字符串化小写

#include <stdio.h>
#include <ctype.h>

char *myStrdown(char *s);

int
main(void)
{
    char s[] = "HELLO";
    char *t = myStrdown(s);
    printf(t);

    return 0;
}

char *
myStrdown(char *s)
{
    if (NULL == s)
        return NULL;

    char *t = s;

    while (*s) {
        if (isupper(*s))
            *s = tolower (*s);
        s++;
    }

    return t;
}