21. 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 things:

• 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" instead.

In the GDB I/O buffer, you can use these special Emacs commands in addition to the standard Shell mode commands:

C-h m

Describe the features of Emacs' GDB Mode.

M-s

Execute to another source line, like the GDB step command; also update the display window to show the current file and location.

M-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.

M-i

Execute one instruction, like the GDB stepi command; update display window accordingly.

M-x gdb-nexti

Execute to next instruction, using the GDB nexti command; update display window accordingly.

C-c C-f

Execute until exit from the selected stack frame, like the GDB finish command.

M-c

Continue execution of your program, like the GDB continue command.

Warning: In Emacs v19, this command is C-c C-p.

M-u

Go up the number of frames indicated by the numeric argument (see section `Numeric Arguments' in The GNU Emacs Manual), like the GDB up command.

Warning: In Emacs v19, this command is C-c C-u.

M-d

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.

C-x &

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 element.

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.