How do I properly compare strings in C?

I am trying to get a program to let a user enter a word or character, store it, and then print it until the user types it again, exiting the program. My code looks like this:

#include <stdio.h>

int main()
{
    char input[40];
    char check[40];
    int i=0;
    printf("Hello!\nPlease enter a word or character:\n");
    gets(input);   /* obsolete function: do not use!! */
    printf("I will now repeat this until you type it back to me.\n");

    while (check != input)
    {
        printf("%s\n", input);
        gets(check);   /* obsolete function: do not use!! */
    }

    printf("Good bye!");
    

    return 0;
}

The problem is that I keep getting the printing of the input string, even when the input by the user (check) matches the original (input). Am I comparing the two incorrectly?

You can't (usefully) compare strings using != or ==, you need to use strcmp:

while (strcmp(check,input) != 0)

The reason for this is because != and == will only compare the base addresses of those strings. Not the contents of the strings themselves.

Ok a few things: gets is unsafe and should be replaced with fgets(input, sizeof(input), stdin) so that you don't get a buffer overflow.

Next, to compare strings, you must use strcmp, where a return value of 0 indicates that the two strings match. Using the equality operators (ie. !=) compares the address of the two strings, as opposed to the individual chars inside them.

And also note that, while in this example it won't cause a problem, fgets stores the newline character, '\n' in the buffers also; gets() does not. If you compared the user input from fgets() to a string literal such as "abc" it would never match (unless the buffer was too small so that the '\n' wouldn't fit in it).