Command completion

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Command completion

GDB can fill in the rest of a word in a command for you, if there is only one possibility; it can also show you, at any time, what the valid possibilities are for the next word in a command. This works for GDB commands, GDB subcommands, and the names of symbols in your program.

Use the Tab key whenever you want GDB to fill out the rest of a word. If there is only one possibility, GDB fills in the word, and waits for you to finish the command (or use Return to enter it). For example, if you type (gdb) info bre, and use the Tab key, GDB fills in the rest of the word ‘breakpoints’, since that is the only info subcommand beginning with ‘bre’.

You can either use Return at this point, to run the info breakpoints command, or use the BACKSPACE key 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 use Return 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 use the Tab key, GDB sounds a bell. You can either supply more characters and try again, or just use the Tab key 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_ and use the Tab key, GDB just sounds the bell. Using the Tab key again displays all the function names in your program that begin with those characters. For example, you type (gdb) b make_ and then use the Tab key. GDB sounds the bell; you use the Tab key again, to see the following display.

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 (in the example, ‘b make_’) so you can finish the command. If you just want to see the list of alternatives in the first place, you can get help by using the command key sequence, M-? rather than using Tab twice.

IMPORTANT:
M-? means using the META key (if there is one, usually the diamond-shaped character is on the key; else, use ESC) and the ‘?’ key. This is a command key sequence with which you may or may not be familiar.

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 use the Tab key or M-? to request word completion, as in the following example.

(gdb) b ’bubble(

Use the M-? command key sequence this point.

bubble(double,double) bubble(int,int)
(gdb) b ’bubble(

In some cases, GDB can tell that completing a name requires using quotes. When this happens, GDB 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

Use the Tab key at this point.GDB alters your input line to the following, and rings a bell.

(gdb) b ’bubble(

In general, GDB 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.

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