如何防止 scanf 失败导致无限循环

该代码应该可以换一美元并且运行良好。但教授说他将随字母一起输入随机数字。它可以很好地处理数字，但是当输入字母时会出现无限循环，有什么建议吗？

#include <stdio.h>
#include <stdlib.h>
#define amtPaid 1
#define SENTINAL -1
int quarters(int numChange);
int dimes(int numChange);
int nickels(int numChange);
int pennies(int numChange);

int main(void)
{
    double amtDue = 0; // how much is paid


while(1){
    printf("\nPlease enter the price less than 1 dollar: ");
    scanf(" %lg", &amtDue);
    int changeReturn = (amtPaid - amtDue) * 100 + 0.5; // convert decimal to whole number
    int updated = 0;    // remaining change after amt of change
    int numberQuarters = quarters(changeReturn); // number of quarters needed

   if(changeReturn >= 0 && changeReturn <= 100){ // if changereturn  is between 0 and 100 execute code

        printf("\nNice!");
        printf("\nWe owe you %i cents" , changeReturn);
        if(numberQuarters >= 0){    // get and print number of quarters
            printf("\nQuarters: %i", numberQuarters);
            updated = changeReturn % 25;
        }
        if(dimes(updated) >= 0){ // get and print number of dimes
            printf("\nDimes: %i", dimes(updated));
            updated = updated % 10;
        }
        if(nickels(updated)>= 0){ // get and print number of nickels
            printf("\nNickels: %i", nickels(updated));
            updated = updated % 5;
        }
        if(pennies(updated) >= 0){ // get and print number pennies
            printf("\nPennies: %i", pennies(updated));
        }

    }

    else if(amtDue == SENTINAL){
        break;
    }

    else {
        printf("That does not make sense to me. please type a valid number");
    }

    printf("\n %g", amtDue);

}
return 0;
}

int quarters(int numChange){
    int numQuarters = 0;

    numQuarters = numChange / 25;

    return numQuarters;
}
int dimes(int numChange){
    int numDimes = 0;

    numDimes = numChange / 10;

    return numDimes;
}
int nickels(numChange){
    int numNickels = 0;

    numNickels = numChange / 5;

    return numNickels;
}
int pennies(numChange){

    return numChange;
}

如果提供的值不是格式说明符的预期值，则使用scanf(), the scanf()将会失败并且不合适的值将会remain在输入缓冲区中，提供feed到下一个scanf()，只会导致连续失败。在这种情况下，您需要clean up在进行下一个输入之前输入缓冲区。你可以使用类似的东西

1. 检查返回值scanf()
2. 如果出现故障，请使用while( getchar() != '\n' );清理输入缓冲区。

也就是说，int nickels(numChange) is now invalid in c (C99向前）。你必须把它做成int明确地。