#if defined(_OPENMP)
#include <omp.h>
#include <parallel/algorithm>
#else
#include <algorithm>
#endif

...

static void get_properties(
   int                nfiles,      //  in: the number of input files
   char*              fname[],     //  in: the input file names
   vec_int_str_type&  vec_ret)     // out: a vector of properties
{
#if defined(_OPENMP)
   omp_set_dynamic(0);
   omp_set_max_active_levels(1);

   #pragma omp parallel
   {
      vec_int_str_type vec_loc;    // thread local copy

      #pragma omp for nowait schedule(static,1)
#endif
      for (int i = 0; i < nfiles; ++i) {
         char line[MAX_LINE_LEN_L+1];
         char* found;
         llil_int_type count;
         FILE* fh = ::fopen(fname[i], "r");
         if (fh == NULL) {
            std::cerr << "Error opening '" << fname[i] << "' : errno=" << errno << "\n";
            continue;
         }
         while ( ::fgets(line, MAX_LINE_LEN_L, fh) != NULL ) {
            found = ::strchr(line, '\t');
            count = ::atoll( &line[found - line + 1] );
            line[found - line] = '\0';  // word
#ifdef MAX_STR_LEN_L
            str_type fixword { { '\0', '\0', '\0', '\0', '\0', '\0' } };
            ::memcpy( fixword.data(), line, found - line );
           #if defined(_OPENMP)
            vec_loc.emplace_back( -count, fixword );
           #else
            vec_ret.emplace_back( -count, fixword );
           #endif
#else
           #if defined(_OPENMP)
            vec_loc.emplace_back( -count, line );
           #else
            vec_ret.emplace_back( -count, line );
           #endif
#endif
         }
         ::fclose(fh);
      }

#if defined(_OPENMP)
      #pragma omp critical
      vec_ret.insert(vec_ret.end(), vec_loc.begin(), vec_loc.end());
   }
#endif

   // Needs to be sorted by word for later sum of adjacent count fields to work
#if defined(_OPENMP)
   __gnu_parallel::sort( vec_ret.begin(), vec_ret.end(),
      [](const int_str_type& left, const int_str_type& right) { return left.second < right.second; }
   );
#else
   std::sort( vec_ret.begin(), vec_ret.end(),
      [](const int_str_type& left, const int_str_type& right) { return left.second < right.second; }
   );
#endif
}