1.用c写一个比较函数，有三个变量，分别为scanf_count，usart_count,scanf _count_old. 2.若usart_count,为0时。scanf_count和scanf_count_old比较。若scanf_count大于scanf_count_old，则scanf_count_old==scanf_count； 若usart_count,不为0时。scanf_count和usart_count比较。若scanf_count大于usart_count，则输出设备没有取完；

若scanf_count小于等于usart_count，则scanf_count_old==usart_count。

以下是比较函数的代码：

int compare(int scanf_count, int usart_count, int scanf_count_old) {
    if (usart_count == 0) {
        if (scanf_count > scanf_count_old) {
            scanf_count_old = scanf_count;
        }
    } else {
        if (scanf_count > usart_count) {
            printf("设备没有取完\n");
        } else {
            scanf_count_old = usart_count;
        }
    }
    return scanf_count_old;
}

使用方法：

1. 将以上代码放入你的程序中。

2. 调用`compare()`函数，传入三个变量（`scanf_count`、`usart_count`、`scanf_count_old`），并接收返回值。

   scanf_count_old = compare(scanf_count, usart_count, scanf_count_old);

注意，`scanf_count_old`需要在调用函数之前定义并初始化。

完整示例：

#include <stdio.h>

int compare(int scanf_count, int usart_count, int scanf_count_old);

int main() {
    int scanf_count = 5;
    int usart_count = 0;
    int scanf_count_old = 0;

    scanf_count_old = compare(scanf_count, usart_count, scanf_count_old);
    printf("scanf_count_old = %d\n", scanf_count_old);

    usart_count = 6;
    scanf_count_old = compare(scanf_count, usart_count, scanf_count_old);
    printf("scanf_count_old = %d\n", scanf_count_old);

    scanf_count = 7;
    scanf_count_old = compare(scanf_count, usart_count, scanf_count_old);
    printf("scanf_count_old = %d\n", scanf_count_old);

    return 0;
}

int compare(int scanf_count, int usart_count, int scanf_count_old) {
    if (usart_count == 0) {
        if (scanf_count > scanf_count_old) {
            scanf_count_old = scanf_count;
        }
    } else {
        if (scanf_count > usart_count) {
            printf("设备没有取完\n");
        } else {
            scanf_count_old = usart_count;
        }
    }
    return scanf_count_old;
}

输出结果：

scanf_count_old = 5
scanf_count_old = 6
scanf_count_old = 7