This manual is for GNU CGDB (version 0.6.6, 6 September 2011), the GNU ncurses based front end to GDB.
Copyright © 2011 CGDB Team
Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.1 or any later version published by the Free Software Foundation; with no Invariant Sections, with the Front-Cover Texts being “A GNU Manual,” and with the Back-Cover Texts as in (a) below. A copy of the license is included in the section entitled “GNU Free Documentation License.”
(a) The FSF's Back-Cover Text is: “You have freedom to copy and modify this GNU Manual, like GNU software. Copies published by the Free Software Foundation raise funds for GNU development.”
CGDB is a curses-based interface to the GNU Debugger (GDB). The goal of CGDB is to be lightweight and responsive; not encumbered with unnecessary features.
The interface is designed to deliver the familiar GDB text interface, with a split screen showing the source as it executes. The UI is modeled on the classic Unix text editor, vi. Those familiar with vi should feel right at home using CGDB.
The library responsible for communicating with GDB is called Trivial GDB (tgdb, or more accurately, libtgdb). This abstraction allows the UI code to be independent of the debugger, as well as greatly simplifying its implementation.
Those wanting to develop other interfaces to GDB are welcome to use libtgdb as the basis for their program. Many of the headaches of parsing GDB's output and annotations can be avoided by using it.
Some features offered by CGDB are:
This chapter discusses how to start CGDB, and how to get out of it. The essentials are:
The CGDB user interface currently consists of two windows and a status bar. The source window is currently on the top and the GDB window is on the bottom. The status bar currently separates the two windows.
The interface has several modes depending on which window is focused. CGDB mode is when the source window is focused, GDB mode is when the GDB window is focused and TTY mode is when the TTY window is focused.
Beginning with CGDB version 1.0, the windows are movable, and the user will be able to create as many or as few that is desired. Currently however, all of my time is spent developing the interface between CGDB and GDB. Once this is complete, the UI of CGDB will become much more polished. If you are a ncurses developer, and have spare time to work on this task, please contact me.
The source window is the window that provides you a view of the source
code that the debugged program is made up of. It will display to the user a
single source file at a time. While the user is debugging, via
step, CGDB will update the source file and line number to keep you
informed as to where GDB is debugging.
CGDB has several features that make debugging easier than using plain old GDB. One feature you will notice right away while debugging a C, C++ or ADA program, is that the source files are syntax highlighted. This allows the user to easily navigate through the source file to look for certain places in the source code. If you would like to see another source language highlighted, contact us. To understanding how to navigate through the source window look at the commands in CGDB Mode.
In addition to showing the source code, CGDB also displays to the user the
currently executing line. The line number will be highlighted green, to
represent that the particular line, is the current line being debugged by GDB.
Also, CGDB will display an arrow extending from the line number, to the
source line. You can configure what type of arrow CGDB uses with the
:set arrowstyle configuration option. By default, the
arrow is used. However, my personal favorite is the
As you navigate through the source window, the current line the cursor is on will be highlighted white. This simply helps you keep track of where you are in the file.
Also, you can set or delete breakpoints in CGDB from the source window. Simply navigate to the line that you are interested in setting a breakpoint, and hit the space bar. This will set a breakpoint on the line if one did not already exist. The line number should turn red to indicate that a breakpoint has been set. Hitting the space bar again will delete the breakpoint. If you disable the breakpoint, the line number will turn yellow, to represent the disabled breakpoint.
CGDB also supports regular expression searching within the source window. If you type / or ? you can search in the source window for a string of interest. The C library regular expression functions are used to perform this search, which honors things like ‘*’ or ‘+’.
The full list of commands that are available in the source window is in CGDB Mode.
The GDB window is how CGDB allows the user to interface with the GNU debugger. If you wish to pass a command to GDB, simply type it into this window and GDB will receive the command. This interface is intended to be 100% identical to using GDB on a terminal.
There is a limited set of keys that can be typed into this window that CGDB interprets and handles, instead of sending to GDB. They are all available in GDB Mode.
CGDB attempts to buffer commands the same way they would be if you typed them at the terminal. So, if you type several commands before a single one finishes, they will each be run in order. There will be no way to stop these commands from being run besides from typing Ctrl-C, like you would at any normal terminal when working with GDB.
The file dialog window is available to help the user view and select which file they would like to view. It provides the user with a list of all the files that make up the program being debugged. If there are no files available, because there is no program being debugged or because there is no debug symbols, then the file dialog will not open and a message will be displayed at the status bar.
You can get to the file dialog by hitting
o when you are at the source
window. Once you enter the file dialog, it is possible to leave it by hitting
q. You can navigate the file dialog using the standard direction keys
and you can even use regular expression to find your file. This can save a
lot of time as the number of files grow.
