# Debugger ## Getting Started To use Debugger, the basic steps are as follows: _Step 1._ Start Debugger by calling `debugger:start()`. The [Monitor window](debugger_chapter.md#monitor) is displayed with information about all debugged processes, interpreted modules, and selected options. Initially there are normally no debugged processes. First, it must be specified which modules that are to be _debugged_ (also called _interpreted_). Proceed as follows: _Step 2._ Select _Module > Interpret..._ in the Monitor window. The [Interpret Modules window](debugger_chapter.md#interpret) is displayed. _Step 3._ Select the appropriate modules from the Interpret Dialog window. > #### Note {: .info } > > Only modules compiled with option `debug_info` set can be interpreted. > Non-interpretable modules are displayed within parenthesis in the Interpret > Modules window. _Step 4._ In the Monitor window, select _Module > the module to be interpreted > View_. The contents of the source file is displayed in the [View Module window](debugger_chapter.md#view). _Step 5._ Set the [breakpoints](debugger_chapter.md#breakpoints), if any. _Step 6._ Start the program to be debugged. This is done the normal way from the Erlang shell. All processes executing code in interpreted modules are displayed in the Monitor window. _Step 7._ To _attach_ to one of these processes, double-click it, or select the process and then choose _Process > Attach_. Attaching to a process opens an [Attach Process window](debugger_chapter.md#attach) for this process. _Step 8._ From the Attach Process window, you can control the process execution, inspect variable values, set breakpoints, and so on. [](){: #breakpoints } ## Breakpoints and Break Dialog Windows Once the appropriate modules are interpreted, breakpoints can be set at relevant locations in the source code. Breakpoints are specified on a line basis. When a process reaches a breakpoint, it stops and waits for commands (_Step_, _Skip_, _Continue_ ...) from the user. > #### Note {: .info } > > When a process reaches a breakpoint, only that process is stopped. Other > processes are not affected. Breakpoints are created and deleted using the _Break_ menu of either the Monitor window, View Module window, or Attach Process window. ### Executable Lines To have an effect, a breakpoint must be set at an _executable line_, which is a line of code containing an executable expression such as a matching or a function call. A blank line or a line containing a comment, function head, or pattern in a `case` statement or `receive` statement is not executable. In the following example, lines 2, 4, 6, 8, and 11 are executable lines: ```erlang 1: is_loaded(Module,Compiled) -> 2: case get_file(Module,Compiled) of 3: {ok,File} -> 4: case code:which(Module) of 5: ?TAG -> 6: {loaded,File}; 7: _ -> 8: unloaded 9: end; 10: false -> 11: false 12: end. ``` ### Status and Trigger Action A breakpoint can be either _active_ or _inactive_. Inactive breakpoints are ignored. Each breakpoint has a _trigger action_ that specifies what is to happen when a process has reached it (and stopped): - _Enable_ \- Breakpoint is to remain active (default). - _Disable_ \- Breakpoint is to be made inactive. - _Delete_ \- Breakpoint is to be deleted. ### Line Breakpoints A line breakpoint is created at a certain line in a module.  Right-click the _Module_ entry to open a popup menu from which the appropriate module can be selected. A line breakpoint can also be created (and deleted) by double-clicking the line when the module is displayed in the View Module window or Attach Process window. ### Conditional Breakpoints A conditional breakpoint is created at a certain line in the module, but a process reaching the breakpoint stops only if a specified condition is true. The condition is specified by the user as a module name `CModule` and a function name `CFunction`. When a process reaches the breakpoint, `CModule:CFunction(Bindings)` is evaluated. If and only if this function call returns `true`, the process stops. If the function call returns `false`, the breakpoint is silently ignored. `Bindings` is a list of variable bindings. To retrieve the value of `Variable` (given as an atom), use function [`int:get_binding(Variable, Bindings)`](`int:get_binding/2`). The function returns `unbound` or `{value,Value}`.  Right-click the _Module_ entry to open a popup menu from which the appropriate module can be selected. _Example:_ A conditional breakpoint calling `c_test:c_break/1` is added at line 6 in module `fact`. Each time the breakpoint is reached, the function is called. When `N` is equal to 3, the function returns `true` and the process stops. Extract from `fact.erl`: ```erlang 5. fac(0) -> 1; 6. fac(N) when N > 0, is_integer(N) -> N * fac(N-1). ``` Definition of `c_test:c_break/1`: ```erlang -module(c_test). -export([c_break/1]). c_break(Bindings) -> case int:get_binding('N', Bindings) of {value, 3} -> true; _ -> false end. ``` ### Function Breakpoints A function breakpoint is a set of line breakpoints, one at the first line of each clause in the specified function.  