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 section Registers). (Value history references, in contrast, are numbers preceded by `$'. See section 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
.
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 section 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.
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.
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