How to get absolute path

I am new for rails and web development, i am stucked at one point.
I am writing a code for uplaoding a file, using multipart=true in my
.rhtml file.
when requested parameter goes to my controller i need full physical path
of that particular file.
How could i achive this , any one have any idea about that. please reply
as soon as possiable.
thanx in advance.