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#### 27.6.4 GUD Customization

On startup, GUD runs one of the following hooks: `gdb-mode-hook`, if you are using GDB; `dbx-mode-hook`, if you are using DBX; `sdb-mode-hook`, if you are using SDB; `xdb-mode-hook`, if you are using XDB; `perldb-mode-hook`, for Perl debugging mode; `pdb-mode-hook`, for PDB; `jdb-mode-hook`, for JDB. See Hooks.

The `gud-def` Lisp macro (see Defining Macros in the Emacs Lisp Reference Manual) provides a convenient way to define an Emacs command that sends a particular command string to the debugger, and set up a key binding for in the GUD interaction buffer:

    (gud-def function cmdstring binding docstring)

This defines a command named function which sends cmdstring to the debugger process, and gives it the documentation string docstring. You can then use the command function in any buffer. If binding is non-`nil`, `gud-def` also binds the command to `C-c binding` in the GUD buffer’s mode and to `C-x C-a binding` generally.

The command string cmdstring may contain certain ‘`%`’-sequences that stand for data to be filled in at the time function is called:

‘`%f`’

  The name of the current source file. If the current buffer is the GUD buffer, then the current source file is the file that the program stopped in.

‘`%l`’

  The number of the current source line. If the current buffer is the GUD buffer, then the current source line is the line that the program stopped in.

‘`%e`’

  In transient-mark-mode the text in the region, if it is active. Otherwise the text of the C lvalue or function-call expression at or adjacent to point.

‘`%a`’

  The text of the hexadecimal address at or adjacent to point.

‘`%p`’

  The numeric argument of the called function, as a decimal number. If the command is used without a numeric argument, ‘`%p`’ stands for the empty string.

  If you don’t use ‘`%p`’ in the command string, the command you define ignores any numeric argument.

‘`%d`’

  The name of the directory of the current source file.

‘`%c`’

  Fully qualified class name derived from the expression surrounding point (jdb only).

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