To open a popup menu from which the appropriate module can be selected, right-click the _Module_ entry. To bring up all functions of the module in the listbox, click the _OK_ button (or press the _Return_ or _Tab_ key) when a module name has been specified,. [](){: #stack_trace } ## Stack Trace The Erlang emulator keeps track of a _stack trace_, information about recent function calls. This information is used if an error occurs, for example: ```erlang 1> catch a+1. {'EXIT',{badarith,[{erlang,'+',[a,1],[]}, {erl_eval,do_apply,6,[{file,"erl_eval.erl"},{line,573}]}, {erl_eval,expr,5,[{file,"erl_eval.erl"},{line,357}]}, {shell,exprs,7,[{file,"shell.erl"},{line,674}]}, {shell,eval_exprs,7,[{file,"shell.erl"},{line,629}]}, {shell,eval_loop,3,[{file,"shell.erl"},{line,614}]}]}} ``` For details about the stack trace, see section [Errors and Error Handling](`e:system:errors.md`) in the Erlang Reference Manual. Debugger emulates the stack trace by keeping track of recently called interpreted functions. (The real stack trace cannot be used, as it shows which functions of Debugger have been called, rather than which interpreted functions.) This information can be used to traverse the chain of function calls, using the _Up_ and _Down_ buttons in the [Attach Process window](debugger_chapter.md#attach). By default, Debugger only saves information about recursive function calls, that is, function calls that have not yet returned a value (option _Stack On, No Tail_). Sometimes, however, it can be useful to save all calls, even tail-recursive calls. This is done with option _Stack On, Tail_. Notice that this option consumes more memory and slows down execution of interpreted functions when there are many tail-recursive calls. To turn off the Debugger stack trace facility, select option _Stack Off_. > #### Note {: .info } > > If an error occurs, the stack trace becomes empty in this case. For information about how to change the stack trace option, see section [Monitor Window](debugger_chapter.md#monitor). [](){: #monitor } ## Monitor Window The Monitor window is the main window of Debugger and displays the following: - A listbox containing the names of all interpreted modules Double-clicking a module brings up the View Module window. - Which options are selected - Information about all debugged processes, that is, all processes that have been or are executing code in interpreted modules  The _Auto Attach_ boxes, _Stack Trace_ label, _Back Trace Size_ label, and _Strings_ box display some options set. For details about these options, see section [Options Menu](debugger_chapter.md#options). ### Process Grid - **_Pid_** - The process identifier. - **_Initial Call_** - The first call to an interpreted function by this process. (`Module:Function/Arity`) - **_Name_** - The registered name, if any. If a registered name is not displayed, it can be that Debugger received information about the process before the name was registered. Try selecting _Edit > Refresh_. - **_Status_** - The current status, one of the following: - **_idle_** - The interpreted function call has returned a value, and the process is no longer executing interpreted code. - **_running_** - The process is running. - **_waiting_** - The process is waiting in a `receive` statement. - **_break_** - The process is stopped at a breakpoint. - **_exit_** - The process has terminated. - **_no_conn_** - There is no connection to the node where the process is located. - **_Information_** - More information, if any. If the process is stopped at a breakpoint, the field contains information about the location `{Module,Line}`. If the process has terminated, the field contains the exit reason. ### File Menu - **_Load Settings..._** - Tries to load and restore Debugger settings from a file previously saved using _Save Settings..._ (see below). Any errors are silently ignored. Notice that settings saved by Erlang/OTP R16B01 or later cannot be read by Erlang/OTP R16B or earlier. - **_Save Settings..._** - Saves Debugger settings to a file. The settings include the set of interpreted files, breakpoints, and the selected options. The settings can be restored in a later Debugger session using _Load Settings..._ (see above). Any errors are silently ignored. - **_Exit_** - Stops Debugger. ### Edit Menu - **_Refresh_** - Updates information about debugged processes. Information about all terminated processes are removed from the window. All Attach Process windows for terminated processes are closed. - **_Kill All_** - Terminates all processes listed in the window using [`exit_signal(Pid, kill)`](`exit_signal/2`). ### Module Menu - **_Interpret..._** - Opens the [Interpret Modules window](debugger_chapter.md#interpret), where new modules to be interpreted can be specified. - **_Delete All_** - Stops interpreting all modules. Processes executing in interpreted modules terminate. For each interpreted module, a corresponding entry is added to the _Module_ menu, with the following submenu: - **_Delete_** - Stops interpreting the selected module. Processes executing in this module terminate. - **_View_** - Opens a [View Module window](debugger_chapter.md#view), displaying the contents of the selected module. ### Process Menu The following menu items apply to the currently selected process, provided it is stopped at a breakpoint (for details, see section [Attach Process window](debugger_chapter.md#attach)): - **_Step_** - **_Next_** - **_Continue_** - **_Finish_** The following menu items apply to the currently selected process: - **_Attach_** - Attaches to the process and open an [Attach Process window](debugger_chapter.md#attach). - **_Kill_** - Terminates the process using [`exit_signal(Pid, kill)`](`exit_signal/2`). ### Break