/* Upper and lower bounds of the partition, packed for easy passing into a thread */
struct bounds_s {
  int upper;
  int lower;
};
typedef struct bounds_s bounds_t;

...

bounds_t global_bounds = { 100, 1 }; /* This would normally come from command line opts */

...

/* This lives in a for loop.
   curr_thr iterates between 0 and total_thr-1.
   global_bounds is the entire partition this process will work on
   thread_bounds is to be populated with the boundaries of this thread's subdivision of the partition */
per_thread_bounds( curr_thr, total_thr, &global_bounds, &thread_bounds );

/* Work out how many subparitions to partition the global domain into,
   and what the bounds of this thread should be */
void per_thread_bounds( int curr_thr, int total_thr, bounds_t const *global_bounds, bounds_t *thread_bounds ) {

  /* How big is the total range we're dealing with? Divide it by the total number of threads
     to find out how big a partition to assign to each worker thread */
  int partition_size;
  partition_size = (int)((global_bounds->upper - global_bounds->lower)/total_thr)+1;

  /* Assign a partition's worth of range in the first instance, and tweak for
     special cases thereafter */
  thread_bounds->lower = global_bounds->lower;
  thread_bounds->upper = global_bounds->lower + ( partition_size - 1 );

  /* Special case: This is not the first CPU (i.e. not the first partition of range to assign
     and is not the last CPU -- so don't need to soak up the remainders to the end of the range */
  if( curr_thr > 0 ) {
    thread_bounds->lower += curr_thr * partition_size;
    thread_bounds->upper += curr_thr * partition_size;
  }
  /* Special case: This is the last thread, should soak up the rest of the partition */
  if( curr_thr == (total_thr-1) ) {
    thread_bounds->upper = global_bounds->upper;
  }

}

partition_size = (int)((global_bounds->upper - global_bounds->lower)/total_thr)+1;

partition_size = ((float)(global_bounds->upper - global_bounds->lower)/(float)total_cpus)+0.5;