How to generate a random int in C?

Question

Is there a function to generate a random int number in C? Or will I have to use a third party library?

Answer 1

If your system supports the arc4random family of functions I would recommend using those instead the standard rand function.

The arc4random family includes:

uint32_t arc4random(void)
void arc4random_buf(void *buf, size_t bytes)
uint32_t arc4random_uniform(uint32_t limit)
void arc4random_stir(void)
void arc4random_addrandom(unsigned char *dat, int datlen)

arc4random returns a random 32-bit unsigned integer.

arc4random_buf puts random content in it's parameter buf : void *. The amount of content is determined by the bytes : size_t parameter.

arc4random_uniform returns a random 32-bit unsigned integer which follows the rule: 0 <= arc4random_uniform(limit) < limit, where limit is also an unsigned 32-bit integer.

arc4random_stir reads data from /dev/urandom and passes the data to arc4random_addrandom to additionally randomize it's internal random number pool.

arc4random_addrandom is used by arc4random_stir to populate it's internal random number pool according to the data passed to it.

If you do not have these functions, but you are on Unix, then you can use this code:

/* This is C, not C++ */
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <errno.h>
#include <unistd.h>
#include <stdlib.h> /* exit */
#include <stdio.h> /* printf */

int urandom_fd = -2;

void urandom_init() {
  urandom_fd = open("/dev/urandom", O_RDONLY);

  if (urandom_fd == -1) {
    int errsv = urandom_fd;
    printf("Error opening [/dev/urandom]: %i\n", errsv);
    exit(1);
  }
}

unsigned long urandom() {
  unsigned long buf_impl;
  unsigned long *buf = &buf_impl;

  if (urandom_fd == -2) {
    urandom_init();
  }

  /* Read 4 bytes, or 32 bits into *buf, which points to buf_impl */
  read(urandom_fd, buf, sizeof(long));
  return buf_impl;
}

The urandom_init function opens the /dev/urandom device, and puts the file descriptor in urandom_fd.

The urandom function is basically the same as a call to rand, except more secure, and it returns a long (easily changeable).

However, /dev/urandom can be a little slow, so it is recommended that you use it as a seed for a different random number generator.

If your system does not have a /dev/urandom, but does have a /dev/random or similar file, then you can simply change the path passed to open in urandom_init. The calls and APIs used in urandom_init and urandom are (I believe) POSIX-compliant, and as such, should work on most, if not all POSIX compliant systems.

Notes: A read from /dev/urandom will NOT block if there is insufficient entropy available, so values generated under such circumstances may be cryptographically insecure. If you are worried about that, then use /dev/random, which will always block if there is insufficient entropy.

If you are on another system(i.e. Windows), then use rand or some internal Windows specific platform-dependent non-portable API.

Wrapper function for urandom, rand, or arc4random calls:

#define RAND_IMPL /* urandom(see large code block) | rand | arc4random */

int myRandom(int bottom, int top){
    return (RAND_IMPL() % (top - bottom)) + bottom;
}

Answer 2

The rand() function in <stdlib.h> returns a pseudo-random integer between 0 and RAND_MAX. You can use srand(unsigned int seed) to set a seed.

It's common practice to use the % operator in conjunction with rand() to get a different range (though bear in mind that this throws off the uniformity somewhat). For example:

/* random int between 0 and 19 */
int r = rand() % 20;

If you really care about uniformity you can do something like this:

/* Returns an integer in the range [0, n).
 *
 * Uses rand(), and so is affected-by/affects the same seed.
 */
int randint(int n) {
  if ((n - 1) == RAND_MAX) {
    return rand();
  } else {
    // Supporting larger values for n would requires an even more
    // elaborate implementation that combines multiple calls to rand()
    assert (n <= RAND_MAX)

    // Chop off all of the values that would cause skew...
    int end = RAND_MAX / n; // truncate skew
    assert (end > 0);
    end *= n;

    // ... and ignore results from rand() that fall above that limit.
    // (Worst case the loop condition should succeed 50% of the time,
    // so we can expect to bail out of this loop pretty quickly.)
    int r;
    while ((r = rand()) >= end);

    return r % n;
  }
}

Answer 3

If you need better quality pseudo random numbers than what stdlib provides, check out Mersenne Twister. It's faster, too. Sample implementations are plentiful, for example here.

Answer 4

Well, STL is C++, not C, so I don't know what you want. If you want C, however, there is the rand() and srand() functions:

int rand(void);

void srand(unsigned seed);

These are both part of ANSI C. There is also the random() function:

long random(void);

But as far as I can tell, random() is not standard ANSI C. A third-party library may not be a bad idea, but it all depends on how random of a number you really need to generate.

Answer 5

Lets go through this. First we use the srand() function to seed the randomizer. Basically, the computer can generate random numbers based on the number that is fed to srand(). If you gave the same seed value, then the same random numbers would be generated every time.

Therefore, we have to seed the randomizer with a value that is always changing. We do this by feeding it the value of the current time with the time() function.

Now, when we call rand(), a new random number will be produced every time.

#include <stdio.h>

int random_number(int min_num, int max_num);

int main(void)
{
    printf("Min : 1 Max : 40 %d\n", random_number(1,40));
    printf("Min : 100 Max : 1000 %d\n",random_number(100,1000));
    return 0;
}

int random_number(int min_num, int max_num)
{
    int result = 0, low_num = 0, hi_num = 0;

    if (min_num < max_num)
    {
        low_num = min_num;
        hi_num = max_num + 1; // include max_num in output
    } else {
        low_num = max_num + 1; // include max_num in output
        hi_num = min_num;
    }

    srand(time(NULL));
    result = (rand() % (hi_num - low_num)) + low_num;
    return result;
}

Answer 6

FWIW, the answer is that yes, there is a stdlib.h function called rand; this function is tuned primarily for speed and distribution, not for unpredictability. Almost all built-in random functions for various languages and frameworks use this function by default. There are also "cryptographic" random number generators that are much less predictable, but run much slower. These should be used in any sort of security-related application.