Obtaining starting adress of a page

I want to obtain the beginning of a memory page, a page that contains a function. In my case I try to achieve the page beginning of main function. Which lies in 0x400a80, I think it is in code section of memory. I would appreciate if you can verify whether it is true or not. As far as I understood, adress 0x400a80 lies in a page. When I show memory segments of my process with pmap, it shows a segment starting with 0x400000 with size 8K, and the next portion goes with 0x601000 with size 4K. I want to obtain adress 0x400000 because it has the address of the main() function. How can I achieve starting adress of a page when I have an adress that resides in that page? Is there any built-in way to do in linux ?