convert string to lowercase in c

1/* Use the tolower Function to Convert String to Lowercase in C */
2#include <stdlib.h>
3#include <stdio.h>
4#include <string.h>
5#include <ctype.h>
6
7int main(){
8    char *str = "THIS STRING LITERAL IS ARBITRARY";
9
10    printf("%s\n", str);
11    for (size_t i = 0; i < strlen(str); ++i) {
12        printf("%c", tolower((unsigned char) str[i]));
13    }
14    printf("\n");
15
16    exit(EXIT_SUCCESS);
17}

1// include all the libraries  used in the program.
2#include <stdio.h>
3#include <cs50.h>
4#include <ctype.h>
5#include <string.h>
6#include <math.h>
7
8// Base Function
9int main(int argc, string argv[])
10{
11    if (argc != 2)
12    {
13        printf("Usage: ./substitution key\n");
14    }
15    else if (strlen(argv[1]) != 26)
16    {
17        printf("Key must contain 26 characters.\n");
18    }
19    
20    string key = argv[1].toupper();
21    string key = argv[1].tolower();
22}