New mechanism: we introduce new fields of struct stream - ifndef and dirty. The former is the nesting level of ifndef that currently protects us, or 0 if there's none. The latter is a boolean - "do we ignore new non-empty stuff shows up right now?". There are five states, more or less matching the old ->constant ones: 1) nothing - dirty = 0, ->protect = NULL, ->ifndef = 0. 2) candidate - dirty = 0, ->protect = <ident>, ->ifndef > 0. 3) checking - dirty = 0, ->protect = <ident>, ->ifndef = 0. 4) maybe - dirty = 1, ->protect = <ident>, ->ifndef > 0. 5) not - dirty = 1, ->protect = NULL, ->ifndef = 0. Rules: - any directive except #ifdef, #ifndef, #else, #endif, #<newline> is considered dirty. - any non-directive (#<not a directive name>...) is considered dirty - any tokens in lines that do not begin with # are considered dirty - #if[n]def with syntax errors is considered dirty. - any nesting errors => stream is non-constant. - any dirty material not under ifndef => stream is non-constant. - the outermost ifndef at the first time we run into dirty material will be the candidate for stream protector. - any subsequent dirty material not under ifndef <protector> => stream is non-constant. We start in "nothing". Transitions: - "nothing": dirty material => "not", see ifndef => "candidate" - "candidate": dirty material => "maybe", leave that ifndef => "nothing" - "maybe": dirty material => "maybe", leave that ifndef => "checking" - "checking": dirty material => "not", same ifndef => "maybe - "not": they check in, but they don't check out... Seeing a nesting error (#else/#elif/#endif out of place/order or unclosed #if... in the end) gets us to "not", no matter which state we were in. That's it. Implementation is actually pretty simple and straightforward; it's less scary than the description above. Signed-off-by: Al Viro <viro@zeniv.linux.org.uk> Signed-off-by: Linus Torvalds <torvalds@osdl.org>