The _OPENMP macro name is defined by OpenMP-compliant implementations as the decimal constant yyyymm, which will be the year and month of the approved specification. This macro must not be the subject of a #define or a #undef preprocessing directive.

#ifdef _OPENMP  
iam = omp_get_thread_num() + index;  
#endif  

If vendors define extensions to OpenMP, they may specify additional predefined macros.