[libvirt] how libvirt communicate with qemu?

i am reading source code, begin with tracing simple command line ,like virsh list and forth. please look at my qurestions 1.how libvirt communicate with qemu? call qemu'lib or open special device/socket? 2. i do not find the implement of function qemuAgentOpen---->qemuAgentInitialize(). i also search qemu-kvm source 3. qemuListAllDomains is the function which corespond to virsh-list. please explain this function . virDomainList(conn, driver->domains.objs, domains, flags); i) it seems driver->domains.objs have gotten all dmains. so when and where driver->domains.objs is assigned. ii)and what is conn,and its definition? thanks.