上週Erlang討論群裡面提到lists的++實現,爭論大多基於猜測,其實開啟程式碼看一下就都明瞭.貼出程式碼截圖後有同學問這程式碼是哪裡找的?   "程式碼去哪裡找?",關於Erlang原始碼閱讀的路線圖江湖上只有一份殘卷了.我覺得"程式碼在哪兒?"這類問題是資訊不對稱造成的,本身難度不大,就像<貧民窟的百萬富翁>裡面的情節:賈馬爾知道市井生活中的零零碎碎卻說不出國徽上的文字,我們就從電影中的這一幕開始本文的探索吧 內景,演播室—夜晚 普瑞姆:這個問題的獎金四千盧比……印度的國徽是三隻獅子,獅子下面寫的是什麼?是否是……
                   A.惟有真理必勝 B.惟有謊言必勝 C.惟有時尚必勝 D.惟有金錢必勝 [普瑞姆假裝困惑的樣子看向觀眾,引他們發笑.] 普瑞姆:你覺得是哪一個呢,賈馬爾?這是我國曆史上最著名的一句話.或許你想給朋友打電話求助吧?
[觀眾哈哈大笑.一滴汗珠從賈馬爾的額頭流下來.普瑞姆喜歡賈馬爾的不安.] 普瑞姆:或者向現場觀眾求助?我憑直覺認為他們可能知道答案.你想怎麼辦?
賈馬爾:是的.
普瑞姆(吃驚):什麼是的?
賈馬爾:求助觀眾.
[普瑞姆吹口哨.舉目望向觀眾席.]
普瑞姆:那麼女士們、先生們,請幫他解難吧.現在請按下你們的選擇鍵.
[燈光轉暗.讓人緊張的音樂聲響起.]
內景,督察辦公室—白天
[督察按暫停鍵.嘆了口氣.]
督察:賈馬爾,我五歲大的女兒都知道答案,你卻不知道.這對一個天才百萬富翁來說,不是很奇怪嗎?怎麼回事?你的作弊同夥跑出去撒尿了是嗎?又或者是他咳得不夠大聲?
[沉默.斯里尼瓦斯警員朝賈馬爾的椅子踢了一腳.]
斯里尼瓦斯警員:督察問你話呢.
賈馬爾:在喬帕蒂海灘吉萬的小吃攤上,炸脆餅多少錢?
督察:什麼?
賈馬爾:一份炸脆餅,多少錢?
斯里尼瓦斯警員(忍不住說):十盧比.
賈馬爾:錯.排燈節過後就是十五盧比了.上個星期四,是誰在達達爾車站外面偷了瓦爾馬警員的自行車?
督察(被逗樂了):你知道是誰偷的?
賈馬爾:朱胡區的每個人都知道.連五歲的小孩兒都知道.  言

歸正傳,我們從程式碼下載開始......

原始碼下載

  對於選擇了Windows安裝包的同學,要特別提示一下:lib目錄中包含了對應類庫的原始碼和ebin,比如kernel,stdlib等等,但ERTS目錄裡面沒有對應原始碼,自己去下載一份來看吧,或者直接線上檢視 https://github.com/erlang/otp/tree/maint/erts

原始碼閱讀工具

      Erlang OTP原始碼量不小,好的工具能幫我們省很多事,比如支援資料夾查詢或者專案內搜尋的,在程式碼之間各種跳轉更是減少很多麻煩.如果是在Windows環境中Everything這樣的工具也是定位檔案利器,Visual studio 閱讀C程式碼體驗真的很棒,當然瞭如果你喜歡在純文字編輯器裡面用正則搞,也無不可;下面是在VS中程式碼截圖:  

Overview

    大體上,otp_src的程式碼如下圖這樣組織的(開啟資料夾就可以看到,算不上什麼Thirty Thousand Feet).與我們每天寫程式碼最息息相關的是ERTS和lib;ERTS(Erlang Run-Time System)包含了Erlang執行時系統的程式碼,是Erlang的基礎設施.lib包含了所有的外圍類庫實現,有些類庫的安排是違反直覺的,不過習慣了就好了,比如file.erl不是在stdlib而是在kernel;gen_server gen_fsm的程式碼實現應該是在kernel吧?錯,它們的程式碼是在stdlib下;但是呢,application.erl是在kernel. Kernel    看一下kernel目錄,是不是有點摸不著頭腦?Erlang執行時是有一個kernel application執行,執行一下appmon我們可以動態看到kernel涉及到的程式碼模組.我們大致可以揣摩到設計者的規劃原則:kernel的範疇包含了application管理,code生命週期管理,IO(檔案IO,網路IO,io_request),HIPE,分散式基礎設施等等,見下面的思維導圖:   上面的劃分方式只是我個人的一種看法,為了方便查閱我把上圖轉成了文字,見下面:  

Kernel

Kernel APP
	kernel.erl
	kernel_config.erl
	kernel.appup.src
	kernel.app.src

application管理
	application_controller.erl
	application_starter.erl
	application_master.hrl
	application_master.erl
	application.erl
	heart.erl

HIPE
	hipe_ext_format.hrl
	hipe_unified_loader.erl

除錯& 日誌
	日誌
		disk_log.erl
		disk_log_sup.erl
		disk_log_server.erl
		disk_log_1.erl
		disk_log.hrl
		error_logger.erl
		wrap_log_reader.erl
	除錯
		error_handler.erl
		erts_debug.erl
		standard_error.erl
		seq_trace.erl
IO
	檔案IO
		file.erl
		file_server.erl
		file_io_server.erl
		ram_file.erl
	網路IO
		gen_sctp.erl
		gen_udp.erl
		gen_tcp.erl
		inet.erl
		inet_config.hrl
		inet_config.erl
		inet_boot.hrl
		inet6_udp.erl
		inet6_tcp_dist.erl
		inet6_tcp.erl
		inet6_sctp.erl
		inet_db.erl
		inet_dns.hrl
		inet_dns.erl
		inet_gethost_native.erl
		inet_udp.erl
		inet_tcp_dist.erl
		inet_tcp.erl
		inet_sctp.erl
		inet_res.hrl
		inet_res.erl
		inet_parse.erl
		inet_int.hrl
		inet_hosts.erl
		inet_dns_record_adts.pl
		erl_reply.erl
		net_kernel.erl
		net_adm.erl
		net.erl
	IO Request
		user_drv.erl
		user.erl
		user_sup.erl
		group.erl

Code生命週期管理
	code.erl
	code_server.erl
	erl_boot_server.erl
	erl_ddll.erl
	
distribute管理
	dist_util.erl
	dist_ac.erl
	Distributed Applications Controller
	erl_distribution.erl
	erl_epmd.erl
	rpc.erl
	pg2.erl
	global_search.erl
	global_group.erl
	global.erl
	auth.erl
OS
	os.erl

stdlib     相比kernel,stdlib恰如起名包含了絕大多數的功能模組,比如lists,ets,各種資料結構實現,當然最重要的是它包含了OTP的gen_server gen_fsm gen_event supervisor以及幕後英雄proc_lib和sys.如果你不嫌棄,這裡有一份略微過時的文件,是我初學Erlang的時候在文件上做的筆記註釋:[Erlang STDLIB 中文註釋版]    特別值得一提的是shell和shell_default,對Erlang Shell好奇的同學看看這裡能找到答案,所謂"EShell裡面靈異的問題"也就有了一個合理的解釋.  其它的模組因為功能特別明確很容易定位到,比如專門處理XML的xmerl,資料庫mnesia等等,輔之以Google,幾乎沒有什麼障礙;

Dive into ERTS

Atom and bifs atom.names  枚舉了ERTS使用的atom,學習一下慣用法還是非常有必要的 bif.tab           bif清單 注意 Use "ubif" for guard BIFs and operators; use "bif" for ordinary BIFs. Basic Type

/*
** Data types:
**
** Eterm: A tagged erlang term (possibly 64 bits)
** BeamInstr: A beam code instruction unit, possibly larger than Eterm, not smaller.
** UInt:  An unsigned integer exactly as large as an Eterm.
** SInt:  A signed integer exactly as large as an eterm and therefor large
**        enough to hold the return value of the signed_val() macro.
** UWord: An unsigned integer at least as large as a void * and also as large
**          or larger than an Eterm
** SWord: A signed integer at least as large as a void * and also as large
**          or larger than an Eterm
** Uint32: An unsigned integer of 32 bits exactly
** Sint32: A signed integer of 32 bits exactly
** Uint16: An unsigned integer of 16 bits exactly
** Sint16: A signed integer of 16 bits exactly.
*/

這裡我們還能看到一些複雜資料結構的內部表示,比如:   兩個例子 看兩個例子吧,第一個例子lists的append是如何實現的,很容易找到lists.erl append(L1, L2) -> L1 ++ L2. 我們發現其實append就是使用的++,那++是在哪裡實現的呢? 比較有趣的一個地方是這兩句:

     copy = last = CONS(hp, CAR(list_val(list)), make_list(hp + 2));
     list = CDR(list_val(list));

有同學說,CAR CDR CONS這三個東西好熟悉啊?對,沒錯,這就是Lisp列表操作的三個基礎原語,分別實現取表頭,取表頭外剩餘部分,表構造(constructs),跳轉到它們的實現,在erl_term.h:

#define CONS(hp, car, cdr) \
        (CAR(hp)=(car), CDR(hp)=(cdr), make_list(hp))

#define CAR(x)  ((x)[0])
#define CDR(x)  ((x)[1])

第二個例子 看看process的定義是什麼樣的 首先在 erl_process.h 找到 Process的定義 typedef struct process Process; 轉到struct process的定義:

struct process {
    ErtsPTabElementCommon common; /* *Need* to be first in struct */

    /* All fields in the PCB that differs between different heap
     * architectures, have been moved to the end of this struct to
     * make sure that as few offsets as possible differ. Different
     * offsets between memory architectures in this struct, means that
     * native code have to use functions instead of constants.
     */

    Eterm* htop;		/* Heap top */
    Eterm* stop;		/* Stack top */
    Eterm* heap;		/* Heap start */
    Eterm* hend;		/* Heap end */
    Uint heap_sz;		/* Size of heap in words */
    Uint min_heap_size;         /* Minimum size of heap (in words). */
    Uint min_vheap_size;        /* Minimum size of virtual heap (in words). */

#if !defined(NO_FPE_SIGNALS) || defined(HIPE)
    volatile unsigned long fp_exception;
#endif

#ifdef HIPE
    /* HiPE-specific process fields. Put it early in struct process,
       to enable smaller & faster addressing modes on the x86. */
    struct hipe_process_state hipe;
#endif

    /*
     * Saved x registers.
     */
    Uint arity;			/* Number of live argument registers (only valid
				 * when process is *not* running).
				 */
    Eterm* arg_reg;		/* Pointer to argument registers. */
    unsigned max_arg_reg;	/* Maximum number of argument registers available. */
    Eterm def_arg_reg[6];	/* Default array for argument registers. */

    BeamInstr* cp;		/* (untagged) Continuation pointer (for threaded code). */
    BeamInstr* i;		/* Program counter for threaded code. */
    Sint catches;		/* Number of catches on stack */
    Sint fcalls;		/* 
				 * Number of reductions left to execute.
				 * Only valid for the current process.
				 */
    Uint32 rcount;		/* suspend count */
    int  schedule_count;	/* Times left to reschedule a low prio process */
    Uint reds;			/* No of reductions for this process  */
    Eterm group_leader;		/* Pid in charge
				   (can be boxed) */
    Uint flags;			/* Trap exit, etc (no trace flags anymore) */
    Eterm fvalue;		/* Exit & Throw value (failure reason) */
    Uint freason;		/* Reason for detected failure */
    Eterm ftrace;		/* Latest exception stack trace dump */

    Process *next;		/* Pointer to next process in run queue */

    struct ErtsNodesMonitor_ *nodes_monitors;

    ErtsSuspendMonitor *suspend_monitors; /* Processes suspended by
					     this process via
					     erlang:suspend_process/1 */

    ErlMessageQueue msg;	/* Message queue */

    union {
	ErtsBifTimer *bif_timers;	/* Bif timers aiming at this process */
	void *terminate;
    } u;

    ProcDict  *dictionary;       /* Process dictionary, may be NULL */

    Uint seq_trace_clock;
    Uint seq_trace_lastcnt;
    Eterm seq_trace_token;	/* Sequential trace token (tuple size 5 see below) */

#ifdef USE_VM_PROBES
    Eterm dt_utag;              /* Place to store the dynamc trace user tag */
    Uint dt_utag_flags;         /* flag field for the dt_utag */
#endif       
    BeamInstr initial[3];	/* Initial module(0), function(1), arity(2), often used instead
				   of pointer to funcinfo instruction, hence the BeamInstr datatype */
    BeamInstr* current;		/* Current Erlang function, part of the funcinfo:
				 * module(0), function(1), arity(2)
				 * (module and functions are tagged atoms;
				 * arity an untagged integer). BeamInstr * because it references code
				 */
    
    /*
     * Information mainly for post-mortem use (erl crash dump).
     */
    Eterm parent;		/* Pid of process that created this process. */
    erts_approx_time_t approx_started; /* Time when started. */

    /* This is the place, where all fields that differs between memory
     * architectures, have gone to.
     */

    Eterm *high_water;
    Eterm *old_hend;            /* Heap pointers for generational GC. */
    Eterm *old_htop;
    Eterm *old_heap;
    Uint16 gen_gcs;		/* Number of (minor) generational GCs. */
    Uint16 max_gen_gcs;		/* Max minor gen GCs before fullsweep. */
    ErlOffHeap off_heap;	/* Off-heap data updated by copy_struct(). */
    ErlHeapFragment* mbuf;	/* Pointer to message buffer list */
    Uint mbuf_sz;		/* Size of all message buffers */
    ErtsPSD *psd;		/* Rarely used process specific data */

    Uint64 bin_vheap_sz;	/* Virtual heap block size for binaries */
    Uint64 bin_vheap_mature;	/* Virtual heap block size for binaries */
    Uint64 bin_old_vheap_sz;	/* Virtual old heap block size for binaries */
    Uint64 bin_old_vheap;	/* Virtual old heap size for binaries */

    ErtsProcSysTaskQs *sys_task_qs;

    erts_smp_atomic32_t state;  /* Process state flags (see ERTS_PSFLG_*) */

#ifdef ERTS_SMP
    ErlMessageInQueue msg_inq;
    ErtsPendExit pending_exit;
    erts_proc_lock_t lock;
    ErtsSchedulerData *scheduler_data;
    Eterm suspendee;
    ErtsPendingSuspend *pending_suspenders;
    erts_smp_atomic_t run_queue;
#ifdef HIPE
    struct hipe_process_state_smp hipe_smp;
#endif
#endif

#ifdef CHECK_FOR_HOLES
    Eterm* last_htop;		/* No need to scan the heap below this point. */
    ErlHeapFragment* last_mbuf;	/* No need to scan beyond this mbuf. */
#endif

#ifdef DEBUG
    Eterm* last_old_htop;	/*
				 * No need to scan the old heap below this point
				 * when looking for invalid pointers into the new heap or
				 * heap fragments.
				 */
#endif

#ifdef FORCE_HEAP_FRAGS
    Uint space_verified;        /* Avoid HAlloc forcing heap fragments when */ 
    Eterm* space_verified_from; /* we rely on available heap space (TestHeap) */
#endif
};

莊子說:"吾生也有涯,而知也無涯.以有涯隨無涯,殆已!",所以各取所需就好,今天就到這裡,且行且珍惜吧