# Debugging

To debug is human. To fix it is divine...

Well, the human part is what this section is about. The divine part is up to you!

When a program does not behave in the way it is supposed to, we say the program contains a bug. A bug is not a compilation error — compilation errors are relatively easy to clear, because the compiler tells you where the problems are. By contrast, bugs are errors that happen during program execution and they can be hard (sometimes very hard!) to detect.

Any program that you think is bug-free is never completely bug-free. All we can try to do is to reduce the number of bugs contained in the program. The process of removing bugs is known as debugging, and the tool that is used for debugging is called the debugger. Anjuta provides a very user-friendly and powerful debugging environment (actually, a GUI wrapper over gdb, a powerful command line debugging tool and standard on Linux).

A debugger tracks and traces the execution of the program and provides various views of information needed to study the execution of the program.

## Start and stop

The debugger is always started with a program loaded. There are two ways to do this:

- Running an executable

- Attaching to a process

Almost every feature of Anjuta is provided by a plugin. If you do not see the menus described here. It is probably because the debugger plugin is not loaded. Go in Edit Preferences General Installed plugins and check that the debugger plugin is loaded. It is an user activatable plugin, so loaded or unloaded on request.

### Running an executable

Running a program with or without the debugger uses the same menu: select Run Run Program to run a program without the debugger and Run Debug Program to run it with the debugger.

In order to better use the debugger, it is strongly recommended that you build your program with debugging information (-g for gcc) and no optimization (-O0 for gcc). This can be done by selecting the Debug configuration before building the program for the first time.

You can select which program to run and its environment using Run Program Parameters…. You can choose:

- The program that you want to debug

  The drop-down menu of the target drop-down combination box is pre-filled with all executables of the current project. But you can alternatively select another executable which is not part of the project. The debugger also accepts libtool executables; these are scripts generated by libtool which wrap real executables.

- The command-line parameters

- The working directory

- Environment variables

  Environment variables displayed in light gray are the current environment variables. You can add, modify or remove environment variables here.

  To debug Anjuta using Anjuta, you can use a different theme to change the look of the instance being debugged; for example, you could add "GTK2_RC_FILES=/usr/share/themes/Crux/gtk-2.0/gtkrc".

- Whether the program should run in a terminal or not. If so, the Anjuta terminal plugin is used.

### Attaching to a Process

It is also possible to attach to a running process and debug it by choosing the menu item Run Debug Process …. A list of all processes running on the system will appear.

#### Attach to Process dialog

Attach to Process dialog

Select the process to attach to and click OK to start the debugger, attach to the selected process and stop it.

It is currently not possible to load symbol information for an attached process. Symbols should be included in the process.

### Stopping the Debugger

Choose the menu item Run Stop Debugger to stop the debugger. This will kill the program which is being debugged. If the debugger has been attached to a running process, the debugger will just detach from it without killing it. You will get a confirmation box if a program is currently attached or running.

### Adding source directories

Choose the menu item Debug Add source paths… to display a dialog box allowing you to add and remove source directories.

Most executables include full file paths, so defining the directories where are the source files is normally not useful. But some executables include only file names without paths; in this case, the debugger will search for source files in all these directories. The order of directories can be important if several files have the same name, so you can change it using the Up and Down buttons. The directory list is sent to the debugger when it is started. After changing it, you need to restart the debugger to cause it to use the new list.

## Execution

Execution of a program in the debugger can be finely controlled. The program can be executed in single steps, or allowed to continue until it encounters a breakpoint. Executing like this is essential for tracking program behavior. Like a video editing session, the program can be executed in slow motion with the ability to go forward, pause, stop, and so on. You cannot go backward yet though.

The methods those can be used to execute a program in the debugger are described in the following sections. These commands are available only when the program has already been started.

### Single stepping (step in)

Choose the menu item Debug Step In or click on the Step In icon in the Debug toolbar to step into a program.

Single stepping executes only one statement of the program (from the place where it has stopped) and then returns control. If the statement that is executed contains one or more functions, the debugger tries to step inside the functions (in the sequence in which the functions are executed). Once the statement is executed and control is passed back, you can study your program's state.

If the Disassembly window has the focus when this command is selected, it executes only one assembler instruction.

### Single stepping (step over)

Choose the menu item Debug Step Over or click on the Step Over icon in the Debug toolbar to step over statements in a program.

Step over is similar to step in, except that it does not step inside any function in the statement being executed. The statement will be executed in one go.

If the Disassembly window has the focus when this command is selected, it executes one assembler instruction but does not stop inside if this instruction is a call to a subroutine.

