gdb.info: Value History

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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

p *$

   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:

p *$.next

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:
     print x
     set x=5
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 +'.