This chapter discusses how to start gdb, and how to get out of it.
The essentials are:
type gdb to start gdb.
type quit or C-d to exit.
4.1. Invoking gdb
Invoke gdb by running the program gdb. Once started,
gdb reads commands from the terminal until you tell it to exit.
You can also run gdb with a variety of arguments and options,
to specify more of your debugging environment at the outset.
The command-line options described here are designed
to cover a variety of situations; in some environments, some of these
options may effectively be unavailable.
The most usual way to start gdb is with one argument,
specifying an executable program:
gdb program
You can also start with both an executable program and a core file
specified:
gdb programcore
You can, instead, specify a process ID as a second argument, if you want
to debug a running process:
gdb program 1234
would attach gdb to process 1234 (unless you also have a file
named 1234; gdb does check for a core file first).
Taking advantage of the second command-line argument requires a fairly
complete operating system; when you use gdb as a remote
debugger attached to a bare board, there may not be any notion of
"process", and there is often no way to get a core dump. gdb
will warn you if it is unable to attach or to read core dumps.
You can optionally have gdb pass any arguments after the
executable file to the inferior using -args. This option stops
option processing.
You can run gdb without printing the front material, which describes
gdb's non-warranty, by specifying -silent:
gdb -silent
You can further control how gdb starts up by using command-line
options. gdb itself can remind you of the options available.
Type
gdb -help
to display all available options and briefly describe their use
(gdb -h is a shorter equivalent).
All options and command line arguments you give are processed
in sequential order. The order makes a difference when the
-x option is used.
4.1.1. Choosing files
When gdb starts, it reads any arguments other than options as
specifying an executable file and core file (or process ID). This is
the same as if the arguments were specified by the -se and
-c (or -p options respectively. (gdb reads the
first argument that does not have an associated option flag as
equivalent to the -se option followed by that argument; and the
second argument that does not have an associated option flag, if any, as
equivalent to the -c/-p option followed by that argument.)
If the second argument begins with a decimal digit, gdb will
first attempt to attach to it as a process, and if that fails, attempt
to open it as a corefile. If you have a corefile whose name begins with
a digit, you can prevent gdb from treating it as a pid by
prefixing it with ./, eg. ./12345.
If gdb has not been configured to included core file support,
such as for most embedded targets, then it will complain about a second
argument and ignore it.
Many options have both long and short forms; both are shown in the
following list. gdb also recognizes the long forms if you truncate
them, so long as enough of the option is present to be unambiguous.
(If you prefer, you can flag option arguments with - rather
than -, though we illustrate the more usual convention.)
-symbols file, -s file
Read symbol table from file file.
-exec file, -e file
Use file file as the executable file to execute when appropriate,
and for examining pure data in conjunction with a core dump.
-se file
Read symbol table from file file and use it as the executable
file.
-core file, -c file
Use file file as a core dump to examine.
-c number, -pid number, -p number
Connect to process ID number, as with the attach command.
If there is no such process, gdb will attempt to open a core
file named number.
Add directory to the path to search for source files.
-m, -mapped
Warning: this option depends on operating system facilities that are not
supported on all systems.
If memory-mapped files are available on your system through the mmap
system call, you can use this option
to have gdb write the symbols from your
program into a reusable file in the current directory. If the program you are debugging is
called /tmp/fred, the mapped symbol file is /tmp/fred.syms.
Future gdb debugging sessions notice the presence of this file,
and can quickly map in symbol information from it, rather than reading
the symbol table from the executable program.
The .syms file is specific to the host machine where gdb
is run. It holds an exact image of the internal gdb symbol
table. It cannot be shared across multiple host platforms.
-r, -readnow
Read each symbol file's entire symbol table immediately, rather than
the default, which is to read it incrementally as it is needed.
This makes startup slower, but makes future operations faster.
You typically combine the -mapped and -readnow options in
order to build a .syms file that contains complete symbol
information. (Refer to Section 17.1 Commands to specify files for information
on .syms files.) A simple gdb invocation to do nothing
but build a .syms file for future use is:
gdb -batch -nx -mapped -readnow programname
4.1.2. Choosing modes
You can run gdb in various alternative modes--for example, in
batch mode or quiet mode.
