Using GDB under GNU Emacs
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.
Using GDB under Emacs is just like using GDB normally except for two
* All "terminal" input and output goes through the Emacs 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.
_Warning:_ If the directory where your program resides is not your
current directory, it can be easy to confuse Emacs about the
location of the source files, in which case the auxiliary display
buffer does not appear to show your source. GDB can find programs
by searching your environment's `PATH' variable, so the GDB input
and output session proceeds normally; but Emacs does not get
enough information back from GDB to locate the source files in
this situation. To avoid this problem, either start GDB mode from
the directory where your program resides, or specify an absolute
file name when prompted for the `M-x gdb' argument.
A similar confusion can result if you use the GDB `file' command to
switch to debugging a program in some other location, from an
existing GDB buffer in Emacs.
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 set the Emacs
variable `gdb-command-name'; for example,
(setq gdb-command-name "mygdb")
(preceded by `M-:' or `ESC :', or typed in the `*scratch*' buffer, or
in your `.emacs' file) makes Emacs call the program named "`mygdb'"
In the GDB I/O buffer, you can use these special Emacs commands in
addition to the standard Shell mode commands:
Describe the features of Emacs' GDB Mode.
Execute to another source line, like the GDB `step' command; also
update the display window to show the current file and location.
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.
Execute one instruction, like the GDB `stepi' command; update
display window accordingly.
Execute to next instruction, using the GDB `nexti' command; update
display window accordingly.
Execute until exit from the selected stack frame, like the GDB
Continue execution of your program, like the GDB `continue'
_Warning:_ In Emacs v19, this command is `C-c C-p'.
Go up the number of frames indicated by the numeric argument
( Numeric Arguments (Emacs)Arguments.), like the GDB `up'
_Warning:_ In Emacs v19, this command is `C-c C-u'.
Go down the number of frames indicated by the numeric argument,
like the GDB `down' command.
_Warning:_ In Emacs v19, this command is `C-c C-d'.
Read the number where the cursor is positioned, and insert it at
the end of the GDB I/O buffer. For example, if you wish to
disassemble code around an address that was displayed earlier,
type `disassemble'; then move the cursor to the address display,
and pick up the argument for `disassemble' by typing `C-x &'.
You can customize this further by defining elements of the list
`gdb-print-command'; once it is defined, you can format or
otherwise process numbers picked up by `C-x &' before they are
inserted. A numeric argument to `C-x &' indicates that you wish
special formatting, and also acts as an index to pick an element
of the list. If the list element is a string, the number to be
inserted is formatted using the Emacs function `format'; otherwise
the number is passed as an argument to the corresponding list
In any source file, the Emacs command `C-x SPC' (`gdb-break') tells
GDB to set a breakpoint on the source line point is on.
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.
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