gdb: Emacs
26 Using GDB under GNU Emacs
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A special interface allows you to use GNU Emacs to view (and edit) the
source files for the program you are debugging with GDB.
To use this interface, use the command 'M-x gdb' in Emacs. Give the
executable file you want to debug as an argument. This command starts
GDB as a subprocess of Emacs, with input and output through a newly
created Emacs buffer.
Running GDB under Emacs can be just like running GDB normally except
for two things:
* All "terminal" input and output goes through an Emacs buffer,
called the GUD buffer.
This applies both to GDB commands and their output, and to the
input and output done by the program you are debugging.
This is useful because it means that you can copy the text of
previous commands and input them again; you can even use parts of
the output in this way.
All the facilities of Emacs' Shell mode are available for
interacting with your program. In particular, you can send signals
the usual way--for example, 'C-c C-c' for an interrupt, 'C-c C-z'
for a stop.
* GDB displays source code through Emacs.
Each time GDB displays a stack frame, Emacs automatically finds the
source file for that frame and puts an arrow ('=>') at the left
margin of the current line. Emacs uses a separate buffer for
source display, and splits the screen to show both your GDB session
and the source.
Explicit GDB 'list' or search commands still produce output as
usual, but you probably have no reason to use them from Emacs.
We call this "text command mode". Emacs 22.1, and later, also uses a
graphical mode, enabled by default, which provides further buffers that
can control the execution and describe the state of your program. 
(Emacs)GDB Graphical Interface.
If you specify an absolute file name when prompted for the 'M-x gdb'
argument, then Emacs sets your current working directory to where your
program resides. If you only specify the file name, then Emacs sets
your current working directory to the directory associated with the
previous buffer. In this case, GDB may find your program by searching
your environment's 'PATH' variable, but on some operating systems it
might not find the source. So, although the GDB input and output
session proceeds normally, the auxiliary buffer does not display the
current source and line of execution.
The initial working directory of GDB is printed on the top line of
the GUD buffer and this serves as a default for the commands that
specify files for GDB to operate on. Commands to Specify Files
Files.
By default, 'M-x gdb' calls the program called 'gdb'. If you need to
call GDB by a different name (for example, if you keep several
configurations around, with different names) you can customize the Emacs
variable 'gud-gdb-command-name' to run the one you want.
In the GUD buffer, you can use these special Emacs commands in
addition to the standard Shell mode commands:
'C-h m'
Describe the features of Emacs' GUD Mode.
'C-c C-s'
Execute to another source line, like the GDB 'step' command; also
update the display window to show the current file and location.
'C-c C-n'
Execute to next source line in this function, skipping all function
calls, like the GDB 'next' command. Then update the display window
to show the current file and location.
'C-c C-i'
Execute one instruction, like the GDB 'stepi' command; update
display window accordingly.
'C-c C-f'
Execute until exit from the selected stack frame, like the GDB
'finish' command.
'C-c C-r'
Continue execution of your program, like the GDB 'continue'
command.
'C-c <'
Go up the number of frames indicated by the numeric argument (
Numeric Arguments (Emacs)Arguments.), like the GDB 'up' command.
'C-c >'
Go down the number of frames indicated by the numeric argument,
like the GDB 'down' command.
In any source file, the Emacs command 'C-x <SPC>' ('gud-break') tells
GDB to set a breakpoint on the source line point is on.
In text command mode, if you type 'M-x speedbar', Emacs displays a
separate frame which shows a backtrace when the GUD buffer is current.
Move point to any frame in the stack and type <RET> to make it become
the current frame and display the associated source in the source
buffer. Alternatively, click 'Mouse-2' to make the selected frame
become the current one. In graphical mode, the speedbar displays watch
expressions.
If you accidentally delete the source-display buffer, an easy way to
get it back is to type the command 'f' in the GDB buffer, to request a
frame display; when you run under Emacs, this recreates the source
buffer if necessary to show you the context of the current frame.
The source files displayed in Emacs are in ordinary Emacs buffers
which are visiting the source files in the usual way. You can edit the
files with these buffers if you wish; but keep in mind that GDB
communicates with Emacs in terms of line numbers. If you add or delete
lines from the text, the line numbers that GDB knows cease to correspond
properly with the code.
A more detailed description of Emacs' interaction with GDB is given
in the Emacs manual ((Emacs)Debuggers).
Info Catalog
gdb: TUI
gdb: Top
gdb: GDB/MI