c - 调用该函数时显示相同的随机数

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我创建了一个方法，根据生成的随机数的值返回一个字符串。当函数位于调用它的同一程序文件中时，这将按预期工作。

但是当我在新的头文件中定义此方法，然后在不同的文件中调用它时，它会返回相同的随机数。

定义方法:

char* newTrafficGenerator(void)
{
    int i;
    int count =0;
    static char currentTrafficType[6];
    i = random_rand();
    printf("Current random no is: %d\n",i);
    if (i>0 && i<32767)
    {
        strncpy(currentTrafficType,"Type A",sizeof(currentTrafficType));
    }
    else if(i>32767 && i<55705)
    {
        strncpy(currentTrafficType,"Type B",sizeof(currentTrafficType));
    }
    else
    {
        strncpy(currentTrafficType,"Type C",sizeof(currentTrafficType));
    }   
    return currentTrafficType;
}

通过以下代码调用:

static void generateInfiniteRandomNumber(void)
{
    int count=0;
    char *c;
    while(count<10)
    {
        c = newTrafficGenerator();
        printf("Type is %s\n",c);
        count++;
    }
}

在同一文件中调用时的输出为:

Current random no is: 18547
Type is Type A
Current random no is: 56401
Type is Type C
Current random no is: 23807
Type is Type A
Current random no is: 37962
Type is Type B
Current random no is: 22764
Type is Type A
Current random no is: 7977
Type is Type A
Current random no is: 31949
Type is Type A
Current random no is: 22714
Type is Type A
Current random no is: 55211
Type is Type B
Current random no is: 16882
Type is Type A

当在新的头文件中定义并调用时输出为:

Current random no is: 1920540202
Type is Type C
Current random no is: 1920540202
Type is Type C
Current random no is: 1920540202
Type is Type C
Current random no is: 1920540202
Type is Type C
Current random no is: 1920540202
Type is Type C
Current random no is: 1920540202
Type is Type C
Current random no is: 1920540202
Type is Type C
Current random no is: 1920540202
Type is Type C
Current random no is: 1920540202
Type is Type C
Current random no is: 1920540202
Type is Type C

Answer 1

@wildplasser 是对的，你的程序中有未定义的行为。在解决该问题之前，所有有关播种或不播种的讨论都是无用的。您需要在 currentTrafficType 中为空终止符留出空间:

static char currentTrafficType[7];

当可以避免时，将返回值放在静态局部变量中通常是不好的做法。它使您的函数不可重入且线程不安全。在您的情况下，您想要返回的数据已经作为字符串文字存在，因此您可以删除整个数组:

/* return type modified from `char *` to `const char *` */
const char* newTrafficGenerator(void)
{
    int i;
    int count =0;

    /* Use pointer rather than array */
    const char *currentTrafficType;

    i = random_rand();
    printf("Current random no is: %d\n",i);
    if (i>0 && i<32767)
    {
        currentTrafficType = "Type A";
    }
    else if(i>32767 && i<55705)
    {
        currentTrafficType = "Type B";
    }
    else
    {
        currentTrafficType = "Type C";
    }   
    return currentTrafficType;
}

字符串文字保证在程序的整个运行期间都存在，因此我们可以通过指针值传递它们，而不是进行缓慢而繁琐的数组复制。请注意，字符串文字是只读的，因此只能分配给 const char * 而不是 char *。