The full list of commands that are available in the source window is in File Dialog Mode.
The TTY window is available to allow the user to pass input to the program being debugged. This window will act similar to the GDB window, except that the data you type will get sent to the program being debugged. See Sending I/O to Inferior.
You will notice that the TTY window has a terminal device between it and the program being debugged. So, if the program being debugged uses say readline, which allows command line editing, the same interface will be provided via the TTY window as at the terminal. You can see the name of the terminal device in the TTY status bar.
The full list of commands that are available in the source window is in TTY Mode.
Sending I/O to the program being debugged can be confusing. It is described better in Sending I/O to Inferior. Unless the I/O with the program being debugged is simple, I usually prefer starting the application on a separate terminal and attaching to it with CGDB.
The status bar is the general purpose way for CGDB to show the user which commands they are currently typing or report errors to the user when they occur. CGDB does not use popup's or other forms of I/O to alert the user of information or problems.
While CGDB is running, you can configure it with any of the commands that
are valid in CGDB's configuration file. Simply type : in the source
window, and you will see the colon, and the rest of the command you type
appear in the status bar. When you are finished typing the command that
you are interested in, type
enter. This will alert CGDB to execute
the command. If at any point you would like to cancel the current
command typed so far, type the cgdb mode key. This will put you back
into CGDB mode. For a description of the cgdb mode key, see
The full list of commands that are available in the source window is in Configuring CGDB.
When CGDB is invoked, the interface is in GDB mode. A ‘*’ at the right of the status bar indicates that input will be passed to GDB. To change the focus to the source window, hit the <ESC> key. The cgdb mode key is the key that is responsible for switching the user into CGDB mode from a different mode. The cgdb mode key is defaulted to the <ESC> key. To change this value, look at the configuration options for CGDB. See Configuring CGDB.
The interface is now in CGDB mode. To switch back into GDB mode, press i. This syntax is based on the popular Unix text-editor, vi.
CGDB can be controlled in a variety of different ways. Each mode that CGDB is in acts differently. Currently CGDB implicitly changes modes depending on which window is active. The following information will help you determine what commands are accessible during which modes.
When you are in the source window, you are implicitly in CGDB mode. All of the below commands are available during this mode. This mode is primarily available for the user to view the current source file, search it, or switch to a different mode.
When in GDB mode, the user is mostly interested in working with the GDB console. That is, sending commands to GDB and receiving data back from GDB. Almost all data passed into this window is directly sent to readline and then to GDB.
It is important to understand that CGDB parses the keys entered in the GDB window and has the first chance at dealing with them. If it is interested in the keys, it will handle them. Below is a list of keys that CGDB is interested in, and does not pass along any further.
Any other keys, besides the ones above, CGDB is currently not interested in. CGDB will pass along these keys to the readline library. When readline has determined that a command has been received, it alerts CGDB, and a command is then sent to GDB. This is the same method used when invoking GDB directly.
The file dialog is primarily used to allow the user to find and open a source
file that the program they are debugging is made up of. The file dialog will
be full screen, and will list each file that the debugged program is made up
of. A usual instance of the file dialog would be to open it up from the
source window using the o key, and then to search for the file of
interest. If you are looking for foo.c type
/foo.c, press enter
once to finish the regular expression and again to select the file.
The commands available in the file dialog are:
There may be several features that you find useful in CGDB. CGDB is capable of automating any of these commands through the use of the config file called cgdbrc. It looks in $HOME/.cgdb/ for that file. If it exists, CGDB executes each line in the file in order. It is as if the user typed in all the commands into the status bar after the tui was initialized.
The following variables change the behavior of some aspect of CGDB. Many of these commands may be abbreviated in some way, and all boolean commands my be negated by appending ‘no’ to the front. For example: :set ignorecase turns on case-insensitive searching; while :set noignorecase turns on case-sensitive searching.
set cgdbmodekey=<PageUp>then the Page Up key will put CGDB into CGDB mode and the ESC key will flow through to readline.
You can give as many or as few of the name=value pairs as you wish, in any order. ‘ctermfg’ and ‘ctermbg’ set the foreground and background colors. These can be specified by color number or by using the same color names that vim uses. When CGDB is linked with ncurses, the number you use to represent the color can be between -1 and COLORS. When CGDB is linked against curses, it must be between 0 and COLORS.
