Function open_memstream 

pub unsafe extern "C" fn open_memstream(
    __bufloc: *mut *mut c_char,
    __sizeloc: *mut usize,
) -> *mut FILE

Open a stream that writes into a malloc’d buffer that is expanded as necessary. *BUFLOC and *SIZELOC are updated with the buffer’s location and the number of characters written on fflush or fclose.