10.8. Value history
Values printed by the print command are saved in the gdb
value history. This allows you to refer to them in other expressions.
Values are kept until the symbol table is re-read or discarded
(for example with the file or symbol-file commands).
When the symbol table changes, the value history is discarded,
since the values may contain pointers back to the types defined in the
symbol table.
The values printed are given history numbers by which you can
refer to them. These are successive integers starting with one.
print shows you the history number assigned to a value by
printing $num = before the value; here num is the
history number.
To refer to any previous value, use $ followed by the value's
history number. The way print labels its output is designed to
remind you of this. Just $ refers to the most recent value in
the history, and $$ refers to the value before that.
$$n refers to the nth value from the end; $$2
is the value just prior to $$, $$1 is equivalent to
$$, and $$0 is equivalent to $.
For example, suppose you have just printed a pointer to a structure and
want to see the contents of the structure. It suffices to type
If you have a chain of structures where the component next points
to the next one, you can print the contents of the next one with this:
You can print successive links in the chain by repeating this
command--which you can do by just typing [RET].
Note that the history records values, not expressions. If the value of
x is 4 and you type these commands:
then the value recorded in the value history by the print command
remains 4 even though the value of x has changed.
- show values
Print the last ten values in the value history, with their item numbers.
This is like p $$9 repeated ten times, except that show
values does not change the history.
- show values n
Print ten history values centered on history item number n.
- show values +
Print ten history values just after the values last printed. If no more
values are available, show values + produces no display.
Pressing [RET] to repeat show values n has exactly the
same effect as show values +.
