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.

Breakpoints are created and deleted using the Break menu of either the Monitor window, View Module window, or Attach Process window.

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.

A line breakpoint is created at a certain line in a module.

Line Break Dialog Window

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.

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). The function returns unbound or {value,Value}.

Conditional Break Dialog Window

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:

5. fac(0) -> 1;
6. fac(N) when N > 0, is_integer(N) -> N * fac(N-1).

Definition of c_test:c_break/1:

-module(c_test).
-export([c_break/1]).

c_break(Bindings) ->
    case int:get_binding('N', Bindings) of
        {value, 3} ->
            true;
        _ ->
            false
    end.

A function breakpoint is a set of line breakpoints, one at the first line of each clause in the specified function.

Function Break Dialog Window

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,.

The Erlang emulator keeps track of a stack trace, information about recent function calls. This information is used if an error occurs, for example:

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 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.

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.

For information about how to change the stack trace option, see section Monitor Window.

The Monitor window is the main window of Debugger and displays the following:

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.

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:

% erlc +debug_info module.erl

An example of how to compile code with debug information from the Erlang shell:

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.

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:

Using the Options menu, you can set which areas to be displayed. By default, all areas except the Trace area are displayed.

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.