Reverse Words of a String

Q.) How do you reverse the words in a string?

"My name is Amit Agarwal"
to
"Agarwal Amit is name My"

A O(n) and 'in space' solution is appreciable.

- First reverse the whole string
- Then reverse each word in the string

Test Cases:
- NULL string
- "" empty string
- "a" string that consists only one word
- "mustafa"
- "a b"
- "Mustafa Veysi Soyvural"
- "Mustafa Veysi Soyvural" more than one space between words

void reverse_words( char s[] ) {
    char *p, temp;
    int len = 0, i = 0, index = 0, word_len = 0;
    if ( NULL == s ) return;
    for ( p = s; *p != '\0'; p++ );
    len = p - s;
    
    /* reverse whole string */
    for ( i = 0; i < (len / 2); i++ ) {
        temp = s[ i ];
        s[ i ] = s[ len - i - 1 ];
        s[ len - i - 1 ] = temp;
    }

    /* reverse each word */
    p = s;
    while ( index < len ) {
        if ( *p != ' ' && *p != '\0' ) p++;
        else {
            word_len = p - &s[ index ];
            /* reverse word */
            for ( i = 0; i < (word_len / 2 ); i++ ) {
                temp = s[ i + index ];
                s[ i + index ] = s[ index + word_len - i - 1 ];
                s[ index + word_len - i - 1 ] = temp;
            }
            index += word_len + 1;
            p++;
        }
    }
}