How to get a statement in GIMPLE?

I am learning gcc-plugin developments. Now i get a function print_gimple_stmt(stderr, stmt, 0, TDF_SLIM)which can print out the detail of statement, like D.2579 = argc + 100;but i need to find a way to store it into a string-like variable, and then i need to use it to do later work. Can anyone help me?