2.2 Conditional Compilation
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.