copy string to lower case.

toLower.c

#include <stdio.h>
#include <string.h>
 
int main() {
    char *s1 = "I Used to BE An UPPER CASE StrinG\n";

    /* alloc memory and copy the string (to preserve the original reference */
    char *s2 = strdup(s1);
    unsigned i;
    for (i = 0; i < strlen(s2); i++)
        s2[i] = tolower(s2[i]);
    
    printf("%s", s1);
    printf("%s", s2);

    free(s2);
}