gdb provides convenience variables that you can use within gdb to hold on to a value and refer to it later. These variables exist entirely within gdb; they are not part of your program, and setting a convenience variable has no direct effect on further execution of your program. That is why you can use them freely.
Convenience variables are prefixed with ‘$’. Any name preceded by ‘$’ can be used for a convenience variable, unless it is one of the predefined machine-specific register names (see Registers). (Value history references, in contrast, are numbers preceded by ‘$’. See Value History.)
You can save a value in a convenience variable with an assignment expression, just as you would set a variable in your program. For example:
set $foo = *object_ptr
would save in $foo
the value contained in the object pointed to by
object_ptr
.
Using a convenience variable for the first time creates it, but its
value is void
until you assign a new value. You can alter the
value with another assignment at any time.
Convenience variables have no fixed types. You can assign a convenience variable any type of value, including structures and arrays, even if that variable already has a value of a different type. The convenience variable, when used as an expression, has the type of its current value.
show convenience
show conv
.
init-if-undefined $
variable =
expressionIf the variable is already defined then the expression is not evaluated so any side-effects do not occur.
One of the ways to use a convenience variable is as a counter to be incremented or a pointer to be advanced. For example, to print a field from successive elements of an array of structures:
set $i = 0 print bar[$i++]->contents
Repeat that command by typing <RET>.
Some convenience variables are created automatically by gdb and given values likely to be useful.
$_
$_
is automatically set by the x
command to
the last address examined (see Examining Memory). Other
commands which provide a default address for x
to examine also
set $_
to that address; these commands include info line
and info breakpoint
. The type of $_
is void *
except when set by the x
command, in which case it is a pointer
to the type of $__
.
$__
$__
is automatically set by the x
command
to the value found in the last address examined. Its type is chosen
to match the format in which the data was printed.
$_exitcode
$_exitcode
is automatically set to the exit code when
the program being debugged terminates.
$_exception
$_exception
is set to the exception object being
thrown at an exception-related catchpoint. See Set Catchpoints.
$_probe_argc
$_probe_arg0...$_probe_arg11
$_sdata
$_sdata
contains extra collected static tracepoint
data. See Tracepoint Action Lists. Note that
$_sdata
could be empty, if not inspecting a trace buffer, or
if extra static tracepoint data has not been collected.
$_siginfo
$_siginfo
contains extra signal information
(see extra signal information). Note that $_siginfo
could be empty, if the application has not yet received any signals.
For example, it will be empty before you execute the run
command.
$_tlb
$_tlb
is automatically set when debugging
applications running on MS-Windows in native mode or connected to
gdbserver that supports the qGetTIBAddr
request.
See General Query Packets.
This variable contains the address of the thread information block.
On HP-UX systems, if you refer to a function or variable name that begins with a dollar sign, gdb searches for a user or system name first, before it searches for a convenience variable.