26 Using gdb under gnu Emacs
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.
listor 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. See GDB Graphical Interface (The gnu Emacs Manual).
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 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. See Commands to Specify 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
stepcommand; 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
nextcommand. Then update the display window to show the current file and location.
- C-c C-i
- Execute one instruction, like the gdb
stepicommand; update display window accordingly.
- C-c C-f
- Execute until exit from the selected stack frame, like the gdb
- C-c C-r
- Continue execution of your program, like the gdb
- C-c <
- Go up the number of frames indicated by the numeric argument
(see Numeric Arguments (The gnu Emacs Manual)),
like the gdb
- C-c >
- Go down the number of frames indicated by the numeric argument, like the
In any source file, the Emacs command C-x <SPC> (
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
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 (see Debuggers (The gnu Emacs Manual)).