The rand() function computes a sequence of pseudo-random integers in the range of 0 to RAND_MAX
(as defined by the header file  stdlib.h).

The srand() function sets its argument seed as the seed for a new sequence of pseudo-random num-bers to be returned by rand().  
These sequences are repeatable by calling srand() with the same seed value.

If no seed value is provided, the functions are automatically seeded with a value of 1.

The sranddev() function initializes a seed, using the random random number device which returns good random numbers.  
However, the rand() function still remains unsuitable for cryptographic use.

The rand_r() function provides the same functionality as rand().  
A pointer to the context value seed must be supplied by the caller.
     


#include "stdio.h"
#include "stdlib.h"

int main(int argc, char *argv[]){
  if (argc != 2){
    printf("seed is needed\n");
    exit(EXIT_FAILURE);
  }
  
  int seed = atoi(argv[1]);
  printf("seed=%d\n",seed);
  int num = 7;
  for(int i=0;i < num;i++){
      printf("%d\n",rand()); 
  }
  printf("\n\n");
  srand(seed);
  for(int i=0;i < num;i++){
      printf("%d\n",rand()); 
  }
  
  printf("\n\n");
  
  seed = seed+1;
  printf("seed=%d\n",seed);
  for(int i=0;i < num;i++){
      printf("%d \n",rand_r(&seed)); 
  }
   
  return 0;
}