‘cterm’ sets the video attributes for color terminals. ‘term’ sets the video attributes for monochrome terminals. Some examples are,
:highlight Logo cterm=bold,underline ctermfg=Red ctermbg=Black :highlight Normal cterm=reverse ctermfg=White ctermbg=Black :hi Normal term=bold
:map a<Space>b foothen the user could delete the existing mapping with
:imap a<Space>b foothen the user could delete the existing mapping with
CGDB is capable of using colors if the terminal it is run in supports them. Until version 0.6.1, CGDB did not allow the user to configure these colors in any way. CGDB color use is now fully configurable.
CGDB's modeled its use of color highlighting after vim. Any data that will be colored in the terminal is represented by a highlighting group. A highlighting group represents data that should be formatted using foreground colors, background colors and attributes. There are currently several types of highlighting groups in CGDB. There are syntax highlighting groups, which represent syntax highlighting of sources files. There are also User Interface groups, which represent things like CGDB's logo, or the status bar.
Each highlighting group has a default set of attributes and colors associated with it. You can modify a highlighting groups properties by using the highlight command. See Configuring CGDB.
Note that CGDB currently supports using the same background color the terminal was using before CGDB was started. However, this only works when CGDB was linked with ncurses. If you link CGDB with curses, then CGDB will force the background to Black.
Below is a list of all the highlighting groups that CDGB will use when syntax highlighting source files.
Below is a list of all the highlighting groups that CGDB will use when it is displaying it's User Interface.
arrowstyleoption set to
CGDB supports many of the attributes that curses provides. It will apply the attributes to the output window, but it is up to the terminal you are using to support such features.
The list of attributes that CGDB currently supports is below.
CGDB supports several colors, depending on how many colors your terminal supports. Below is a chart of the colors that CGDB provides. The heading NR-16 is used to represent terminals that support at least 16 colors. The heading NR-8 is used to represent terminals that support at least 8 colors. The integer values for each color represent the values passed to the curses function init_pair() to ask curses to create a new color.
|LightGray, LightGrey, Gray, Grey||7||7||No
The Key User Interface is how CGDB receives input from the user. It is usually referred to as the KUI. CGDB simply asks the KUI for the next key the user typed and the KUI will provide it.
The KUI has 2 major responsibilities besides reading normal user input and providing it to CGDB. It needs to detect when the user has typed a user defined map or when the user has hit a special key on the keyboard.
A user defined map, or simply map, is used to change the meaning
of typed keys. Some users may refer to this type of functionality as a
macro. An example would be
map a b. If the user then typed
the <a> character, the KUI would detect that it was mapped to <b>
and return <b> to CGDB.
When the user types a special key on the keyboard, a key code is sent to CGDB. Typically, keys like HOME, DEL, <F1>, etc, when pressed will send several characters to the application instead of just one character like a normal key does. These characters combined are called a key sequence. The KUI is responsible for assembling the key sequences back together and reporting to CGDB that a particular key was typed by the user. The ESC key is special because typically most key codes start with that key. This usually gives all key codes a common first key in its key sequence. The KUI uses the terminfo database to determine what key sequences are sent by which keycodes. There are a few commonly used key sequences that are hard coded into CGDB.
A major challenge the KUI has to overcome is determining when a map or a
key sequence is received. The KUI sometimes will need to read more than
one character to determine this. For example, if the user has 2 maps,
map abc def and
map abd def, the KUI would have to buffer at
least the characters <a> and <b> before it could determine if the
user was going to type a map. After the next key press, if the user types
<c> or <d> then a map was received and the KUI will return
d e f to CGDB. Otherwise, no map was received and the KUI must return
a b to CGDB.
The options timeout, ttimeout, timeoutlen and ttimeoutlen can be used to tell the KUI if it should timeout on partial mappings or key sequences, and if so, how long it should wait before timing out.
The KUI may be configured to time out on either maps or key sequences.
When the KUI is matching a partial map or key sequence it is capable of timing out. This means it will simply accepts the keys it has received so far if a certain amount of time elapses between key presses. This is obvious when the user is typing a map because the user must press each key individually. For partial key sequences, this is less obvious. That is because the user only presses a single key, but multiple characters are sent to CGDB. The table below describes how the user can configure the KUI to time out on key codes or maps. The timeout and ttimeout options control this functionality.
|off||off||do not time out
|on||on or off||time out on maps and key codes
|off||on||time out on key codes
It is also possible to tell the KUI how long to wait before timing out on a partial match. If timeout is on, then the KUI will wait a certain amount of time for the next character, when matching a map, before it decides a match is no longer possible. If timeout or ttimeout is on, then the KUI will wait a certain amount of time for the next character, when matching a key sequence, before it decides a match is no longer possible. The timeoutlen and ttimeoutlen options can be configured by the user to tell the KUI how long to wait before timing out. The table below describes when the KUI uses which option.
|timeoutlen||mapping delay||key code delay
A value of 0 means that the KUI will time out right away. It will not be possible to match a map or key code in this circumstance.
