Length is increased while reversing a string

Question

I'm working through "The C Programming Language" and trying to reverse a C-style string. My understanding is that a string literal like "voldemort" is represented as a character array (each of which has size 1) of 9 characters followed by a null character:

v o l d e m o r t \0
_ _ _ _ _ _ _ _ _ _

My idea to reverse the string is to find the length and then use that to declare an array of the appropriate length and define it as the reverse keeping a null at the end.

t r o m e d l o v \0
_ _ _ _ _ _ _ _ _ _

However, in my reversed string the length is suddenly longer and I'm not having any luck figuring out why.

#include <stdio.h>

int mystrlen(char s[]);
void reverse(char original[], char reversed[], int len);

int mystrlen(char s[]) {
    int i = 0;
    while (s[i] != '\0') {
        ++i;
    }
    return i;
}

void reverse(char original[], char reversed[], int len) {
    int i, c;
    for (i = 0; i < len; ++i) {
        reversed[i] = original[len-i-1];
    }
}

int main() {
    int len;
    char test[] = "voldemort";
    len = mystrlen(test);
    char tset[len+1];
    reverse(test, tset, len);
    printf("Original: %s\n", test);
    printf("Original Length: %d\n", mystrlen(test)); // 9
    printf("Original Size: %lu\n\n", sizeof(test)); // 10

    printf("Reversed: %s\n", tset);
    printf("Reversed Length: %d\n", mystrlen(tset)); // 13 (expected: 9)
    printf("Reversed Size: %lu\n", sizeof(tset));  // 10
    return 0;
}

Answer 1

Once you reverse the string need to NUL terminate

   void reverse(char original[], char reversed[], int len) {
        int i, c;
        for (i = 0; i < len; ++i) {
            reversed[i] = original[len-i-1];
        }
        reversed[i] = '\0'; //this line required
    }

Output:

Original: voldemort
Original Length: 9
Original Size: 10

Reversed: tromedlov
Reversed Length: 9
Reversed Size: 10