Export a predefined constant to an application

I am writing a thread migration system in C. And I am not able to make a feature in my package backward compatible, so I am thinking of predefining a constant like MYPACK_VER so that an application of my package can do this:
#if MYPACK_VER > 3.0
use the new format of a feature
#else
use the old format of a feature
#endif
So it is somewhat similiar to what a compiler would do - for example, GCC defines a VERSION macro so that I guess if a C programmer wants to use a feature that is only provided after GCC version 3.4.4, he can enclose that feature with #if VERSION>3.4.4 / #endif.
How can I do that in my thread migration system?

I'll try to answer your question assuming that what you're asking can be rephrazed as:

I am updating a thread migration system in C, and am not able to make a specific feature in my package backward compatible. I'm thinking of doing this by using a defined constant such as "MYPACK_VER" so that an application using my package can do the following:

Code:

#if MYPACK_VER > 3.0
use the new format of the feature
#else
use the old format of the feature
#endif

Given this interpretation of your question, the answer is quite simple -- define the symbol MYPACK_VER in the header file for yor package, as in:

Code:

#define MYPACK_VER 3.1 /* versions 3.0 and lower do it the old way */

Applications that depend on the library can then do something like the following:

Code:

#include "mypack.h"    /* use the thread migration package */
#ifndef MYPACK_VER
#define MYPACK_VER 1 /* If the version is not defined, it's an early version */
#endif
...
#if MYPACK_VER > 3.0
use new facility
#else
use old facility
#endif

Does this help?