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3. GDB Commands

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).


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3.1 Command Syntax

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 command step accepts an argument which is the number of times to step, as in `step 5'. You can also use the step command with no arguments. Some commands 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 equivalent to step even though there are other commands whose names start with s. You can test abbreviations by using them as arguments to the help command.

A blank line as input to GDB (typing just RET) means to repeat the previous command. Certain commands (for example, run) will not repeat this way; these are commands whose unintentional repetition might cause trouble and which you are unlikely to want to repeat. User-defined commands can disable this feature; see dont-repeat.

The list and 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 more (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).

The Ctrl-o binding is useful for repeating a complex sequence of commands. This command accepts the current line, like RET, and then fetches the next line relative to the current line from the history for editing.


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3.2 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 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 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-?
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.

For more information about overloaded functions, see C++ Expressions. You can use the command set overload-resolution off to disable overload resolution; see GDB Features for C++.

When completing in an expression which looks up a field in a structure, GDB also tries(2) to limit completions to the field names available in the type of the left-hand-side:

 
(gdb) p gdb_stdout.M-?
magic      to_delete  to_fputs   to_put     to_rewind  
to_data    to_flush   to_isatty  to_read    to_write   

This is because the gdb_stdout is a variable of the type struct ui_file that is defined in GDB sources as follows:

 
struct ui_file
{
   int *magic;
   ui_file_flush_ftype *to_flush;
   ui_file_write_ftype *to_write;
   ui_file_fputs_ftype *to_fputs;
   ui_file_read_ftype *to_read;
   ui_file_delete_ftype *to_delete;
   ui_file_isatty_ftype *to_isatty;
   ui_file_rewind_ftype *to_rewind;
   ui_file_put_ftype *to_put;
   void *to_data;
}

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3.3 Getting Help

You can always ask GDB itself for information on its commands, using the command help.

help
h

You can use help (abbreviated h) with no arguments to display a short list of named classes of commands:

 
(gdb) help
List of classes of commands:

aliases -- Aliases of other commands
breakpoints -- Making program stop at certain points
data -- Examining data
files -- Specifying and examining files
internals -- Maintenance commands
obscure -- Obscure features
running -- Running the program
stack -- Examining the stack
status -- Status inquiries
support -- Support facilities
tracepoints -- Tracing of program execution without
               stopping the program
user-defined -- User-defined commands

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)
help class

Using one of the general help classes as an argument, you can get a list of the individual commands in that class. For example, here is the help display for the class status:

 
(gdb) help status
Status inquiries.

List of commands:

info -- Generic command for showing things
        about the program being debugged
show -- Generic command for showing things
        about the debugger

Type "help" followed by command name for full
documentation.
Command name abbreviations are allowed if unambiguous.
(gdb)
help command

With a command name as help argument, GDB displays a short paragraph on how to use that command.

apropos args

The apropos command searches through all of the GDB commands, and their documentation, for the regular expression specified in args. It prints out all matches found. For example:

 
apropos reload

results in:

 
set symbol-reloading -- Set dynamic symbol table reloading
                        multiple times in one run
show symbol-reloading -- Show dynamic symbol table reloading
                        multiple times in one run
complete args

The complete args command lists all the possible completions for the beginning of a command. Use args to specify the beginning of the command you want completed. For example:

 
complete i

results in:

 
if
ignore
info
inspect

This is intended for use by GNU Emacs.

In addition to help, you can use the GDB commands info and 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 under info and under show in the Index point to all the sub-commands. See section Index.

info

This command (abbreviated i) is for describing the state of your program. For example, you can show the arguments passed to a function 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 info sub-commands with help info.

set

You can assign the result of an expression to an environment variable with set. For example, you can set the GDB prompt to a $-sign with set prompt $.

show

In contrast to info, show is 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 show radix.

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 set commands:

show version

Show what version of GDB is running. You should include this information in GDB bug-reports. If multiple versions of GDB are in use at your site, you may need to determine which version of GDB you are running; as GDB evolves, new commands are introduced, and old ones may wither away. Also, many system vendors ship variant versions of GDB, and there are variant versions of GDB in GNU/Linux distributions as well. The version number is the same as the one announced when you start GDB.

show copying
info copying

Display information about permission for copying GDB.

show warranty
info warranty

Display the GNU "NO WARRANTY" statement, or a warranty, if your version of GDB comes with one.


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