2.2 Conditional Compilation
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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.