19. Controlling GDB

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

19.1 Prompt
19.2 Command editing
19.3 Command history
19.4 Screen size
19.5 Numbers
19.6 Configuring the current ABI
19.7 Optional warnings and messages
19.8 Optional messages about internal happenings

19.1 Prompt

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.

Note: set prompt does not add a space for you after the prompt you set. This allows you to set a prompt which ends in a space or a prompt that does not.

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'

19.2 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 GNU 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.

19.3 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' (`./_gdb_history' on MS-DOS) 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 GNU 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.

19.4 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 terminal driver software. For example, on Unix GDB uses 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.

19.5 Numbers

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 input-radix base

Set the default base for numeric input. 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.

set output-radix base

Set the default base for numeric display. Supported choices for base are decimal 8, 10, or 16. base must itself be specified either unambiguously or using the current default radix.

show input-radix

Display the current default base for numeric input.

show output-radix

Display the current default base for numeric display.

19.6 Configuring the current ABI

GDB can determine the ABI (Application Binary Interface) of your application automatically. However, sometimes you need to override its conclusions. Use these commands to manage GDB's view of the current ABI.

One GDB configuration can debug binaries for multiple operating system targets, either via remote debugging or native emulation. GDB will autodetect the OS ABI (Operating System ABI) in use, but you can override its conclusion using the set osabi command. One example where this is useful is in debugging of binaries which use an alternate C library (e.g. UCLIBC for GNU/Linux) which does not have the same identifying marks that the standard C library for your platform provides.

show osabi

Show the OS ABI currently in use.

set osabi

With no argument, show the list of registered available OS ABI's.

set osabi abi

Set the current OS ABI to abi.

Generally, the way that an argument of type float is passed to a function depends on whether the function is prototyped. For a prototyped (i.e. ANSI/ISO style) function, float arguments are passed unchanged, according to the architecture's convention for float. For unprototyped (i.e. K&R style) functions, float arguments are first promoted to type double and then passed.

Unfortunately, some forms of debug information do not reliably indicate whether a function is prototyped. If GDB calls a function that is not marked as prototyped, it consults set coerce-float-to-double.

set coerce-float-to-double

set coerce-float-to-double on

Arguments of type float will be promoted to double when passed to an unprototyped function. This is the default setting.

set coerce-float-to-double off

Arguments of type float will be passed directly to unprototyped functions.

GDB needs to know the ABI used for your program's C++ objects. The correct C++ ABI depends on which C++ compiler was used to build your application. GDB only fully supports programs with a single C++ ABI; if your program contains code using multiple C++ ABI's or if GDB can not identify your program's ABI correctly, you can tell GDB which ABI to use. Currently supported ABI's include "gnu-v2", for g++ versions before 3.0, "gnu-v3", for g++ versions 3.0 and later, and "hpaCC" for the HP ANSI C++ compiler. Other C++ compilers may use the "gnu-v2" or "gnu-v3" ABI's as well. The default setting is "auto".

show cp-abi

Show the C++ ABI currently in use.

set cp-abi

With no argument, show the list of supported C++ ABI's.

set cp-abi abi

set cp-abi auto

Set the current C++ ABI to abi, or return to automatic detection.

19.7 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. This 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 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.

19.8 Optional messages about internal happenings

set debug arch

Turns on or off display of gdbarch debugging info. The default is off

show debug arch

Displays the current state of displaying gdbarch debugging info.

set debug event

Turns on or off display of GDB event debugging info. The default is off.

show debug event

Displays the current state of displaying GDB event debugging info.

set debug expression

Turns on or off display of GDB expression debugging info. The default is off.

show debug expression

Displays the current state of displaying GDB expression debugging info.

set debug frame

Turns on or off display of GDB frame debugging info. The default is off.

show debug frame

Displays the current state of displaying GDB frame debugging info.

set debug overload

Turns on or off display of GDB C++ overload debugging info. This includes info such as ranking of functions, etc. The default is off.

show debug overload

Displays the current state of displaying GDB C++ overload debugging info.

set debug remote

Turns on or off display of reports on all packets sent back and forth across the serial line to the remote machine. The info is printed on the GDB standard output stream. The default is off.

show debug remote

Displays the state of display of remote packets.

set debug serial

Turns on or off display of GDB serial debugging info. The default is off.

show debug serial

Displays the current state of displaying GDB serial debugging info.

set debug target

Turns on or off display of GDB target debugging info. This info includes what is going on at the target level of GDB, as it happens. The default is off.

show debug target

Displays the current state of displaying GDB target debugging info.

set debug varobj

Turns on or off display of GDB variable object debugging info. The default is off.

show debug varobj

Displays the current state of displaying GDB variable object debugging info.