#if with no expression

Gast 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!


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#define XYZ

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#define WITHB

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#ifdef XYZ

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 #ifdef WITHA

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        if (count < 1000) {

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            save[count] = (short)IP;

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            count++;

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        }

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 #elif WITHB                           //<-- ERROR: #if with no expression

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        if (count < 1000) {

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            save[count] = temp;

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            count++;

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        }

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 #endif

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#endif
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Rolf Magnus 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.
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Ralf Guest #1413809 
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 #elif WITHB                           //<-- ERROR: #if with no expression

This expression equals to

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 #else if WITHB == TRUE                          //<-- ERROR: #if with no expression


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:

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 #else ifdef WITHB


Regards,

Ralf
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Rolf Magnus 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
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Gast 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!


1
#define WITHB 1
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Clifford S. #1414276 
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):
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#define XYZ

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#define WITHB

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#if defined XYZ

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 #if defined WITHA

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        if (count < 1000) {

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            save[count] = (short)IP;

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            count++;

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        }

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 #elif defined WITHB

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        if (count < 1000) {

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            save[count] = temp;

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            count++;

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        }

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 #endif

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#endif


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
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