What is the best way to know if a value is defined in an enum?

I read that it is difficult to find out if an element is in an enumeration. So what would be the best way ?

For example, the following code comes from the Linux kernel 2.6.32:

enum v4l2_colorfx {
    V4L2_COLORFX_NONE    = 0,
   开发者_开发知识库 V4L2_COLORFX_BW      = 1,
    V4L2_COLORFX_SEPIA   = 2,
};

And this one from the 2.6.38 version:

enum v4l2_colorfx {
    V4L2_COLORFX_NONE        = 0,
    V4L2_COLORFX_BW          = 1,
    V4L2_COLORFX_SEPIA       = 2,
    V4L2_COLORFX_NEGATIVE    = 3,
    V4L2_COLORFX_EMBOSS      = 4,
    V4L2_COLORFX_SKETCH      = 5,
    V4L2_COLORFX_SKY_BLUE    = 6,
    V4L2_COLORFX_GRASS_GREEN = 7,
    V4L2_COLORFX_SKIN_WHITEN = 8,
    V4L2_COLORFX_VIVID       = 9,
};

How would you check if V4L2_COLORFX_NEGATIVE is defined ? Would #ifndef V4L2_COLORFX_NEGATIVE be okay ?

You would have to look at a compiler macro in the wider context (for example the version of linux, I don't know what's available) or some other piece of information at compile time. ifndef is for checking if compiler macros are defined, not symbols in code.

Check the version of linux in /usr/include/linux/version.h ( you need to install kernel headers though )

it contains something like :

#define LINUX_VERSION_CODE 132640
#define KERNEL_VERSION(a,b,c) (((a) << 16) + ((b) << 8) + (c))

So you can use this :

#if LINUX_VERSION_CODE >= KERNEL_VERSION( 2, 6, 38 )