Syntactic Symbols Top Frequently Asked Questions
C and its derivative languages are highly complex creatures. Often, ambiguous code situations arise that require CC Mode to scan large portions of the buffer to determine syntactic context. Such pathological code can cause CC Mode to perform fairly badly. This section identifies some of the coding styles to watch out for, and suggests some workarounds that you can use to improve performance.
Because CC Mode has to scan the buffer backwards from the current insertion point, and because C's syntax is fairly difficult to parse in the backwards direction, CC Mode often tries to find the nearest position higher up in the buffer from which to begin a forward scan. The farther this position is from the current insertion point, the slower the mode gets. Some coding styles can even force CC Mode to scan from the beginning of the buffer for every line of code!
One of the simplest things you can do to reduce scan time, is make sure
any brace that opens a top-level construct always appears in the
leftmost column. This is actually an Emacs constraint, as embodied in
beginning-of-defun function which CC Mode uses
heavily. If you insist on hanging top-level open braces on the right
side of the line, then you might want to set the variable
defun-prompt-regexp to something reasonable , however that ``something
reasonable'' is difficult to define, so CC Mode doesn't do it
A special note about
defun-prompt-regexp in Java mode: while much
of the early sample Java code seems to encourage a style where the brace
that opens a class is hung on the right side of the line, this is not a
good style to pursue in Emacs. CC Mode comes with a variable
c-Java-defun-prompt-regexp which tries to define a regular
expression usable for this style, but there are problems with it. In
some cases it can cause
beginning-of-defun to hang. For this reason,
it is not used by default, but if you feel adventurous, you can set
defun-prompt-regexp to it in your mode hook. In any event,
setting and rely on
defun-prompt-regexp will definitely slow
You will probably notice pathological behavior from CC Mode when working in files containing large amounts of C preprocessor macros. This is because Emacs cannot skip backwards over these lines as quickly as it can comment.
Previous versions of CC Mode had potential performance problems
when recognizing K&R style function argument declarations. This was
because there are ambiguities in the C syntax when K&R style argument
lists are used. CC Mode has adopted BOCM's convention for
limiting the search: it assumes that argdecls are indented at least one
space, and that the function headers are not indented at all. With
current versions of CC Mode, user customization of
c-recognize-knr-p is deprecated. Just don't put argdecls in
You might want to investigate the speed-ups contained in the
cc-lobotomy.el', which comes as part of the CC Mode
distribution, but is completely unsupported.
As mentioned previous, CC Mode always trades speed for accuracy,
however it is recognized that sometimes you need speed and can sacrifice
some accuracy in indentation. The file `
hacks that will ``dumb down'' CC Mode in some specific ways, making
that trade-off of accurancy for speed. I won't go into details of its
use here; you should read the comments at the top of the file, and look
at the variable
cc-lobotomy-pith-list for details.
 such as the output of
 e.g. a function in C, or outermost class definition in C++ or Java.
 Note that this variable is only defined in Emacs 19.
 This has been observed in Emacs 19.34 and XEmacs 19.15.
 It is hard to distinguish them from top-level declarations.Syntactic Symbols Top Frequently Asked Questions