I inherited some code which contains far too many ...

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I inherited some code which contains far too many #ifdef's for my taste. How can I preprocess the code to leave only one conditional compilation set, without running it through the preprocessor and expanding all of the #include's and #define's as well?

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There are programs floating around called unifdef, rmifdef, and scpp (``selective C preprocessor'') which do exactly this

2016-01-29, 1134👍, 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, 1190👍, 0💬

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, 1177👍, 0💬

I a'm getting strange syntax errors inside lines I have ifdeffed out.
I a'm getting strange syntax errors inside lines I have ifdeffed out. Under ANSI C, the text inside a ``turned off'' #if, #ifdef, or #ifndef must still consist of ``valid preprocessing tokens.'' This means that the characters " and ' must each be paired just as in real C code, and the pairs mustn't ... 2016-01-29, 1177👍, 0💬

How can I list all of the predefined identifiers?
How can I list all of the predefined identifiers? There's no standard way, although it is a common need. gcc provides a -dM option which works with -E, and other compilers may provide something similar. If the compiler documentation is unhelpful, the most expedient way is probably to extract printab... 2016-01-27, 1160👍, 0💬

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, 1159👍, 0💬

How can I use a macro argument inside a string literal in the macro expansion?
How can I use a macro argument inside a string literal in the macro expansion? Some pre-ANSI compilers/preprocessors interpreted macro definitions like #define TRACE(var, fmt) printf("TRACE: var = fmt\n", var) such that invocations like TRACE(i, %d); were expanded as printf("TRACE: i = %d\n", i); In... 2016-01-22, 1158👍, 0💬

I inherited some code which contains far too many ...
I inherited some code which contains far too many #ifdef's for my taste. How can I preprocess the code to leave only one conditional compilation set, without running it through the preprocessor and expanding all of the #include's and #define's as well? There are programs floating around called unifd... 2016-01-29, 1135👍, 0💬

I have some old code that tries to construct identifiers with a macro like ...
I have some old code that tries to construct identifiers with a macro like #define Paste(a, b) a/**/b but it doesn't work any more. It was an undocumented feature of some early preprocessor implementations (notably Reiser's) that comments disappeared entirely and could therefore be used for token pa... 2016-01-27, 1122👍, 0💬

I have an old macro that doesn't seem to work any more....
I have an old macro #define CTRL(c) ('c' & 037) that doesn't seem to work any more. The intended use of this macro is in code like tchars.t_eofc = CTRL(D); which is expected to expand to tchars.t_eofc = ('D' & 037); based on the assumption that the actual value of the parameter c wil... 2016-01-25, 1080👍, 0💬

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