If a dynamic library is loaded during the step, the program will not stop at the end of the step. But it will run until it finds a breakpoint or you stop it.

### Single stepping (step out)

Choose the menu item Debug Step Out or click on the Step Out icon in the Debug toolbar to step out in a program.

Step out will execute the current function until it returns. The program will be stopped once it exits from the function. Step out is not really single stepping, because it does not only execute a single statement — it executes the whole function until that function returns to the calling function.

### Run/Continue

Choose the menu item Debug Run/Continue or click on the Run/Continue icon in the Debug toolbar to continue the execution of a program.

This option continues the execution of the program until a breakpoint is encountered, or the program exits.

### Run To

Choose the menu item Debug Run to Cursor or click on the Run to Cursor icon in the Debug toolbar to run until the line at the cursor is reached.

This option continues the execution of the program until the line or the address (if the Disassembly window has the focus) at the cursor is reached.

### Stop Program

While the program is running and has control, no debugging tasks can be performed. To obtain control while the program is running, choose the menu item Debug Pause Program or click on the Pause Program icon in the Debug toolbar. This will interrupt the program and return control to the debugger.

## Breakpoints

When debugging a program, it is useful to be able to stop the execution of the program at a particular point, so that the state of the program can be examined at that location. Breakpoints enable this to happen. Breakpoints can be set at different locations in a source file and then the program is allowed to run. When a breakpoint is encountered, the execution of the program is suspended, enabling expressions to be evaluated, variables to be inspected, the stack trace to be studied, and so on.

### Listing Breakpoints

The Breakpoint list window can be opened by choosing View Breakpoints

#### Breakpoint view

Breakpoint view

The view has the following columns:

Location

  This is the position of the breakpoint in the source file. When the breakpoint is set by the debugger, you can get additional information such as the function containing the breakpoint.

Address

  This field is filled with the address of the breakpoint when it is set by the debugger.

Type

  This always displays "breakpoint". It will be used in a future release supporting different kinds of breakpoints.

Condition

  This is the breakpoint condition. It is empty if no condition is defined.

Pass count

  When the debugger is not running, this column displays the pass count of the breakpoint, or zero if it is not defined.

  When the debugger is running, if the pass count is zero, this column displays the number of times this breakpoint has been triggered. If the pass count is not zero, both numbers are displayed. The pass count is the second number.

State

  This is the state of the breakpoint. It may be

  - Pending: the breakpoint is not set in your program.

  - Permanent: the breakpoint is set (its id number in given inside parentheses).

  - Temporary: the breakpoint is set but will be deleted if reached.

All breakpoints are kept across Anjuta sessions even if they correspond to a non-existing location. In this case their states are set as pending. The interface tries to set them each time the program is started or a new dynamic library is loaded.

### Adding or Setting Breakpoints

Choose the menu item Debug Add Breakpoint… or Add Breakpoint… in the breakpoint list popup menu to open the add breakpoint dialog.

#### Breakpoint add dialog

Breakpoint add dialog

Enter the location and, optionally, the break condition and the pass count; then click OK to set the breakpoint.

Location

  Enter the location at which to set the breakpoint. It can be specified in any of the following formats:

  1. File_name:Line_number

  2. Function_name

  3. File:Function_name

  The first format is obvious — the location refers to the line number Line_number in the source file File. The second format refers to the first line of the function Function_name. The third is similar to the second, except that this notation is used where there is more than one function with the name Function_name in the program. It is possible to differentiate between them by providing the File, so the notation refers to the function Function_name in the file File.

Condition

  The Condition parameter displays a associated expression which should evaluate to a Boolean value — that is, the evaluation of the expression should result in either TRUE(1) or FALSE(0). If the final evaluation value is not a Boolean value, then it will be appropriately type cast to a Boolean.

  Every time the breakpoint is encountered during your program's execution, the break condition will be evaluated. The debugger will break the execution only if the evaluation results in a TRUE value, otherwise it will continue the execution as though there had been no breakpoint.

  The default value, if you leave this field blank, of Condition is TRUE. The debugger will always break the execution at the breakpoint location.

Pass Count

  The Pass count is an integer (unsigned) value which tells the debugger to skip the breakpoint that number of times before it is considered. Pass count has a higher priority than the Break condition. Only when the Pass count reaches zero will the debugger evaluate the Break condition (if any condition is present). If there is no condition, the debugger will break the execution once the Pass count counts down to zero.

  The default value, if you leave this field blank, of the Pass count is zero. The breakpoint will be considered when it is first encountered.

A breakpoint may also be set by selecting a line in the editor and choosing the menu item Debug Toggle Breakpoint in the editor context menu or the Toggle Breakpoint icon in the Debug toolbar.

Breakpoints can be added even if the debugger is not started, or in a dynamic library which is not loaded yet. But they cannot be added while a program is running under control of the debugger.

### Editing Breakpoints

You can change the condition and the pass count by selecting the breakpoint in the breakpoint list and clicking Edit Breakpoint in the popup menu.

Edit the entries as required and click on OK to commit the changes.

### Deleting Breakpoints

Select the breakpoint in the breakpoint list view and click on Remove Breakpoint to delete it.

A existing breakpoint may also be deleted by selecting its line in the editor and choosing the menu item Debug Toggle Breakpoint, the editor context menu item Toggle Breakpoint or the Toggle Breakpoint icon in the Debug toolbar.

You can remove all breakpoints by clicking on Debug Remove All Breakpoints or on Remove All Breakpoints in the breakpoint list context menu.

### Enabling or Disabling Breakpoints

Click on the Enable column of the Breakpoint list window or in the context menu item Enable Breakpoint to enable or disable the selected breakpoint. The breakpoint will be enabled or disabled, depending on its current state.

To disable all breakpoints, click on Debug Disable All Breakpoints or on Disable All Breakpoints in the breakpoint list context menu.

## Expressions

### Listing local variables

When a program is running under the control of the debugger, the Locals list window can be opened by choosing View Locals.

Local variables of the current frame (the current function in which the program control is present) are displayed in the Locals window. During execution of the program (e.g. during single stepping), local variables will be updated. If any variable value was changed in the last debugging step, its value will be highlighted in red. Variables are presented in a tree form for easy view.

#### Local variables

Local variables

The value of a local variable can be modified by selecting it and clicking in the Value column.

gdb is more and more often used as a back end for a graphical front end. It has been improved recently in this area. It is recommended that you use the latest version (6.6) of gdb. With older versions, gdb can crash when the front end asks for an pointer with an invalid (but unused) value.

### Listing watched expressions

Inspecting or evaluating an expression provides the result only once. To continuously monitor some variables or expressions, use expression watches.

Choose the menu item View Watches to display the Watches window.

### Adding an expression to watch

Right-click on the Watches window to open the context menu. Choose the menu item Add Watch…. Alternatively, you can use Debug Add Watch… A dialog prompting for the expression will appear. Enter the expression and click OK.

By default watch expressions are updated automatically each time the program is stopped. This can be changed in the add watch dialog or later using the Automatic update menu item in the Watches window context menu. A watched expression can be updated manually by choosing Update Watch or Update All in the context menu.

It is not necessary to have the debugger running to add a new watch expression. If the debugger is not running or the corresponding expression cannot be found, the front end will try to create the watch expression each time the program is stopped.

### Removing a watched expression

Right-click the watch expression that you want to remove in the Watches window, then choose the context menu item Remove to remove it.

All watches can be removed by choosing Remove All in the context menu of the Watches window.

### Evaluating expressions

When control is returned from a program — possibly at a breakpoint — it is possible to evaluate expressions or inspect the values of variables in the program. Choose the menu item Debug Inspect/Evaluate …, Inspect/Evaluate … in the context menu of the editor or click the Inspect button on the Debug Toolbar.

A window will appear showing the evaluation of the expression highlighted in the editor. You can evaluate a new expression by clicking on the name column and entering the new one.

You can add the expression to the list of watched expressions directly by clicking on the Add button.

### Inspecting expressions in editor window

When the debugger is active with the program stopped, you can get the value of an expression just by putting the mouse pointer over it and staying there for a short time. If the expression is already present in the Locals or Watches window, its value is displayed in a tooltip.

This capability is currently working only for simple variables, not for structures or arrays.

## Stack Trace

### Stack Window

The stack trace window shows the contents of the program stack. It lists all functions and their arguments in the sequence they were called. There is also a number representing each call. This number is called the frame. Each call in the trace exists in a different frame. This starts from frame 0 (the last function called) and grows higher as function nesting becomes deeper.

Choose the menu item View Stack to open the stack trace for the program being debugged.

#### Stack trace window

Stack trace window

A small arrow points to the currently selected frame in the stack trace. By default, this will be frame 0, the last function called. All evaluation and inspection of expressions or variables will be with reference to this selected frame. The scope of variables or expressions being evaluated will be limited to the selected frame only. The same applies for new expressions only in the watch.

