Input Validation for a String in C - c

Question

I'm trying to make a loop that does not stop until the user inputs a string. For ex. if the user inputs a number or a letter it will say Invalid input until the user enters a string.
But for some strange reason, when I run my code and I input a string, the program continues to loop the block of code. Here's the output
#include <stdio.h>
int main() {
char name[100];
char letter[100];
lt:
printf("\033[0;33m");
printf("\nEnter your Name:\ni.e. Miguel\n");
printf("\033[0m");
scanf("%s", name);
if (name != letter) {
printf("\033[0;31m");
printf("Invalid input");
printf("\033[0m");
goto lt;
}
return0;
}
I've tried the goto function to loop the code but seems like that isn't working.

Answer 1

Uninitialized variables:
char letter[100];
letter is used uninitialized in your code. It's contents are indeterminate. Variables declared with automatic storage aren't implicitly initialised to 0.
Comparing strings:
if (name != letter)
This only compares the pointer addresses (and I believe it invokes undefined behavior), not the contents of the what those pointers point to.
The C standard library provides a function strcmp that compares two strings.
Note: It's declared in string.h.
Buffer overflow vulnerability:
scanf("%s", name);
is similar to using gets (in terms of limiting input). scanf will happily continue to read from stdin until it sees a whitespace, ignore everything to the right hand side of the whitespace, (which leads to more problems down the road) and potentially overflow the buffer.
You could use a field width to limit input:
scanf("%99s", name);
Or even better, use fgets.
fgets(name, sizeof(name), stdin);
It will read at most n - 1 characters and null-terminate the string.
Side-note: fgets will store the \n character in the buffer, which might not be what you want. Here's one way to remove it¹:
name[strcspn(name, "\n\r")] = '\0';
scanf returns the number of successful conversions:
Ignoring the return value of scanf gainsays the 6th commandment of Henry Spencer's "The Ten Commandments for C Programmers":
If a function be advertised to return an error code in the event of
difficulties, thou shalt check for that code, yea, even though the
checks triple the size of thy code and produce aches in thy typing
fingers, for if thou thinkest ``it cannot happen to me'', the gods
shall surely punish thee for thy arrogance.
if (scanf("%99s", name) != 1) {
handle the error here..
}
Using goto:
While that is a legal use, it's discouraged. goto should not be used for such purposes.
Instead, as #SimonGoater suggested, use a while loop.
if the user inputs a number or a letter it will say Invalid input
until the user enters a string.
So the requirements are:
Names should be greater than 1 character.
Names shouldn't have any numbers in them. (But my friends have names with numbers, and even special characters. :-( )
Note:
You can't have a string with one char, unless that char is a null-byte.
Possible Solutions:
As strlen doesn't include the null-terminator, we can use it to calculate the length of the string, and if it equals 1, handle the error accordingly.
Iterate through the buffer character by character, and if you find a number or a special character, handle the error accordingly.
[1] You might want to give this a read for more:
https://codereview.stackexchange.com/q/67608

Answer 2

#include <stdio.h>
#include<conio.h>
#include<string.h>
int main(){
char name[100];
char letter[100]="amir";
lt:
printf("\033[0;33m");
printf("\nEnter your Name:\ni.e. Miguel\n");
printf("\033[0m \n");
scanf("%s",name);
if(strcmp(name, letter) != 0){
printf("\033[0;31m");
printf("Invalid input");
printf("\033[0m");
goto lt;
}
return 0;
}