Simple Loops and String Length in C

问题:

I'm pretty new to C. Writing in Visual Studio 2015, I'm trying to safely prompt a user for a string by using fgets. I want to use fgets to get the string, check if the string is too long, and reprompt the user if it is until they enter a good string. Here is my code

/*
* Nick Gilbert
* COS317 Lab 2 Task 2
*/
#include "stdafx.h"
int main()
{
    char str[10];
    int isValid = 0;
    while (isValid == 0) {
        printf("Please enter a password: ");
        fgets(str, 10, stdin);
        if (strlen(str) == 9 && str[8] != '\n') { //http://stackoverflow.com/questions/21691843/how-to-correctly-input-a-string-in-c
            printf("Error! String is too long\n\n");
            memset(&str[0], 0, sizeof(str));
        }
        else {
            printf(str);
            isValid = 1;
        }
    }
    printf("Press 'Enter' to continue...");
    getchar();
}

However, when I run this and enter a bad string, the excess characters get fed into the next fgets automatically!

How can I fix this to do what I want it to do?

回答1:

If the string read in by fgets doesn't end with a newline, call fgets in a loop until it does, then prompt the user again.

    if (strlen(str) > 0 && str[strlen(str)-1] != '\n') {
        printf("Error! String is too long\n\n");
        do {
            fgets(str, 10, stdin);
        } while (strlen(str) > 0 && str[strlen(str)-1] != '\n') {
    }

Also, never pass a variable at the first argument to printf, particularly if the contents of that variable comes from user entered data. Doing so can lead to a format string vulnerability.

回答2:

Try this:

#include "stdafx.h"
int main()
{
    char str[10];
    int isValid = 0;
    while (isValid == 0) {
        printf("Please enter a password: ");
        fgets(str, str, stdin);
        if (strlen(str) == 9 && str[8] != '\n') { //http://stackoverflow.com/questions/21691843/how-to-correctly-input-a-string-in-c
            printf("Error! String is too long\n\n");
            memset(str, 0, sizeof(str));
        }
        else {
            printf("%s",str);
            isValid = 1;
        }
    }
    printf("Press 'Enter' to continue...");
    getchar();
}

In addition:

While using memset() you can directly use the array_name rather &array_name[0].