c char to lower case

#include <stdio.h>
#include <ctype.h>
int main()
{
    char c, result;

    c = 'M';
    result = tolower(c);
    printf("tolower(%c) = %c\n", c, result); // tolower(M) = m

    c = 'm';
    result = tolower(c);
    printf("tolower(%c) = %c\n", c, result); // tolower(m) = m

    c = '+';
    result = tolower(c);
    printf("tolower(%c) = %c\n", c, result); // tolower(+) = +

    return 0;
}