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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 the following example declaration.
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
exp
Add the expression, exp,
to the list of expressions to display each time your program stops. See
Expressions.
display
does not repeat if you press Return
again after using it.
display/fmt exp
For fmt
specifying only a display format and not a size or count, add the expression
exp
to the auto-display list but arrange to display it each time in the specified
format, fmt.
See 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’. See
Examining memory.
For example, ‘display/i
$pc’ can be helpful,
to see the machine instruction about to be executed each time execution
stops (‘$pc’
isa common name for the program counter; see Registers).
undisplay
dnums
...
delete display
dnums
...
Remove item numbers dnums
from the list of expressions to display.
undisplay
does not repeat if you use Return
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.
enable display
dnums
...
Enable display of item numbers,
dnums.
It becomes effective once again in auto display of its expression, until
you specify otherwise.
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.
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