26 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.

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. See GDB Graphical Interface in 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 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 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 (see Numeric Arguments in The GNU Emacs Manual ), 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.