Red Hat ENTERPRISE LINUX 4 - DEVELOPER TOOLS GUIDE, Manual

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Chapter 10. Examining Data 71
The addresses and contents printed by the
x
command are not saved in the value history because there
is often too much of them and they would get in the way. Instead, gdb makes these values available
for subsequent use in expressions as values of the convenience variables
$_
and
$__
. After an
x
command, the last address examined is available for use in expressions in the convenience variable
$_
. The contents of that address, as examined, are available in the convenience variable
$__
.
If the
x
command has a repeat count, the address and contents saved are from the last memory unit
printed; this is not the same as the last address printed if several units were printed on the last line of
output.
10.6. 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 on how elaborate your format specification is--it uses
x
if you
specify a unit size, or one of the two formats (
i
and
s
) that are only supported by
x
; otherwise it uses
print
.
display
expr
Add the expression
expr
to the list of expressions to display each time your program stops. Refer
to Section 10.1 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
. Refer to Section
10.4 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
. Refer to Section 10.5 Examining 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; (refer to Section 10.10
Registers ).
undisplay
dnums
...
delete display
dnums
...
Remove item numbers
dnums
from the list of expressions to display.
undisplay
does not repeat if you press [RET] after using it. (Otherwise you would just get the
error
No display number ...
.)