Debugging with GDB (February 2008)

18 Debugging with GDB
You can either press
h
RET
i
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
h
RET
i
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
h
TAB
i
, GDB sounds
a bell. You can either supply more characters and try again, or just press
h
TAB
i
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_
h
TAB
i
GDB just sounds the bell. Typing
h
TAB
i
again displays all the function
names in your program that begin with those characters, for example:
((gdb)) b make_
h
TAB
i
GDB sounds bell; press
h
TAB
i
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
h
TAB
i
twice. M-? means
h
META
i
?. You can type this either by holding down
a key designated as the
h
META
i
shift on your keyboard (if there is one) while typing ?, or as
h
ESC
i
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
h
TAB
i
or M-? to request word completion:
((gdb)) b ’bubble( M-?
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
h
TAB
i
GDB alters your input line to the following, and rings a bell: