erlang init boot 浅析

说明：遇到一个OTP的问题，疑惑了很久，也就有了这篇文章。

目的：跟踪Erlang的启动过程

 

参考这里 http://erlangdisplay.javaeye.com/blog/315497 ，直接从erts-5.7.5/src/init.erl 的boot/1函数开始（关于这个链接给出的结论以后会继续研究分析）：

 

-spec boot([binary()]) -> no_return().
boot(BootArgs) ->
    register(init, self()),
    process_flag(trap_exit, true),
    start_on_load_handler_process(),
    {Start0,Flags,Args} = parse_boot_args(BootArgs),
    Start = map(fun prepare_run_args/1, Start0),
    Flags0 = flags_to_atoms_again(Flags),
    boot(Start,Flags0,Args).

 

do_boot(Init,Flags,Start) ->
    process_flag(trap_exit,true),
    {Pgm0,Nodes,Id,Path} = prim_load_flags(Flags),
    Root = b2s(get_flag('-root',Flags)),
    PathFls = path_flags(Flags),
    Pgm = b2s(Pgm0),
    _Pid = start_prim_loader(Init,b2a(Id),Pgm,bs2as(Nodes),
                 bs2ss(Path),PathFls),
    BootFile = bootfile(Flags,Root),
    BootList = get_boot(BootFile,Root),
    LoadMode = b2a(get_flag('-mode',Flags,false)),
    Deb = b2a(get_flag('-init_debug',Flags,false)),
    BootVars = get_flag_args('-boot_var',Flags),
    ParallelLoad =
    (Pgm =:= "efile") and (erlang:system_info(thread_pool_size) > 0),

    PathChoice = code_path_choice(),
    eval_script(BootList,Init,PathFls,{Root,BootVars},Path,
        {true,LoadMode,ParallelLoad},Deb,PathChoice),

    %% To help identifying Purify windows that pop up,
    %% print the node name into the Purify log.
    (catch erlang:system_info({purify, "Node: " ++ atom_to_list(node())})),

    start_em(Start).

 

boot/3的基本流程为 start_prim_loader/6 → eval_script/8 → start_em/1

1． start_prim_loader/6启动了erl_prim_loader模块，负责代码加载（远程或者本地等）

2． eval_script/8 负责启动一些关键的进程

来看个eval_script/8参数的例子：

{[{preLoaded,[erl_prim_loader,erlang,init,otp_ring0,

              prim_file,prim_inet,prim_zip,zlib]},

  {progress,preloaded},

  {path,["$ROOT/lib/kernel-2.13.5/ebin",

         "$ROOT/lib/stdlib-1.16.5/ebin"]},

  {primLoad,[error_handler]},

  {kernel_load_completed},

  {progress,kernel_load_completed},

  {path,["$ROOT/lib/kernel-2.13.5/ebin"]},

  {primLoad,[application,application_controller,

             application_master,application_starter,auth,code,

             code_server,disk_log,disk_log_1,disk_log_server,

             disk_log_sup,dist_ac,dist_util,erl_boot_server,erl_ddll,

             erl_distribution,erl_epmd,erl_reply|...]},

  {path,["$ROOT/lib/stdlib-1.16.5/ebin"]},

  {primLoad,[array,base64,beam_lib,c,calendar,dets,

             dets_server,dets_sup,dets_utils,dets_v8,dets_v9,dict,

             digraph,digraph_utils,edlin,edlin_expand|...]},

  {progress,modules_loaded},

  {path,["$ROOT/lib/kernel-2.13.5/ebin",

         "$ROOT/lib/stdlib-1.16.5/ebin"]},

  {kernelProcess,heart,{heart,start,[]}},

  {kernelProcess,error_logger,{error_logger,start_link,[]}},

  {kernelProcess,application_controller,

                 {application_controller,start,

                                         [{application,kernel,

                                                       [{description,[...]},{vsn,…},{…}|…]}]}},

  {progress,init_kernel_started},

  {apply,{application,load,

                      [{application,stdlib,[{description,...},{...}|...]}]}},

  {progress,applications_loaded},

  {apply,{application,start_boot,[kernel,permanent]}},

  {apply,{application,start_boot,[stdlib,permanent]}},

  {apply,{c,erlangrc,[]}},

  {progress,started}],

 <0.0.0>,

 {[],[]},

 {"/usr/local/lib/erlang",[]},

 false,

 {true,false,false},

 false,relaxed}

 

查看eval_script/8的实现（代码就不贴了），发现启动了上面参数中 kernelProcess对应键值的进程，如：

 

{kernelProcess,heart,{heart,start,[]}},

  {kernelProcess,error_logger,{error_logger,start_link,[]}},

  {kernelProcess,application_controller,

                 {application_controller,start,

                                         [{application,kernel,

 

 

eval_script([{kernelProcess,Server,{Mod,Fun,Args}}|CfgL],Init,
        PathFs,Vars,P,Ph,Deb,PathChoice) ->
    debug(Deb,{start,Server}),
    start_in_kernel(Server,Mod,Fun,Args,Init),
    eval_script(CfgL,Init,PathFs,Vars,P,Ph,Deb,PathChoice);

 

start_in_kernel(Server,Mod,Fun,Args,Init) ->
    Res = apply(Mod,Fun,Args),
    Init ! {self(),started,{Server,Res}},
    receive
    {Init,ok,Pid} ->
        unlink(Pid),  %% Just for sure…
        ok;
    {Init,ignore} ->
        ignore
    end.

 

最终产生一条消息给init：

Init ! {self(),started,{Server,Res}},

 

{BootPid,started,KernelPid} ->

        boot_loop(BootPid, new_kernelpid(KernelPid, BootPid, State));

 

init收到消息后，就将该进程加入到kernel pids的队列中了。

 

new_kernelpid({Name,{ok,Pid}},BootPid,State) when is_pid(Pid) ->
    link(Pid),
    BootPid ! {self(),ok,Pid},
    Kernel = State#state.kernel,
    State#state{kernel = [{Name,Pid}|Kernel]};
new_kernelpid({_Name,ignore},BootPid,State) ->
    BootPid ! {self(),ignore},
    State;
new_kernelpid({Name,What},BootPid,State) ->
    erlang:display({"could not start kernel pid",Name,What}),
    clear_system(BootPid,State),
    crash("could not start kernel pid", [Name, What]).

 

 

{kernelProcess,application_controller,

                 {application_controller,start,

                                         [{application,kernel,

 

application_controller启动后会启动kernel app，至此erlang的启动过程就大致的有个了解了，而关于application的启动过程就不在本次分析范围之内了。

 

结论：

 

系统从init<0.0.0>开始，一些关键的进程如：error_logger、application_controller、erl_prim_load等都会预先启动并加入到kernel pids中。

 

附言：

本次跟踪自然又离不开余锋老大(http://blog.yufeng.info/ )的指点，init.erl是preloaded模块：

{preLoaded,[erl_prim_loader,erlang,init,otp_ring0,

              prim_file,prim_inet,prim_zip,zlib]},

 

而由于本文之前的无知，导致我去尝试修改init.erl。 如果大家需要跟踪init.erl的启动细节，需要修改源码包中的 erts/preloaded/src/init.erl、make && make install，由于此时系统还没跑起来，所以error_logger io之类的模块是没有用的，幸运的是preloaded里面有 prim_file，可以通过 prim_file:write_file/2把一些状态写入到文件中以便查看。