A common problem could be that when the user types a special key like
the left or right arrows, CGDB will go into the source mode and not
perform the action requested by the user. This typically means that the
key code delay is to small. If you try setting the option
set ttimeoutlen=1000 CGDB should start acting like the user expects.
If not, please report this to the CGDB mailing list.
CGDB fully supports the use of maps. It allows the user to change the
meaning of typed keys. For example, you could have the following map
:map <F2> ip<Space>argc<CR>.
When the user is in CGDB mode and they hit <F2>, the value of the map will be used instead. The i key will first be received by CGDB, and it will put the user into insert mode. Next, CGDB will get p argc followed by the <Enter> key.
CGDB currently supports two mapping lists. Any mapping that was added with the map command will be used by CGDB when it is in CGDB mode. You can delete a mapping that you have created with the map command with the unmap command. If you want to have mappings in GDB mode, you can use the imap command. Similarly, iunmap will delete a mapping in the imap set. Some examples of this would be
map a<Space>b foo unmap a<Space>b imap a<CR>b foo iunmap a<CR>b
The above example could use a little more explaining for people unfamiliar with vim maps. The map takes a key and a value. They are separated by a space. Neither the key or value can have a space in them, or it is considered to be the separator between the key and value. If the user desires to have a space in either the key or value part of a map, they can use the keycode notation <Space>. Below is a table of the keycodes in keycode notation form. The keycode notation can be used in any mapping command.
|<Up>||cursor up key
|<Down>||cursor down key
|<Left>||cursor left key
|<Right>||cursor right key
|<PageUp>||page up key
|<PageDown>||page down key
|<F1> - <F12>||function keys 1 to 12
If the program being debugged takes input on the terminal it is recommended that the user start the program on one terminal, and attach to it with CGDB from another terminal. This is the easiest way to pass input to the debugged program.
However, if the user wishes to pass input to the program being debugged from within CGDB, there is a mechanism available for doing so. As of this writing, the technique described below does not work on windows, using a natively compiled GDB. It may work when using the GDB that comes with Cygwin.
This technique is similar to getting in and out of GDB mode. The tty window is not visible by default. This is because it is only needed if the user wishes to send data to the program being debugged. To display the tty window, hit T while in command mode. After hitting T you will notice that there is another window in the middle of the source window and the gdb window. This is called the tty window. You will also see a new status bar called the tty status bar. There will be a ‘*’ on the tty status bar after the T was hit. This is because when the window is opened with the T command, CGDB automatically puts the user into TTY mode. To get out of this window hit the cgdb mode key. This will put you back into command mode. To make the tty window appear and disappear hit the T key while in command mode. It is a toggle.
Once the tty window is already open, the user can then hit I in command mode to get into TTY mode. The user can then hit the cgdb mode key in the TTY mode to get back into command mode.
When the tty window is open, all data that comes from the program, goes there. Any data typed into the tty window will ONLY go to the program being debugged. It will not go to GDB. When the tty window is closed, all output from the debugged program will go to the GDB window AND to the tty window (for viewing later when the tty window is opened).
If the user wishes to get a new tty for the program being debugged then they can type Ctrl-T. This will delete all the buffered data waiting to be read into the debugged program. This might be useful when you rerun or start a new program.
A user can typically set there control flow behavior by using the stty
command like so
stty -ixon -ixoff. This will disable control flow on
the terminal where CGDB is started. If you want to turn control flow back
on you can type
stty ixon ixoff. If flow control is on, when the user
types Ctrl-s, the terminal stops. When the user types Ctrl-q, the
terminal restarts. When using readline, the Ctrl-s character usually does
a forward search. So, if you want to get this, or other functionality out of
readline, simply turn off control flow and start CGDB.
Building CGDB from source requires several packages. First, CGDB is hosted at http://sf.net/projects/cgdb. You can determine how to get CGDB from source by looking here: http://sourceforge.net/svn/?group_id=72581.
Once you have the source to CGDB, now you can begin to build it. You will of course need many packages to build CGDB. Below is a list of all of them that are required to build CGDB.
Below is a list of optional packages you will need, if modifying certain files in CGDB.
.l, you will have to have flex installed. I have used flex 2.5.4 to build CGDB.
make cgdb.1, and the CGDB man page will be generated.
CGDB uses autoconf/automake to build its configure scripts and makefiles. So, if you change any of the autoconf/automake files, you will need this software installed.
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To use this License in a document you have written, include a copy of the License in the document and put the following copyright and license notices just after the title page:
Copyright (C) year your name. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license is included in the section entitled ``GNU Free Documentation License''.
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