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You can abbreviate a GDB command to the first few letters of the command name, if that abbreviation is unambiguous; and you can repeat certain GDB commands by typing just RET. You can also use the TAB key to get GDB to fill out the rest of a word in a command (or to show you the alternatives available, if there is more than one possibility).
A GDB command is a single line of input. There is no limit on
how long it can be. It starts with a command name, which is followed by
arguments whose meaning depends on the command name. For example, the
step accepts an argument which is the number of times to
step, as in `step 5'. You can also use the
with no arguments. Some command names do not allow any arguments.
GDB command names may always be truncated if that abbreviation is
unambiguous. Other possible command abbreviations are listed in the
documentation for individual commands. In some cases, even ambiguous
abbreviations are allowed; for example,
s is specially defined as
step even though there are other commands whose
names start with
s. You can test abbreviations by using them as
arguments to the
A blank line as input to GDB (typing just RET) means to
repeat the previous command. Certain commands (for example,
will not repeat this way; these are commands whose unintentional
repetition might cause trouble and which you are unlikely to want to
x commands, when you repeat them with
RET, construct new arguments rather than repeating
exactly as typed. This permits easy scanning of source or memory.
GDB can also use RET in another way: to partition lengthy
output, in a way similar to the common utility
(see section Screen size). Since it is easy to press one
RET too many in this situation, GDB disables command
repetition after any command that generates this sort of display.
Any text from a # to the end of the line is a comment; it does nothing. This is useful mainly in command files (see section Command files).
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 what the valid possibilities are for the next word in a command, at any time. This works for GDB commands, GDB subcommands, and the names of symbols in your program.
Press 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 press RET to enter it). For example, if you type
(gdb) info bre TAB
GDB fills in the rest of the word `breakpoints', since that is
info subcommand beginning with `bre':
(gdb) info breakpoints
You can either press RET at this point, to run the
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
name(int), or the version that takes a
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-? 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 TAB 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.
You can always ask GDB itself for information on its commands, using the
h) with no arguments to display a short list of named classes of commands:
(gdb) help List of classes of commands: running -- Running the program stack -- Examining the stack data -- Examining data breakpoints -- Making program stop at certain points files -- Specifying and examining files status -- Status inquiries support -- Support facilities user-defined -- User-defined commands aliases -- Aliases of other commands obscure -- Obscure features Type "help" followed by a class name for a list of commands in that class. Type "help" followed by command name for full documentation. Command name abbreviations are allowed if unambiguous. (gdb)
(gdb) help status Status inquiries. List of commands: show -- Generic command for showing things set with "set" info -- Generic command for printing status Type "help" followed by command name for full documentation. Command name abbreviations are allowed if unambiguous. (gdb)
helpargument, GDB displays a short paragraph on how to use that command.
In addition to
help, you can use the GDB commands
show to inquire about the state of your program, or the state
of GDB itself. Each command supports many topics of inquiry; this
manual introduces each of them in the appropriate context. The listings
info and under
show in the Index point to
all the sub-commands. See section Index.
i) is for describing the state of your program. For example, you can list the arguments given to your program with
info args, list the registers currently in use with
info registers, or list the breakpoints you have set with
info breakpoints. You can get a complete list of the
showis for describing the state of GDB itself. You can change most of the things you can
show, by using the related command
set; for example, you can control what number system is used for displays with
set radix, or simply inquire which is currently in use with
To display all the settable parameters and their current
values, you can use
show with no arguments; you may also use
info set. Both commands produce the same display.
Here are three miscellaneous
show subcommands, all of which are
exceptional in lacking corresponding
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