27.6.5.7 Watch Expressions

If you want to see how a variable changes each time your program stops, move point into the variable name and click on the watch icon in the tool bar (gud-watch) or type C-x C-a C-w. If you specify a prefix argument, you can enter the variable name in the minibuffer.

Each watch expression is displayed in the speedbar (see Speedbar). Complex data types, such as arrays, structures and unions are represented in a tree format. Leaves and simple data types show the name of the expression and its value and, when the speedbar frame is selected, display the type as a tooltip. Higher levels show the name, type and address value for pointers and just the name and type otherwise. Root expressions also display the frame address as a tooltip to help identify the frame in which they were defined.

To expand or contract a complex data type, click mouse-2 or press SPC on the tag to the left of the expression. Emacs asks for confirmation before expanding the expression if its number of immediate children exceeds the value of the variable gdb-max-children.

To delete a complex watch expression, move point to the root expression in the speedbar and type D (gdb-var-delete).

To edit a variable with a simple data type, or a simple element of a complex data type, move point there in the speedbar and type RET (gdb-edit-value). Or you can click mouse-2 on a value to edit it. Either way, this reads the new value using the minibuffer.

If you set the variable gdb-show-changed-values to non-nil (the default value), Emacs uses font-lock-warning-face to highlight values that have recently changed and shadow face to make variables which have gone out of scope less noticeable. When a variable goes out of scope you can’t edit its value.

If the variable gdb-delete-out-of-scope is non-nil (the default value), Emacs automatically deletes watch expressions which go out of scope. Sometimes, when re-entering the same function, it may be useful to set this value to nil so that you don’t need to recreate the watch expression.

If the variable gdb-use-colon-colon-notation is non-nil, Emacs uses the ‘function::variable’ format. This allows the user to display watch expressions which share the same variable name. The default value is nil.

To automatically raise the speedbar every time the display of watch expressions updates, set gdb-speedbar-auto-raise to non-nil. This can be useful if you are debugging with a full screen Emacs frame.