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Can I use an #ifdef in a #define line, to define something two different ways, like this?
#define a b \
#ifdef whatever
c d
#else
e f g
#endif

✍: Guest

No. You can't ``run the preprocessor on itself,'' so to speak. What you can do is use one of two completely separate #define lines, depending on the #ifdef setting:

#ifdef whatever
#define a b c d
#else
#define a b e f g
#endif

2016-02-03, 1035👍, 0💬

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Can I use an ifdef in a #define line ...
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I a'm getting strange syntax errors inside lines I have ifdeffed out.
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Is there anything like an ifdef for typedefs?
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I mm compiling a program, and I seem to be missing one of the header files it requires
I mm compiling a program, and I seem to be missing one of the header files it requires There are several situations, depending on what sort of header file it is that's ``missing''. If the missing header file is truly a standard one (that is, one defined by the ANSI C Standard, such as <stdio.... 2016-02-05, 1001👍, 0💬

Does the sizeof operator work in preprocessor if directives?
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How can I list all of the predefined identifiers?
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How can I use a preprocessorif expression to ? ....
How can I use a preprocessor #if expression to tell whether a machine's byte order is big-endian or little-endian? You probably can't. The usual techniques for detecting endianness involve pointers or arrays of char, or maybe unions, but preprocessor arithmetic uses only long integers, and there is ... 2016-02-01, 977👍, 0💬

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