Function hyper_io_set_userdata 

#[unsafe(no_mangle)]
pub extern "C" fn hyper_io_set_userdata(
    io: *mut hyper_io,
    data: *mut c_void,
)

Available on crate feature ffi and hyper_unstable_ffi only.

Set the user data pointer for this IO to some value.

This value is passed as an argument to the read and write callbacks.