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 = (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 supported only 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 . The display command doesn't repeat if you press Enter 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 is a common name for the program counter; see Registers ).
- undisplay dnums... or delete display dnums...
Remove item numbers dnums from the list of expressions
The undisplay command doesn't repeat if you press Enter after using it. (Otherwise you'd just get the error No display number ....)
- disable display dnums...
- Disable the display of item numbers dnums. A disabled display item isn't printed automatically, but isn't forgotten; it may be enabled again later.
- enable display dnums...
- Enable the display of item numbers dnums. It becomes effective once again in auto display of its expression, until you specify otherwise.
- 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 that wouldn't be displayed right now because they refer to automatic variables not currently available.
If a display expression refers to local variables, it doesn't 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 isn't 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 where there's 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.