I observed that rand() library function when it is called just once within a loop, it almost always produces positive numbers.
for (i = 0; i <
May be you could try rather a tricky approach by ensuring that the value returned by sum of 2 rand() never exceeds the value of RAND_MAX. A possible approach could be sum = rand()/2 + rand()/2; This would ensure that for a 16 bit compiler with RAND_MAX value of 32767 even if both rand happens to return 32767, even then (32767/2 = 16383) 16383+16383 = 32766, thus would not result in negative sum.