<stdlib.h>中的rand()函数返回一个介于0和RAND_MAX之间的伪随机整数。你可以使用srand(unsigned int seed)来设置种子。

通常的做法是将%操作符与rand()结合使用以获得不同的范围(但请记住，这在一定程度上破坏了一致性)。例如:

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

如果你真的在乎一致性，你可以这样做:

/* 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;
  }
}

如果您需要比stdlib提供的质量更好的伪随机数，请检查Mersenne Twister。它也更快。示例实现有很多，例如这里。

如果你需要安全的随机字符或整数:

正如在如何在各种编程语言中安全地生成随机数中提到的，你会想要做以下事情之一:

使用libsodium的randombytes API 重新实现你自己需要的libsodium的sysrandom实现，非常小心 更广泛地说，使用/dev/urandom，而不是/dev/random。而不是OpenSSL(或其他用户空间prng)。

例如:

#include "sodium.h"

int foo()
{
    char myString[32];
    uint32_t myInt;

    if (sodium_init() < 0) {
        /* panic! the library couldn't be initialized, it is not safe to use */
        return 1; 
    }


    /* myString will be an array of 32 random bytes, not null-terminated */        
    randombytes_buf(myString, 32);

    /* myInt will be a random number between 0 and 9 */
    myInt = randombytes_uniform(10);
}

Randombytes_uniform()是加密安全且无偏倚的。

<stdlib.h>中的rand()函数返回一个介于0和RAND_MAX之间的伪随机整数。你可以使用srand(unsigned int seed)来设置种子。

通常的做法是将%操作符与rand()结合使用以获得不同的范围(但请记住，这在一定程度上破坏了一致性)。例如:

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

如果你真的在乎一致性，你可以这样做:

/* 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;
  }
}

#include<stdio.h>
#include<stdlib.h>
#include<time.h>

//generate number in range [min,max)
int random(int min, int max){
    int number = min + rand() % (max - min);
    return number; 
}

//Driver code
int main(){
    srand(time(NULL));
    for(int i = 1; i <= 10; i++){
        printf("%d\t", random(10, 100));
    }
    return 0;
}

我们来看看这个。首先，我们使用srand()函数来为随机化器播种。基本上，计算机可以根据输入给srand()的数字生成随机数。如果你给出相同的种子值，那么每次都会生成相同的随机数。

因此，我们必须用一个总是在变化的值来给随机化器播种。我们通过time()函数将当前时间的值提供给它来实现这一点。

现在，当我们调用rand()时，每次都会产生一个新的随机数。

    #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;
    }