gdb: Auto Display
10.7 Automatic Display
======================
If you find that you want to print the value of an expression frequently
(to see how it changes), you might want to add it to the "automatic
display list" so that GDB prints its value each time your program stops.
Each expression added to the list is given a number to identify it; to
remove an expression from the list, you specify that number. The
automatic display looks like this:
2: foo = 38
3: bar[5] = (struct hack *) 0x3804
This display shows item numbers, expressions and their current values.
As with displays you request manually using 'x' or 'print', you can
specify the output format you prefer; in fact, 'display' decides whether
to use 'print' or 'x' depending your format specification--it uses 'x'
if you specify either the 'i' or 's' format, or a unit size; otherwise
it uses 'print'.
'display EXPR'
Add the expression EXPR to the list of expressions to display each
time your program stops. Expressions Expressions.
'display' does not repeat if you press <RET> again after using it.
'display/FMT EXPR'
For FMT specifying only a display format and not a size or count,
add the expression EXPR to the auto-display list but arrange to
display it each time in the specified format FMT. Output
Formats Output Formats.
'display/FMT ADDR'
For FMT 'i' or 's', or including a unit-size or a number of units,
add the expression ADDR as a memory address to be examined each
time your program stops. Examining means in effect doing 'x/FMT
ADDR'. Examining Memory Memory.
For example, 'display/i $pc' can be helpful, to see the machine
instruction about to be executed each time execution stops ('$pc' is a
common name for the program counter; Registers Registers.).
'undisplay DNUMS...'
'delete display DNUMS...'
Remove items from the list of expressions to display. Specify the
numbers of the displays that you want affected with the command
argument DNUMS. It can be a single display number, one of the
numbers shown in the first field of the 'info display' display; or
it could be a range of display numbers, as in '2-4'.
'undisplay' does not repeat if you press <RET> after using it.
(Otherwise you would just get the error 'No display number ...'.)
'disable display DNUMS...'
Disable the display of item numbers DNUMS. A disabled display item
is not printed automatically, but is not forgotten. It may be
enabled again later. Specify the numbers of the displays that you
want affected with the command argument DNUMS. It can be a single
display number, one of the numbers shown in the first field of the
'info display' display; or it could be a range of display numbers,
as in '2-4'.
'enable display DNUMS...'
Enable display of item numbers DNUMS. It becomes effective once
again in auto display of its expression, until you specify
otherwise. Specify the numbers of the displays that you want
affected with the command argument DNUMS. It can be a single
display number, one of the numbers shown in the first field of the
'info display' display; or it could be a range of display numbers,
as in '2-4'.
'display'
Display the current values of the expressions on the list, just as
is done when your program stops.
'info display'
Print the list of expressions previously set up to display
automatically, each one with its item number, but without showing
the values. This includes disabled expressions, which are marked
as such. It also includes expressions which would not be displayed
right now because they refer to automatic variables not currently
available.
If a display expression refers to local variables, then it does not
make sense outside the lexical context for which it was set up. Such an
expression is disabled when execution enters a context where one of its
variables is not defined. For example, if you give the command 'display
last_char' while inside a function with an argument 'last_char', GDB
displays this argument while your program continues to stop inside that
function. When it stops elsewhere--where there is no variable
'last_char'--the display is disabled automatically. The next time your
program stops where 'last_char' is meaningful, you can enable the
display expression once again.