<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;
  }
}

您可以使用悬浮指针的概念。

指向已删除(或释放)内存位置的指针称为悬浮指针。

它将在打印时显示随机值。

这是一个在你选择的两个数字之间得到一个随机数的好方法。

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

    #define randnum(min, max) \
        ((rand() % (int)(((max) + 1) - (min))) + (min))

int main()
{
    srand(time(NULL));

    printf("%d\n", randnum(1, 70));
}

第一次输出:39

第二次输出:61

第三次输出:65

您可以将randnum后面的值更改为您选择的任何数字，它将在这两个数字之间为您生成一个随机数。

<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128个安全随机位，符合RFC 1750的解决方案是读取已知可以生成可用熵位的硬件源(例如旋转磁盘)。更好的是，好的实现应该使用混合函数组合多个源，并最终通过重新映射或删除输出来消除输出分布的倾斜。

如果你需要更多的比特，你需要做的就是从128个安全随机比特的序列开始，并将其拉伸到所需的长度，将其映射到人类可读的文本等等。

如果你想在C中生成一个安全的随机数，我将遵循这里的源代码:

https://wiki.sei.cmu.edu/confluence/display/c/MSC30-C.+Do+not+use+the+rand%28%29+function+for+generating+pseudorandom+numbers

注意，对于Windows bbcryptgenrandom是使用的，而不是CryptGenRandom，在过去的20年里已经变得不安全。您可以亲自确认BCryptGenRandom符合RFC 1750。

For POSIX-compliant operating systems, e.g. Ubuntu (a flavor of Linux), you can simply read from /dev/urandom or /dev/random, which is a file-like interface to a device that generates bits of entropy by combining multiple sources in an RFC 1750 compliant fashion. You can read a desired number of bytes from these "files" with read or fread just like you would any other file, but note that reads from /dev/random will block until a enough new bits of entropy are available, whereas /dev/urandom will not, which can be a security issue. You can get around that by checking the size of the available entropy pool, either my reading from entropy_avail, or by using ioctl.

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;
    }