pset2 making a whole string uppercase for use in substitution.c

Question

I am trying to use the following helper function inside my substitution.c script:

string upper(string key)
{
    int n = strlen(key);
    char key_upper[n];

    for (int i = 0; i < n; i++)
    {
        if (key[i] >= 'a' && key[i] <= 'z')
        {
            key_upper[i] = key[i] - 32;
        }
        else
        {
            key_upper[i] = key[i];
        }
    }
    return key_upper;
}

The whole point of this was to take a string and make every character uppercase. I tried making a separate script with this as the main function and it seems to work when I print key_upper but not when I return it. I get the following error:

clang -ggdb3 -O0 -std=c11 -Wall -Werror -Wextra -Wno-sign-compare -Wno-unused-parameter -Wno-unused-variable -Wshadow    substitution.c  -lcrypt -lcs50 -lm -o substitution
substitution.c:64:12: error: address of stack memory associated with local variable 'key_upper' returned [-Werror,-Wreturn-stack-address]
    return key_upper;
           ^~~~~~~~~
1 error generated.
<builtin>: recipe for target 'substitution' failed
make: *** [substitution] Error 1

Before that, I tried to to set key_upper to key, and then modify the entries that had lowercase characters in them, but when I used this function on a string, it also modified the original string! Either of two things would solve my problem: 1.(Preferred) Fix the script as written. I'm not sure why there is a memory error? I'm not sure what this means. It would be really nice if someone could explain this to me like they would to a child because I am completely lost. 2. Can I make a copy of an existing variable and update the copy without it redefining the original variable?

Answer 1

Apparently you can make a copy of a string with strcopy. This at least got rid of the error and it now compiles.