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Controlling GDB

You can alter the way GDB interacts with you by using the set command. For commands controlling how GDB displays data, see section Print settings; other settings are described here.


GDB indicates its readiness to read a command by printing a string called the prompt. This string is normally `(gdb)'. You can change the prompt string with the set prompt command. For instance, when debugging GDB with GDB, it is useful to change the prompt in one of the GDB sessions so that you can always tell which one you are talking to.

set prompt newprompt
Directs GDB to use newprompt as its prompt string henceforth.
show prompt
Prints a line of the form: `Gdb's prompt is: your-prompt'

Command editing

GDB reads its input commands via the readline interface. This GNU library provides consistent behavior for programs which provide a command line interface to the user. Advantages are emacs-style or vi-style inline editing of commands, csh-like history substitution, and a storage and recall of command history across debugging sessions.

You may control the behavior of command line editing in GDB with the command set.

set editing
set editing on
Enable command line editing (enabled by default).

set editing off
Disable command line editing.

show editing
Show whether command line editing is enabled.

Command history

GDB can keep track of the commands you type during your debugging sessions, so that you can be certain of precisely what happened. Use these commands to manage the GDB command history facility.

set history filename fname
Set the name of the GDB command history file to fname. This is the file where GDB reads an initial command history list, and where it writes the command history from this session when it exits. You can access this list through history expansion or through the history command editing characters listed below. This file defaults to the value of the environment variable GDBHISTFILE, or to `./.gdb_history' if this variable is not set.

set history save
set history save on
Record command history in a file, whose name may be specified with the set history filename command. By default, this option is disabled.

set history save off
Stop recording command history in a file.

set history size size
Set the number of commands which GDB keeps in its history list. This defaults to the value of the environment variable HISTSIZE, or to 256 if this variable is not set.

History expansion assigns special meaning to the character !.

Since ! is also the logical not operator in C, history expansion is off by default. If you decide to enable history expansion with the set history expansion on command, you may sometimes need to follow ! (when it is used as logical not, in an expression) with a space or a tab to prevent it from being expanded. The readline history facilities do not attempt substitution on the strings != and !(, even when history expansion is enabled.

The commands to control history expansion are:

set history expansion on
set history expansion
Enable history expansion. History expansion is off by default.

set history expansion off
Disable history expansion.

The readline code comes with more complete documentation of editing and history expansion features. Users unfamiliar with emacs or vi may wish to read it.

show history
show history filename
show history save
show history size
show history expansion
These commands display the state of the GDB history parameters. show history by itself displays all four states.

show commands
Display the last ten commands in the command history.

show commands n
Print ten commands centered on command number n.

show commands +
Print ten commands just after the commands last printed.

Screen size

Certain commands to GDB may produce large amounts of information output to the screen. To help you read all of it, GDB pauses and asks you for input at the end of each page of output. Type RET when you want to continue the output, or q to discard the remaining output. Also, the screen width setting determines when to wrap lines of output. Depending on what is being printed, GDB tries to break the line at a readable place, rather than simply letting it overflow onto the following line.

Normally GDB knows the size of the screen from the termcap data base together with the value of the TERM environment variable and the stty rows and stty cols settings. If this is not correct, you can override it with the set height and set width commands:

set height lpp
show height
set width cpl
show width
These set commands specify a screen height of lpp lines and a screen width of cpl characters. The associated show commands display the current settings.

If you specify a height of zero lines, GDB does not pause during output no matter how long the output is. This is useful if output is to a file or to an editor buffer.

Likewise, you can specify `set width 0' to prevent GDB from wrapping its output.


You can always enter numbers in octal, decimal, or hexadecimal in GDB by the usual conventions: octal numbers begin with `0', decimal numbers end with `.', and hexadecimal numbers begin with `0x'. Numbers that begin with none of these are, by default, entered in base 10; likewise, the default display for numbers--when no particular format is specified--is base 10. You can change the default base for both input and output with the set radix command.

set radix base
Set the default base for numeric input and display. Supported choices for base are decimal 8, 10, or 16. base must itself be specified either unambiguously or using the current default radix; for example, any of

set radix 012
set radix 10.
set radix 0xa

sets the base to decimal. On the other hand, `set radix 10' leaves the radix unchanged no matter what it was.

show radix
Display the current default base for numeric input and display.

Optional warnings and messages

By default, GDB is silent about its inner workings. If you are running on a slow machine, you may want to use the set verbose command. It makes GDB tell you when it does a lengthy internal operation, so you will not think it has crashed.

Currently, the messages controlled by set verbose are those which announce that the symbol table for a source file is being read; see symbol-file in section Commands to specify files.

set verbose on
Enables GDB output of certain informational messages.

set verbose off
Disables GDB output of certain informational messages.

show verbose
Displays whether set verbose is on or off.

By default, if GDB encounters bugs in the symbol table of an object file, it is silent; but if you are debugging a compiler, you may find this information useful (see section Errors reading symbol files).

set complaints limit
Permits GDB to output limit complaints about each type of unusual symbols before becoming silent about the problem. Set limit to zero to suppress all complaints; set it to a large number to prevent complaints from being suppressed.

show complaints
Displays how many symbol complaints GDB is permitted to produce.

By default, GDB is cautious, and asks what sometimes seems to be a lot of stupid questions to confirm certain commands. For example, if you try to run a program which is already running:

(gdb) run
The program being debugged has been started already.
Start it from the beginning? (y or n)

If you are willing to unflinchingly face the consequences of your own commands, you can disable this "feature":

set confirm off
Disables confirmation requests.

set confirm on
Enables confirmation requests (the default).

show confirm
Displays state of confirmation requests.

Some systems allow individual object files that make up your program to be replaced without stopping and restarting your program. For example, in VxWorks you can simply recompile a defective object file and keep on running. If you are running on one of these systems, you can allow GDB to reload the symbols for automatically relinked modules:

set symbol-reloading on
Replace symbol definitions for the corresponding source file when an object file with a particular name is seen again.

set symbol-reloading off
Do not replace symbol definitions when re-encountering object files of the same name. This is the default state; if you are not running on a system that permits automatically relinking modules, you should leave symbol-reloading off, since otherwise GDB may discard symbols when linking large programs, that may contain several modules (from different directories or libraries) with the same name.

show symbol-reloading
Show the current on or off setting.

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