GDB fills in the rest of the word 'breakpoints', since that is the only info subcommand beginning with 'bre':

((gdb)) info breakpoints

You can either press RET at this point, to run the info breakpoints command, or backspace and enter something else, if 'breakpoints' does not look like the command you expected. (If you were sure you wanted info breakpoints in the first place, you might as well just type RET immediately after 'info bre', to exploit command abbreviations rather than command completion.)

If there is more than one possibility for the next word when you press TAB , GDB sounds a bell. You can either supply more characters and try again, or just press TAB a second time; GDB displays all the possible completions for that word. For example, you might want to set a breakpoint on a subroutine whose name begins with 'make_', but when you type b make_TAB GDB just sounds the bell. Typing TAB again displays all the function names in your program that begin with those characters, for example:

((gdb)) b make_TAB

GDB sounds bell; press TAB again, to see:

make_a_section_from_file make_environ make_abs_section make_function_type make_blockvector make_pointer_type make_cleanup make_reference_type make_command make_symbol_completion_list

((gdb)) b make_

After displaying the available possibilities, GDB copies your partial input ('b make_' in the example) so you can finish the command.

If you just want to see the list of alternatives in the first place, you can press M-?rather than pressing TAB twice. M-?means META?. You can type this either by holding down a key designated as the META shift on your keyboard (if there is one) while typing ?, or as ESC followed by ?.

Sometimes the string you need, while logically a “word”, may contain parentheses or other characters that GDB normally excludes from its notion of a word. To permit word completion to work in this situation, you may enclose words in ' (single quote marks) in GDB commands.

The most likely situation where you might need this is in typing the name of a C++ function. This is because C++ allows function overloading (multiple definitions of the same function, distinguished by argument type). For example, when you want to set a breakpoint you may need to distinguish whether you mean the version of name that takes an int parameter, name(int), or the version that takes a float parameter, name(float). To use the word-completion facilities in this situation, type a single quote ' at the beginning of the function name. This alerts GDB that it may need to consider more information than usual when you press TAB or M-?to request word completion:

((gdb)) b 'bubble( M-?

34 GDB Commands