HP gnu source-level debugger 5992-4701 manual Automatic display, See Output formats

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.

8.6 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. See “Expressions” (page 83).
	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.
	See“Output formats” (page 86).
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”
	(page 87).

For example, `display/i $pc' can be helpful, to view the machine instruction about to be executed each time execution stops (`$pc' is a common name for the program counter; see “Registers” (page 98)).

undisplay dnums..., delete Remove item numbers dnums from the list of

display dnums...expressions to display. undisplay does not repeat if you press RET after using it. (Otherwise you would just get the error 'No display number

...'.)

8.6 Automatic display 89