C++ `srand()` function producing a pattern?

New to C++ and following the beginner's tutorial here. Refer to the section titled Random Numbers in C++. Using exactly the code given:

#include <iostream>
#include <ctime>
#include <cstdlib>

using namespace std;

int main () {
   int i,j;

   // set the seed
   srand( (unsigned)time( NULL ) );

   /* generate 10  random numbers. */
   for( i = 0; i < 10; i++ ) {
      // generate actual random number
      j = rand();
      cout <<" Random Number : " << j << endl;
   }

   return 0;
}

I'm seeding srand() with time() (compiled with g++) and so the generated result should be completely random. However, here's the result I'm getting:

$ ./a.out
 Random Number : 1028986599
 Random Number : 491960102
 Random Number : 561393364
 Random Number : 1442607477
 Random Number : 813491309
 Random Number : 1467533561
 Random Number : 986873932
 Random Number : 1373969343
 Random Number : 411091610
 Random Number : 761796871
$ ./a.out
 Random Number : 1029003406
 Random Number : 774435351
 Random Number : 36559790
 Random Number : 280067488
 Random Number : 1957600239
 Random Number : 1937744833
 Random Number : 1087901476
 Random Number : 684336574
 Random Number : 1869869533
 Random Number : 621550933
$ ./a.out
 Random Number : 1029020213
 Random Number : 1056910600
 Random Number : 1659209863
 Random Number : 1265011146
 Random Number : 954225522
 Random Number : 260472458
 Random Number : 1188929020
 Random Number : 2142187452
 Random Number : 1181163809
 Random Number : 481304995

As you can see from the first number generated on each ./a.out execution, the first number in the 10-loop is increasing on each execution. And it seems to always be about 1.02 million. Further testing suggests this pattern always holds and it's not a coincidence.

I can only assume that it's increasing due to the seed time(), which is always increasing. But that suggests the rand() function isn't truly random and is predictable.

Solution

But that suggests the rand() function isn't truly random and is predictable.

Yes, that's absolutely correct. Typically, rand is implement with a very simple pseudo-random number generator. The use of rand is not appropriate when numbers that are truly random or unpredictable are required.

Under the hood, your implementation probably uses a Linear congruential generator and your three examples are all inside the same linear interval, at least for the first output.