IO not appropriate in the context of a sorting routine, and in any case the proper headers are not included, so gcc emits an undefined symbol error. Remove the "printf" statement. --- CombBLAS_beta_16_2/usort/include/usort/seqUtils.tcc +++ CombBLAS_beta_16_2/usort/include/usort/seqUtils.tcc @@ -142,7 +142,6 @@ } if ( (a[nmax]==a[nmin]) && (ctr==0) ) { - printf("All the numbers are identical, the list is sorted\n"); return; }