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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, 1619👍, 0💬

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I am using header files which accompany two different third-party libraries...
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Does the sizeof operator work in preprocessor if directives?
Does the sizeof operator work in preprocessor if directives? No. Preprocessing happens during an earlier phase of compilation, before type names have been parsed. Instead of sizeof, consider using the predefined constants in ANSI's <limits.h>, if applicable, or perhaps a ``configure'' script.... 2016-02-03, 1627👍, 0💬

Can I use an ifdef in a #define line ...
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 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 s... 2016-02-03, 1620👍, 0💬

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, 1599👍, 0💬

I am including the right header file for the library function I am using, but the linker keeps saying its undefined.
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Is there anything like an ifdef for typedefs?
Is there anything like an ifdef for typedefs? Unfortunately, no. (There can't be, because types and typedefs haven't been parsed at preprocessing time.) You may have to keep sets of preprocessor macros (e.g. MY_TYPE_DEFINED) recording whether certain typedefs have been declared. 2016-02-01, 1573👍, 0💬

How can I list all of the predefined identifiers?
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I a'm getting strange syntax errors inside lines I have ifdeffed out.
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