Completion is normally done in the minibuffer (see Completion), but you can also complete symbol names in ordinary Emacs buffers.
In programming language modes, type C-M-i or M-<TAB> to complete the partial symbol before point. On graphical displays, the M-<TAB> key is usually reserved by the window manager for switching graphical windows, so you should type C-M-i or <ESC> <TAB> instead.
In most programming language modes, C-M-i (or
M-<TAB>) invokes the command
which generates its completion list in a flexible way. If Semantic
mode is enabled, it tries to use the Semantic parser data for
completion (see Semantic). If Semantic mode is not enabled or
fails at performing completion, it tries to complete using the
selected tags table (see Tags). If in Emacs Lisp mode, it
performs completion using the function, variable, or property names
defined in the current Emacs session.
In all other respects, in-buffer symbol completion behaves like minibuffer completion. For instance, if Emacs cannot complete to a unique symbol, it displays a list of completion alternatives in another window. See Completion.
In Text mode and related modes, M-<TAB> completes words based on the spell-checker's dictionary. See Spelling.