[HACKERS] How can i get record by data block not by sql?

[姜头]

How can i get record by data block not by sql?
 I want to read and write lots of data by data blocks and write a record to a 
appointed data blocks,so i can form a disk-resident tree by recording the block 
address. But i don't know  how to implement in postgresql. 
 Is there system function can do this?  
 Can someone help me?? Thank you very very much!!!!1