### Setting the current frame

Double-clicking on any frame in the stack trace sets that frame as the currently selected frame. The arrow will point to the frame, indicating that it has been selected as the current frame. Alternatively, open the context menu by right-clicking on the Stack trace window, and choose the menu item Set current frame to set the frame.

Changing the stack frame changes the Locals window content, but not the Watches window as each expression is evaluated in the frame used when it was defined.

## Thread

### Thread Window

The Thread window shows all threads used by the program and indicates the current thread. Choose the menu item View Thread to open this window.

A small arrow points to the current thread. When the program is stopped, the current thread is the thread which has been interrupted. Each thread has its own stack frame, so changing the current thread changes the current stack frame.

### Setting the current thread

Double-clicking on any thread in the thread list will set that thread as the currently selected thread. The arrow will point to the thread, indicating that it has been selected as the current thread. Alternatively, open the context menu by right-clicking on the Thread list window, and choose the menu item Set current thread to set the thread.

Changing the thread changes the value of CPU registers and the current stack frame, so the Locals window changes too.

## CPU

### Register window

It is possible to examine the contents of the internal registers of the CPU (microprocessor). Choose the menu item View Registers. A window listing all available registers in the microprocessor and their corresponding contents will appear.

If any register value was changed in the last debugging step, its value will be highlighted in red. It is possible to change one register value by selecting it and clicking in the value column.

### Memory window

Choose the menu item View Memory to open the memory window for the program being debugged. This window shows the contents of all memory.

The first column displays memory addresses in hexadecimal, the second displays memory contents in hexadecimal and the last column shows memory contents in ASCII.

The addressing space of even a 32-bit microprocessor is quite large (4 gigabytes), so it is very difficult to go to a particular address with the scrollbar. But you can click on the right mouse button to display a context menu and select Goto address to get a small edit box where you can enter an address in hexadecimal.

### Disassembly window

Choose the menu item View Disassembly to open the disassembly window for the program being debugged.

The first column displays addresses in hexadecimal. In the second column, you can have a label starting at the beginning of the line and ending with a colon or a assembly-language instruction indented by 4 space characters.

Again, as the addressing space is very large, the scrollbar is quite useless. You can click on the right mouse button to display the context menu and select Goto address to get a small edit box where you can enter an address in hexadecimal. The position in the disassembly window will be changed to the program counter value when the program is stopped.

## Others

There are a number of other debugger features used less frequently and not very well integrated in the new front end but which are still working.

### Dynamically loaded Libraries

To obtain a list of the dynamic libraries used by a program, choose the menu item Debug Info Shared Libraries. This will bring open a window which will list all shared libraries the program has loaded and their locations in memory. It also shows whether each library's symbol table is loaded or not (Yes/No).

#### Shared Libraries window

Shared Libraries window

### Kernel Signals

Kernel signals are a way of signaling between processes in Linux. The list of signals available for a program can be displayed by choosing the menu item Debug Info Kernel Signals. A window will open which lists all signals available in the system along with a brief description of each signal.

#### Kernel Signals window

Kernel Signals window

There are three columns which specify what to do when a signal is received:

1. Stop — this tells the debugger whether to stop the program execution (and return control) when the program receives this signal.

2. Print — this tells the debugger whether to display the received signal.

3. Pass — this tells the debugger whether to pass the signal to the program.

The context menu that is displayed when you click on the right mouse button has all its items disabled because the corresponding functions are not implemented yet.

### Information about used files

You can get some information about the files used by the debugged program by choosing the menu item Debug Information Target files. A window will open displaying this information.

### Information about debugged program

You can get some information about the debugged program by choosing the menu item Debug Information Program. A window will open displaying this information.

### Information about kernel structure

You can get some information about kernel data for the current process by choosing the menu item Debug Information Kernel user struct. A window will open displaying this information.

### Information about global variables

You can list all global variables by choosing the menu item Debug Information Info Global Variable. A window will open listing all global variables.

### Information about the current frame

You can obtain information about the currently selected frame by choosing the menu item Debug Information Info Current Frame. A window will open describing the current frame.

### Information about the current function arguments

You can obtain information about the arguments of the current function by choosing the menu item Debug Information Arguments. A window will open describing the arguments.

### User command

To send directly a command to the back end choose the menu item Debug Debugger command. This will bring a small dialog where you can enter commands that will be send to the debugger when you press Return.

The front end sends this command directly to the back end without performing any checks. By example if you set a breakpoint like this, it will not appear in the breakpoint list window. It is better to avoid using this command unless you know exactly what you are doing.