The debugger can fill in the rest of a word in a command for you, if there is only one possibility; it can also show you what the valid possibilities are for the next word in a command, at any time. This works for commands, subcommands, and the names of symbols in your program.
Press the Tab key whenever you want the debugger to fill out the rest of a word. If there is only one possibility, the debugger fills in the word, and waits for you to finish the command (or press Enter to enter it). For example, if you type
(gdb) info bre Tab
The debugger 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 Enter 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 Enter 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, the debugger sounds a bell. You can either supply more characters and try again, or just press Tab a second time; the debugger 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 the debugger 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
the debugger 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, the debugger 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 the debugger 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 the debugger 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 the debugger that it may need to consider more information than usual when you press Tab or Meta-? to request word completion:
(gdb) b 'bubble( Meta-? bubble(double,double) bubble(int,int) (gdb) b 'bubble(
In some cases, the debugger can tell that completing a name requires using quotes. When this happens, the debugger inserts the quote for you (while completing as much as it can) if you do not type the quote in the first place:
(gdb) b bub Tab
the debugger alters your input line to the following, and rings a bell:
(gdb) b 'bubble(
In general, the debugger can tell that a quote is needed (and inserts it) if you have not yet started typing the argument list when you ask for completion on an overloaded symbol.