Test for GNU extension

Question

I want to make a macro to return the real part of a complex number (which will work with double, float and long double types). The GNU C extension __real__ seems to fit the bill (although it is not portable unfortunately). I am trying the following:

#include <complex.h>
#if defined(__real__)
#define MYREAL(z) (__real__ z)
#endif

However it seems that the __real__ extension is not defined as a usual macro, so the defined(__real__) test fails, even though it is available. Does anyone know how to test for the existence of __real__ to make a proper macro for this?

Also, if anyone knows of a portable way to do this, I'd be interested in that solution as well.

Where does `__real__` come from? Maybe that header defines an include-once guard. — tadman, Dec 18 '20 at 05:04
[`creal()`](https://en.cppreference.com/w/c/numeric/complex/creal) should handle whatever you throw at it. — tadman, Dec 18 '20 at 05:05
It's perhaps not a dup, but [this question](https://stackoverflow.com/q/54079257/1270789) says it cannot easily be done. — Ken Y-N, Dec 18 '20 at 05:06
creal() is defined for double, there are other functions (crealf, creall) for float and long double — vibe, Dec 18 '20 at 05:27

score 4 · Accepted Answer · answered Dec 18 '20 at 05:32

4

Per manual:

To test for the availability of these features in conditional compilation, check for a predefined macro __GNUC__, which is always defined under GCC.

Hence:

#ifdef __GNUC__

#define MYREAL(z) (__real__(z))

#endif

answered Dec 18 '20 at 05:32

selbie

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score 3 · Answer 2 · answered Dec 18 '20 at 06:59

3

Also, if anyone knows of a portable way to do this, I'd be interested in that solution as well.

That would be the creal() macro in <tgmath.h>, which works for all complex types.

answered Dec 18 '20 at 06:59

Shawn

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Test for GNU extension

2 Answers2