char *last;
char *my_tok(char *first, char c)
{
        int pos=0;

        last=first; /* 'last' now points to "abc def ghi", so becomes red */
        first=last; /* because of the statement above, 'last' is already equal to 'first' */

        /* Increment 'pos' until we find c or NULL */
        while(first[pos] && (first[pos] != c)) pos++;

        // pos will now be '3', because first[3] == ' '

        first[pos]='\0';
        pos++;

        // 'last' currently points to "abc\0def ghi"
        last += pos;
        // Now 4 ahead, pointing to "def ghi".
        // Since it's now different, I've made it green again.

        // Return a pointer to where we started, which still points to
        // "abc def ghi", but changed to "abc\0def ghi".
        // The variable 'last' knows where we left off, pointing to "def ghi".
        return(first);
}

int main() {
        char buf[]="abc def ghi";
        char *tok=my_tok(buf, ' ');
}

char *my_tok(char *first, char c)
{
        int pos=0;

        //last=first;        /* Since 'first' is NULL, this DOES NOT happen: */

        /* Right now, 'first' points to NULL. */
       first=last;
        /* 'first' now points to "def ghi" instead. */

        /* Increment 'pos' until we find c or NULL */
        while(first[pos] && (first[pos] != c)) pos++;

        // pos is now 3 again, since first[3] == ' '

        first[pos]='\0'; // replace that ' ' with '\0'
        pos++; // Increment once, to include that '\0' in the length
        // pos is now 4.  "abc\0" is exactly 4 chars.

        // currently, 'last' points to "def\0ghi"
        last += pos;
        // last now points to "ghi".

        // Return a pointer to where we started, "def\0ghi".
        // 'last' remembers where we left off, four further ahead at "ghi".
        return(first);
}

int main(void) {
        char buf[128]="abc def ghi";
        char *tok=my_tok(buf, ' ');

        while(tok != NULL)
        {
                fprintf(stderr, "tok=%s\n", tok);
                tok=my_tok(NULL, ' ');
        }
}