c - error : conflicting types, 错误:形参2的类型不完整

Question

I'm writing a code that calculates the difference in days between 2 given dates. Note that each month is considered to be equal to 30 days and each year equal to 360 days.

I'm getting the following warning/errors and I don't seem to understand why:

Warning&Errors

这是我的代码:

#include <stdio.h>

int dif_days(struct Date Date1, struct Date Date2);

struct Date
{
    int Day;
    int Month;
    int Year;
};

int main()
{
    struct Date Date1;
    struct Date Date2;

    printf("\n Please enter the first date: ");
    scanf("%d %d %d ", &Date1.Day, &Date1.Month, &Date1.Year);

    printf("\n Please enter the second date: ");
    scanf("%d %d %d ", &Date2.Day, &Date2.Month, &Date2.Year);

    int diff = dif_days(Date1, Date2);
    printf("\n The difference in days is: %d \n", diff);

    return 0;
}

int dif_days(struct Date Date1, struct Date Date2)
{
    // variable declaration 
    int difference;
    int Day, Month, Year; // The final days/months/years

    // for the days
    if (Date1.Day > Date2.Day)
        Day = Date1.Day - Date2.Day;
    else    
        Day = Date2.Day - Date1.Day;

    // for the months
    if (Date1.Month > Date2.Month) 
        Month = Date1.Month - Date2.Month;
    else
        Month = Date2.Month - Date1.Month;

    // for the years
    if (Date1.Year > Date2.Year)
        Year = Date1.Year - Date2.Year;
    else        
        Year = Date2.Year - Date1.Year;       

    difference = Day + Month*30 + Year*360;                

    return difference;
}

Answer 1

在 dif_days() 的函数原型(prototype)中使用它之前，您需要声明 struct Date。

您可以移动 struct 的整个定义，使其位于函数原型(prototype)之前，或者您可以通过在函数原型(prototype):

struct Date;

此外，您还需要从 scanf() 格式字符串中删除尾随的空白字符。 This plays havoc with interactive input, and does not do what people usually seem to expect.请注意，%d 指令会自动忽略前导空白字符，事实上，唯一不会忽略前导空白的 scanf() 指令是 %c、%n 和 %[]。

当我们讨论 scanf() 主题时，您应该检查调用 scanf() 返回的值以确保输入符合预期.一旦格式字符串中的匹配失败，scanf() 将继续前进，而不会在其余变量中存储任何内容。当代码尝试使用不确定的值时，无效输入将导致未定义的行为。如果输入了三个数字，此处 scanf() 将返回 3，对于基本验证，您可以在继续之前检查用户是否确实输入了三个数字。