preprocessor directives

[rahul8143]

hello,
how to interprete following #if in code? e.g. if i have a code
like
#if 0
somefunc();
else
somefunc();
#endif

also what is meaning of
#if !defined(__ENVVAR__)?

 

[Mike Wahler]

hello,
how to interprete following #if in code? e.g. if i have a code
like
#if 0
somefunc();
else
#else

somefunc();
#endif

The above (after my correction) means that only
the second call to 'somefunc()' (after the #else)
will become part of the translation unit. The
first call is excluded.

also what is meaning of
#if !defined(__ENVVAR__)?

It means that if, at this point, the preprocessor has not
encountered a definition of '__ENVVAR__', then everything
after the #if, up to the next #endif, #else, or #elif
will become part of the translation unit. If a definition
of '__ENVVAR' was already encountered, the inverse would
be true (everything between the #if and the next #endif,
#else, or #elif would be excluded).

This is all basic stuff. Which C textbook(s) are you
reading?

-Mike

 

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[Martin Ambuhl]

hello,
how to interprete following #if in code? e.g. if i have a code
like
#if 0

if 0 is non-zero (never), use the lines that follow up to the #endif,
#else, or #elif
(This is the normal way to comment out code, for example.)

somefunc();
else

'else' here means "please abort the compilation or at least warn about
this error".

somefunc();
#endif

also what is meaning of
#if !defined(__ENVVAR__)

['?' deleted from the above]

if __ENVVAR__ is not defined, use the lines that follow up to the
#endif, #else, or #elif

 

[Kuku]

it means, if FALSE then execute somefunc() otherwise the second
somefunc().

 

[Eric Sosman]

if 0 is non-zero (never), use the lines that follow up to the #endif,
#else, or #elif
(This is the normal way to comment out code, for example.)



'else' here means "please abort the compilation or at least warn about
this error".

Why? The material between `#if 0' and `#endif' must
be a valid stream of tokens, but need not be valid C.