2. Numbers

//------------------------------------------------------------------
#include <stdlib.h> // rand() srand()

...

/*
    rand() function is not guaranted to produce random values
*/

int random;
  
// the same value between 0 and RAND_MAX
// for every run
random = rand();

...

//------------------------------------------------------------------
...

float random;

// the same value between 0 and 1.0
// for every run
random = (float) rand() / RAND_MAX;

...

//------------------------------------------------------------------
...

#define N 42

...

int random;

// the same value between 0 and N -1
// for every run
random = (int)( N * (double)rand() / (RAND_MAX + 1.0) );

...

//------------------------------------------------------------------
#include <stdlib.h> // rand() srand()
#include <time.h> // time()

...

int random;

// seeds a new sequence of random number
srand(time(NULL));

// a different value between 0 and RAND_MAX
// for every run
random = rand();

...

//------------------------------------------------------------------
/*
    under Linux /dev/random or /dev/urandom
*/

#include <stdio.h> // FILE *, fopen(), fread(), fclose()

...

FILE * fp;

unsigned int random;

if ((fp = fopen("/dev/random","r")) == NULL) {
  perror("fopen");
  exit(1);
}

fread(&random, sizeof(random), 1, fp);
printf("random number %u from /dev/random\n", random);

fclose(fp);

...

//------------------------------------------------------------------