opengl - GLEW - why should I define GLEW_STATIC?

Question

I am using a statically linked library for GLEW and therefore I have to define GLEW_STATIC to compile my programs that are using GLEW. But I have never understood why this is necessary. So could anyone explain the reason?

Answer 1

When a function is implemented in a static library that must be specified. The extern keyword provides linkage information. A function which is imported from a dynamic library has to be marked by extern __declspec(dllimport). GLEW_STATIC is a preprocessor definition which activates the first case.

The relevant code part in the "glew.h" file (verison 2.1.0) is:

/*
 * GLEW_STATIC is defined for static library.
 * GLEW_BUILD  is defined for building the DLL library.
 */

#ifdef GLEW_STATIC
  define GLEWAPI extern
#else
  ifdef GLEW_BUILD
    define GLEWAPI extern __declspec(dllexport)
  else
    define GLEWAPI extern __declspec(dllimport)
  endif
#endif