problem of subdomain

[freed1985]

How can I point a subdomain to my a folder?

for example

my web root is /var/www/virtual/maindomain.com/htdocs

I want to point my subdomain to /var/www/virtual/maindomain.com/htdocs/sub
but when I set point /sub ,It point to /var/www/virtual/maindomain.com/sub/htdocs
and then I set it to /htdocs/sub,but It pointed to /var/www/virtual/maindomain.com/htdocs/sub/htdocs.

Now how can I do to make it to /var/www/virtual/maindomain.com/htdocs/sub with out the htdocs folder?