#if with no expression
Guest
#1413782
Hi, I've got the following code fragment and getting the error as indicated. I have no clue why it doesn't work, it seems to be alright for me... Google didn't help either, just tons of kernel compilation bug reports etc. but no solution. Any help is appreciated!
Guest
#1413789
never heard of something called "elif". I just know "elsif". Try that
Guest
#1413808
#elif is not the same as a combination of #else and #ifdef, but rather #else and #if. And #if requires an expression, which is missing if the macro isn't defined.
Guest
#1413809
This expression equals to
but since you only defined WITHB (without a value) it is expanded to 0, which equals FALSE. Try either to define WITHB with a value or change your if-statement as follows:
Regards, Ralf
Guest
#1413812
> ... which is missing if the macro isn't defined.
Actually I meant "which is missing if the maco is defined, but empty".
If it's not defined, that's ok.
So try:
#define WITHB 1
Guest
#1413823
Wow, thats quick :-) I tried #elsif before, but that resulted in some weired behaviour. Thanks to Ralf and Rolf's solution it works as intended now! Thanks very much for your help!
Rolf Magnus wrote: > #elif is not the same as a combination of #else and #ifdef, but rather > #else and #if. And #if requires an expression, which is missing if the > macro isn't defined. Rolf makes a good point, but a better solution is to use the improved preprocessor syntax introduced in ANSI C in 1989! Despite its 20 year history, this K&R syntax is still prevalent; I have no idea why. This is what you need (already suggested by Stefan Ernst rather more succinctly and without the rant):
That way you never need to assign a value to the macros; their mere existence is enough. I have no idea what "Guest" was talking about; in what language is #elsif valid I wonder!? Certainly non-standard, and not documented in the GNU C preprocessor: http://gcc.gnu.org/onlinedocs/cpp/Index-of-Directives.html#Index-of-Directives ReplyPlease log in before posting. |