Menu The items in this menu are used to create and delete breakpoints. For details, see section [Breakpoints](debugger_chapter.md#breakpoints). - **_Line Break..._** - Sets a line breakpoint. - **_Conditional Break..._** - Sets a conditional breakpoint. - **_Function Break..._** - Sets a function breakpoint. - **_Enable All_** - Enables all breakpoints. - **_Disable All_** - Disables all breakpoints. - **_Delete All_** - Removes all breakpoints. For each breakpoint, a corresponding entry is added to the _Break_ menu, from which it is possible to disable, enable, or delete the breakpoint, and to change its trigger action. [](){: #options } ### Options Menu - **_Trace Window_** - Sets the areas to be visible in an [Attach Process window](debugger_chapter.md#attach). Does not affect existing Attach Process windows. - **_Auto Attach_** - Sets the events a debugged process is to be attached to automatically. Affects existing debugged processes. - _First Call_ \- The first time a process calls a function in an interpreted module. - _On Exit_ \- At process termination. - _On Break_ \- When a process reaches a breakpoint. - **_Stack Trace_** - Sets the stack trace option, see section [Stack Trace](debugger_chapter.md#stack_trace). Does not affect existing debugged processes. - _Stack On, Tail_ \- Saves information about all current calls. - _Stack On, No Tail_ \- Saves information about current calls, discarding previous information when a tail recursive call is made. - _Stack Off_ \- Does not save any information about current calls. - **_Strings_** - Sets the integer lists to be printed as strings. Does not affect existing debugged processes. - _Use range of +pc flag_ \- Uses the printable character range set by the [`erl(1)`](`e:erts:erl_cmd.md`) flag [`+pc`](`e:erts:erl_cmd.md#printable_character_range`). - **_Back Trace Size..._** - Sets how many call frames to be fetched when inspecting the call stack from the Attach Process window. Does not affect existing Attach Process windows. ### Windows Menu Contains a menu item for each open Debugger window. Selecting one of the items raises the corresponding window. ### Help Menu - **_Help_** - Shows the Debugger documentation. This function requires a web browser. [](){: #interpret } ## Interpret Modules Window The Interpret Modules window is used for selecting which modules to interpret. Initially, the window displays the modules (`erl` files) and subdirectories of the current working directory. Interpretable modules are modules for which a `.beam` file, compiled with option `debug_info` set, is located in the same directory as the source code, or in an `ebin` directory next to it. Modules for which these requirements are not fulfilled are not interpretable and are therefore displayed within parentheses. Option `debug_info` causes _debug information_ or _abstract code_ to be added to the `.beam` file. This increases the file size and makes it possible to reconstruct the source code. It is therefore recommended not to include debug information in code aimed for target systems. An example of how to compile code with debug information using `erlc`: ```erlang % erlc +debug_info module.erl ``` An example of how to compile code with debug information from the Erlang shell: ```text 4> c(module, debug_info). ```  To browse the file hierarchy and interpret the appropriate modules, either select a module name and click _Choose_ (or press carriage return), or double-click the module name. Interpreted modules have the type `erl src`. To interpret all displayed modules in the chosen directory, click _All_. To close the window, click _Done_. > #### Note {: .info } > > When Debugger is started in global mode (which is the default, see > `debugger:start/0`), modules added (or deleted) for interpretation are added > (or deleted) on all known Erlang nodes. [](){: #attach } ## Attach Process Window From an Attach Process window, you can interact with a debugged process. One window is opened for each process that has been attached to. Notice that when attaching to a process, its execution is automatically stopped.  The window is divided into the following five parts: - The Code area, displaying the code being executed. The code is indented and each line is prefixed with its line number. If the process execution is stopped, the current line is marked with `-->`. An existing break point at a line is marked with a stop symbol. In the example shown in the illustration, the execution stopped at line 6, before the execution of `fac/1`. Active breakpoints are displayed in red and inactive breakpoints in blue. - The Button area, with buttons for quick access to frequently used functions in the _Process_ menu. - The Evaluator area, where you can evaluate functions within the context of the debugged process, if that process execution is stopped. - The Bindings area, displaying all variables bindings. If you click a variable name, the value is displayed in the Evaluator area. Double-click a variable name to open a window where the variable value can be edited. Notice however that pid, port, reference, or fun values cannot be edited unless they can be represented in the running system. - The Trace area, which displays a trace output for the process. - **`++ (N) `** - Function call, where `N` is the call level and `L` the line number. - **`-- (N)`** - Function return value . - **`==> Pid : Msg`** - The message `Msg` is sent to process `Pid`. - **`<== Msg`** - The message `Msg` is received. - **`++ (N) receive`** - Waiting in a `receive`. - **`++ (N) receive with timeout`** - Waiting in a `receive...after`. The Trace area also displays Back Trace, a summary of the current function calls on the stack. Using the _Options_ menu, you can set which areas to be displayed. By default, all areas except the Trace area are displayed. ### File Menu - **_Close_** - Closes this window and detach from the process. ### Edit Menu - **_Go to line..._** - Goes to a specified line number. - **_Search..._** - Searches for a specified string. ### Process Menu - **_Step_** - Executes the current code line, stepping into any (interpreted) function calls. - **_Next_** - Executes the current code line and stop at the next line. - **_Continue_** - Continues the execution. - **_Finish_** - Continues the execution until the current function returns. - **_Skip_** - Skips the current code line and stop at the next line. If used on the last line in a function body, the function returns `skipped`. - **_Time Out_** - Simulates a time-out when executing a `receive...after` statement. - **_Stop_** - Stops the execution of a running process, that is, make the process stop at a breakpoint. The command takes effect (visibly) the next time the process receives a message. - **_Where_** - Verifies that the current location of the execution is visible in the code area. - **_Kill_** - Terminates the process using [`exit_signal(Pid, kill)`](`exit_signal/2`). - **_Messages_** - Inspects the message queue of the process. The queue is displayed in the Evaluator area. - **_Back Trace_** - Displays the back trace of the process, a summary of the current function calls on the stack, in the Trace area. Requires that the Trace area is visible and that the Stack Trace option is _Stack On, Tail_ or _Stack On, No Tail_. - **_Up_** - Inspects the previous function call on the stack, showing the location and variable bindings. - **_Down_** - Inspects the next function call on the stack, showing the location and variable bindings. ### Options Menu - **_Trace Window_** - Sets which areas are to be visible. Does not affect other Attach Process windows. - **_Stack Trace_** - Same as in the [Monitor window](debugger_chapter.md#monitor), but only affects the debugged process the window is attached to. - **_Strings_** - Same as in the [Monitor window](debugger_chapter.md#monitor), but only affects the debugged process the window is attached to. - **_Back Trace Size..._** - Sets how many call frames are to be fetched when inspecting the call stack. Does not affect other Attach Process windows. ### Break, Windows, and Help Menus The _Break_, _Windows_, and _Help_ menus are the same as in the [Monitor Window](debugger_chapter.md#monitor), except that the _Breaks_ menu applies only to local breakpoints. [](){: #view } ## View Module Window The View Module window displays the contents of an interpreted module and makes it possible to set breakpoints.  The source code is indented and each line is prefixed with its line number. Clicking a line highlights it and selects it to be the target of the breakpoint functions available from the _Break_ menu. To set a line breakpoint on a line, double-click it. To remove the breakpoint, double-click the line with an existing breakpoint. Breakpoints are marked with a stop symbol. ### File and Edit Menus The _File_ and _Edit_ menus are the same as in the [Attach Process Window](debugger_chapter.md#attach). ### Break, Windows, and Help Menus The _Break_, _Windows_, and _Help_ menus are the same as in the [Monitor Window](debugger_chapter.md#monitor), except that the _Break_ menu applies only to local breakpoints. ## Performance Execution of interpreted code is naturally slower than for regularly compiled modules. Using Debugger also increases the number of processes in the system, as for each debugged process another process (the meta process) is created. It is also worth to keep in mind that programs with timers can behave differently when debugged. This is especially true when stopping the execution of a process (for example, at a breakpoint). Time-outs can then occur in other processes that continue execution as normal. ## Code Loading Mechanism Code loading works almost as usual, except that interpreted modules are also stored in a database and debugged processes use only this stored code. Reinterpreting an interpreted module results in the new version being stored as well, but does not affect existing processes executing an older version of the code. This means that the code replacement mechanism of Erlang does not work for debugged processes. ## Debugging Remote Nodes By using `debugger:start/1`, you can specify if Debugger is to be started in local or global mode: ```text debugger:start(local | global) ``` If `debugger:start/0` is called, Debugger starts in global mode. In local mode, code is interpreted only on the current node. In global mode, code is interpreted on all known nodes. Processes on other nodes executing interpreted code are automatically displayed in the Monitor window and can be attached to like any other debugged process. > #### Note {: .info } > > It not recommended to start Debugger in global mode on more than one > node in a network, as the nodes interfere with each other, leading to > inconsistent behavior.