Typedefs, defined constants and macros, and function declarations may change between versions. A client is always written to a single, specific API version, and thus expects the header files to define everything according to that API. Failure of a header file to define values correctly will result in either compiler warnings (e.g. if the pointer type of a function argument changes) or fatal errors (e.g. if the number of arguments to a function changes, or the fields of a structure change). For example, in VERSION_1, kadm5_get_policy took a pointer to a pointer to a structure, and in VERSION_2 it takes a pointer to a structure; that would generate a warning if not correct. In VERSION_1, kadm5_randkey_principal accepted three arguments but in VERSION_2 accepts four; that would generate a fatal error.
The header file defines everything correctly based on the value of the USE_KADM5_API_VERSION constant. The constant can be assigned to an integer corresponding to any supported API version, and defaults to the newest version. The header files then simply use an #ifdef to include the right definitions:
#if USE_KADM5_API_VERSION == 1 kadm5_ret_t kadm5_get_principal(void *server_handle, krb5_principal principal, kadm5_principal_ent_t *ent); #else kadm5_ret_t kadm5_get_principal(void *server_handle, krb5_principal principal, kadm5_principal_ent_t ent, long mask); #endif