unix - C - How to ENSURE different random number generation in C when program is executed within the same second? -

for assignment, supposed ensure that, when execute program within same second, should return different numbers. however, i've read other posts , can't quite figure out how within same second. if run code:

int main() {         srand(time(null));      for(count = 0; count < 10; count++)     {         printf("%d\n", rand()%20 + 1);     }      return 0; } 

i end same result when executed within same second. know how mix results within same second? i'm operating in unix environment if makes difference. thanks.

to prevent 2 programs generating same pseudo-random numbers rand(), must, @ minimum use different seeds srand().

the source seeds 2 runs of program derived either

1) same source - mechanism unique generation.
2) random source , chance of same seed generation tolerable low.

1a #1, time() used, definition, programs start in same second, simplistic use of fails.

1b attempting create file both program access write "i started seed - 12345, if generate seed try again".

1c approach use finer time hinted (@will) - that's better finer resolution may not enough.

2a platforms provide random number via system function call, since depends on various esoteric system events , slow compared rand(), useful seeding srand(). not systems provide this.
[edit] did not see unix tag till later.
/dev/random & /dev/urandom (@dietrich epp) provide feature.

2b 1 can use variant human reaction time like

printf("press enter\n"); unsigned u = 0; while (!keyboard_hit()) u++; srand(u); 

both: combining (via exclusive-or ^) various sources pid(), time(), reaction_time(), etc. helps. (see @nodakai)

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even employing various mechanism prevent different seeding of random number generator, 2 different runs still generate same sequence: 1 in 20^10 (10,240,000,000,000) times per op's code. after these random numbers, coming same sequence happen.

to absolutely prevent duplication, 2 program must communicate - @ least in 1 direction. maybe ever program first write common file, sequence generated, next inspect , insure sequence makes different.

// pseudo code n = 1; repeat {   srand(time()^n^pid());   n++;   generate_random_number_sequence();   attempt exclusive r/w access shared file.   if (file opened) {     read file;     if (different sequence) {       write new sequence , fclose()       if (no i/o errors) {         done - exit       }     }     fclose()   }   maybe sleep fraction of second   maybe quit if repeated }