Preprocessor Macros

Some common preprocessor macros

Include Guard

For most modern compilers (MS Visual C++, GCC, Clang/llvm), you can just add a #pragma once at the top.
Note that this is not defined in the standard.
See Wikipedia: pragma once#Portability to see a list of compilers which support this.

For older compilers:

#ifndef GRANDPARENT_H
#define GRANDPARENT_H
... contents of grandparent.h
#endif /* !GRANDPARENT_H */

GLSL

#define PI 3.141592653589793
#define PI_2 1.5707963267948966
#define DEG2RAD 0.017453292519943295
#define RAD2DEG 57.29577951308232

#define sign(x) (int((x) > 0) - int((x) < 0))
#define imin(x, y) (int(x) * int(int(x) < int(y)) + int(y) * int(int(x) >= int(y)))
#define imax(x, y) (int(x) * int(int(x) > int(y)) + int(y) * int(int(x) <= int(y)))
#define iabs(x) ((x) * int((x) >= 0) - (x) * int((x) < 0))