn on or off break on throw

An empty command will execute the previous command.

Changing frames with f i::Int will change the prompt to $i|debug>. Stepping commands will not work until you return to f 1, but a subset of normal commands will continue to work.

In addition to these debugging commands, you can type ` to enter "evaluation mode" indicated by a prompt $i|julia>. In evaluation mode, any expression you type is executed in the debug context. For example, if you have a local variable named n, then once in evaluation mode typing n will show you the value of n rather than advancing to the next line.

Hit backspace as the first character of the line to return to "debug mode."

Breakpoints

To add and manipulate breakpoints, either the bp add command in the debug interface or the JuliaInterpreter breakpoint API, documented here can be used.

It is common to want to run a function until a breakpoint is hit. Therefore, the "shortcut macro" @run is provided which is equivalent of starting the debug mode with @enter and then executing the continue command (c):

julia> using Debugger

julia> breakpoint(abs);

julia> @run sin(2.0)
Hit breakpoint:
[1/2] abs(x) at float.jl:522
>522  abs(x::Float64) = abs_float(x)

  x::Float64 = 2.0  (arg)

→ (abs_float)(2.0)
1|debug> bt
>[1] abs(x) at float.jl:522
 [2] sin(x) at special/trig.jl:30

Breakpoint on error

It is possible to halt execution when an error is thrown. This is done by calling the exported function break_on(:error).

julia> using Debugger

julia> break_on(:error)

julia> f() = "αβ"[2];

julia> @run f()
Breaking for error:
ERROR: StringIndexError("αβ", 2)
[1/4] string_index_err(s, i) at strings/string.jl:12
>12  @noinline string_index_err(s::AbstractString, i::Integer) =

  s::String = "αβ"  (arg)
  i::Int64  = 2     (arg)

→ (throw)(StringIndexError("αβ", 2))
1|debug> bt
>[1] string_index_err(s, i) at strings/string.jl:12
 [2] getindex_continued(s, i, u) at strings/string.jl:218
 [3] getindex(s, i) at strings/string.jl:211
 [4] f() at REPL[5]:1

julia> JuliaInterpreter.break_off(:error)

julia> @run f()
ERROR: StringIndexError("αβ", 2)
Stacktrace:
[...]

Place breakpoints in source code

It is sometimes more convenient to choose in the source code when to break. This is done for instance in Matlab/Octave with keyboard, and in R with browser(). You can use the @bp macro to do this:

julia> using Debugger

julia> function f(x)
           if x < 0
               @bp
           else
               println("All good!")
           end
       end
f (generic function with 1 method)

julia> @run f(2)
All good!

julia> @run f(-2)
Hit breakpoint:
[1/1] f(x) at REPL[6]:2
 1  function f(x)
 2      if x < 0
>3          @bp
 4      else
 5          println("All good!")
 6      end
 7  end

  x::Int64 = -2  (arg)

→ return
1|debug> bt
>[1] f(x) at REPL[6]:3

Compiled mode

In order to fully support breakpoints, the debugger interprets all code, even code that is stepped over. Currently, there are cases where the interpreter is too slow for this to be feasible. A workaround is to use "compiled mode" which is toggled by pressing C in the debug REPL mode (note the change of prompt color). When using compiled mode, code that is stepped over will be executed by the normal julia compiler and run just as fast as normally. The drawback is that breakpoints in compiled code that is stepped over are missed.

To split the difference between these two extremes, one can fine tune which modules are compiled and which are not. For example, to compile all code in Base, even when not in C mode, and hence break on all specified points in user code, issue the following commands on the REPL before @enter:

using JuliaInterpreter, MethodAnalysis
union!(JuliaInterpreter.compiled_modules, child_modules(Base))

Additional imported modules can also always be compiled with:

union!(JuliaInterpreter.compiled_modules, SomePackage)

Configuration

UI options are set with Debugger.config(; kwargs...); calling it without arguments shows the current settings. The available options are:

• theme::String: syntax highlighting theme, a Highlights.jl theme name (default: "Monokai Dark")
• highlight::Bool: syntax highlight source code and variables (default: true)
• context_lines::Int: number of source lines shown above and below the current line (default: 4)
• vartypes::Symbol: how variable types are displayed: :compact (long type parameters elided, e.g. Dict{…}), :none, :types or :full (default: :compact); can also be cycled with the T key in the debugger
• max_vars::Int: maximum number of variables shown in the automatic status display (default: 15)
• sticky::Bool: "full screen" mode — the debugger runs on the terminal's alternate screen and redraws the status in place instead of scrolling (default: true); can also be toggled with the S key
• charset::Symbol: :unicode or :ascii (default: :unicode)
• menus::Bool: use the interactive menus for bp, f and w (default: true)

The older entry points Debugger.set_theme(theme) and Debugger.set_highlight(false) still work.