Simple Loops and String Length in C - c

Question

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?

Answer 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.

Answer 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].