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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.

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.