gdb: Value History
10.10 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 +'.
Info Catalog
gdb: Pretty Printing
gdb: Data
gdb: Convenience Vars