-nx, -n
Do not execute commands found in any initialization files. Normally,
gdb executes the commands in these files after all the command
options and arguments have been processed.
Refer to Section 22.3 Command files.
-quiet, -silent, -q
"Quiet". Do not print the introductory and copyright messages. These
messages are also suppressed in batch mode.
-batch
Run in batch mode. Exit with status 0 after processing all the
command files specified with -x (and all commands from
initialization files, if not inhibited with -n). Exit with
nonzero status if an error occurs in executing the gdb commands
in the command files.
Batch mode may be useful for running gdb as a filter, for
example to download and run a program on another computer; in order to
make this more useful, the message
Program exited normally.
(which is ordinarily issued whenever a program running under
gdb control terminates) is not issued when running in batch
mode.
-nowindows, -nw
"No windows". If gdb comes with a graphical user interface
(GUI) built in, then this option tells gdb to only use the command-line
interface. If no GUI is available, this option has no effect.
-windows, -w
If gdb includes a GUI, then this option requires it to be
used if possible.
-cd directory
Run gdb using directory as its working directory,
instead of the current directory.
-fullname, -f
gnu Emacs sets this option when it runs gdb as a
subprocess. It tells gdb to output the full file name and line
number in a standard, recognizable fashion each time a stack frame is
displayed (which includes each time your program stops). This
recognizable format looks like two \032 characters, followed by
the file name, line number and character position separated by colons,
and a newline. The Emacs-to-gdb interface program uses the two
\032 characters as a signal to display the source code for the
frame.
-epoch
The Epoch Emacs-gdb interface sets this option when it runs
gdb as a subprocess. It tells gdb to modify its print
routines so as to allow Epoch to display values of expressions in a
separate window.
-annotate level
This option sets the annotation level inside gdb. Its
effect is identical to using set annotate level
(refer to Chapter 27 gdb Annotations).
Annotation level controls how much information does gdb print
together with its prompt, values of expressions, source lines, and other
types of output. Level 0 is the normal, level 1 is for use when
gdb is run as a subprocess of gnu Emacs, level 2 is the
maximum annotation suitable for programs that control gdb.
-async
Use the asynchronous event loop for the command-line interface.
gdb processes all events, such as user keyboard input, via a
special event loop. This allows gdb to accept and process user
commands in parallel with the debugged process being
run[1], so you don't need to wait for
control to return to gdb before you type the next command.
(Note: as of version 5.1, the target side of the asynchronous
operation is not yet in place, so -async does not work fully
yet.)
When the standard input is connected to a terminal device, gdb
uses the asynchronous event loop by default, unless disabled by the
-noasync option.
-noasync
Disable the asynchronous event loop for the command-line interface.
-args
Change interpretation of command line so that arguments following the
executable file are passed as command line arguments to the inferior.
This option stops option processing.
-baud bps, -b bps
Set the line speed (baud rate or bits per second) of any serial
interface used by gdb for remote debugging.
-tty device, -t device
Run using device for your program's standard input and output.
-tui
Activate the Terminal User Interface when starting.
The Terminal User Interface manages several text windows on the terminal,
showing source, assembly, registers and gdb command outputs
(refer to Chapter 24 gdb Text User Interface).
Do not use this option if you run gdb from Emacs
(refer to Chapter 25 Using gdb under gnu Emacs).
-interpreter interp
Use the interpreter interp for interface with the controlling
program or device. This option is meant to be set by programs which
communicate with gdb using it as a back end
(refer to Chapter 23 Command Interpreters.
-interpreter=mi (or -interpreter=mi2) causes
gdb to use the current gdb/mi interface
(refer to Chapter 26 The gdb/mi Interface). The previous gdb/mi
interface, included in gdb version 5.3, can be selected with
-interpreter=mi1. Earlier gdb/mi interfaces
are not supported.
-write
Open the executable and core files for both reading and writing. This
is equivalent to the set write on command inside gdb
(refer to Section 16.6 Patching programs).
-statistics
This option causes gdb to print statistics about time and
memory usage after it completes each command and returns to the prompt.
-version
This option causes gdb to print its version number and
no-warranty blurb, and exit.
