Unix in a Nutshell, 4th Edition

Convenience Variables and Machine Registers

GDB lets you create convenience variables . These are variables you can use to store values as you need them. Their names begin with a $ and consist of alphanumeric characters and underscores. They should start with a letter or underscore. (Note that values in the value history have names that are numeric.) You might want to use a convenience variable as an array index:

    (gdb) set $j = 0
    (gdb)print data[$j++]

After these two commands, simply hitting the ENTER key repeats the last command, stepping through the array one element at a time.

GDB predefines several convenience variables. It also enables you to access the machine registers using predefined register names. Register names vary with machine architecture, of course, but there are four predefined registers available on every architecture. The following list summarizes the convenience variables and predefined registers. The last four entries in the list are the registers that are always available.

$	The most recent value in the value history.
$ n	Item n in the value history.
$$	The next to last item in the value history.
$$ n	Item n in the value history, counting from the end.
$_	The address last printed by the x command.
$_ _	The contents of the address last printed by the x command.
$_exitcode	The exit status that the debuggee returned when it exited.
$bpnum	The breakpoint number of the most recently set breakpoint.
$cdir	The compilation directory ...