What is x86_thread_state64_t & thread_act_t?

I was looking into source code of StarRuntime and I came across two terms x86_thread_state64_t & thread_act_t.

I tried to Google it but there is not much documentation. I 开发者_如何学运维see it being used consistently in apple open source projects? Can someone explain me what are they?

x86_thread_state64_t should be something like this

struct x86_thread_state64_t {
    uint64_t rax, rbx, rcx, rdx,
             rdi, rsi, rbp, rsp,
             r8, r9, r10, r11,
             r12, r13, r14, r15,
             rip, rflags,
             cs, fs, gs;
};

and

thread_act_t is

struct thread  *thread_act_t;