在Erlang-china的郵件列表上看到這樣的問題:

我的服務經常發生這樣的錯誤,舉例:
Error in process <0.33.0> with exit value: {badarg,[{erlang,’++’,[undefined,[{“37”}]]},{groups,doWork,1},
{groups,doWork,1},{groups,manage_clients,1}]}

大意明白,但問題是我使用匹配機制時沒考慮到多個函式”doWork/1″出錯無法定位到其中一個,這該如何是好?
Erlang是否會像其它語言一樣提示某一行出錯?

這個問題確實很常見, Erlang的執行期沒有給出出錯的具體行數, 這給我們定位問題帶來了很大的麻煩.

這裡我給出另外一個方案, 利用erlang現有的模組來實現的: cover + dbg

原理就是cover編譯過的模組會在每行執行前, 先執行ets:update_counter(cover_internal_data_table,{bump,Mod,Fun,1,1,Line},1) 來更新模組某行的執行次數.
那麼我們只要擷取 ets:update_counter這個動作, 我們就知道改模組最後的執行行, 也就是異常所在的行.

Ok, 原理介紹完畢, 上菜.

[[email protected] ~]# cat line.erl

-module(line).
-export([dbg/1]).
-include_lib("stdlib/include/ms_transform.hrl").

dbg(Mod)->
    cover:compile(Mod),
    dbg:tracer(),
    dbg:p(all, [call]),
    dbg:tpl(ets,
            update_counter,
            dbg:fun2ms(fun([_,{bump,Mod,_,_,_,_},1]) ->
                               return_trace()
                       end)),
    ok.

[[email protected] ~]# cat hello.erl

-module(hello).
-export([start/0]).

start()->
    a=a,
    A=2,
    C=3,
    A=C-1,
    C=A+1,
    io:format("hello world~n",[]),
    test(C),
    ok.


test(C)->
    A=4,
    A=C,  % Error is on this line.
    ok.

我們可以看到這個hello模組會在hello:test發生異常, A=C這個地方是具體位置. 現在讓我們找到行號:

[[email protected] ~]# erl
Erlang R13B02 (erts-5.7.3)  [64-bit] [smp:2:2] [rq:2] [async-threads:0] [hipe] [kernel-poll:false]

Eshell V5.7.3  (abort with ^G)
1> line:dbg(hello).
ok
2> hello:start().
hello world
** exception error: no match of right hand side value 3
     in function  hello:test/1
     in call from hello:start/0
4> (<0.34.0>) call ets:update_counter(cover_internal_data_table,{bump,hello,start,0,1,5},1)
(<0.34.0>) returned from ets:update_counter/3 -> 1
(<0.34.0>) call ets:update_counter(cover_internal_data_table,{bump,hello,start,0,1,6},1)
(<0.34.0>) returned from ets:update_counter/3 -> 1
(<0.34.0>) call ets:update_counter(cover_internal_data_table,{bump,hello,start,0,1,7},1)
(<0.34.0>) returned from ets:update_counter/3 -> 1
(<0.34.0>) call ets:update_counter(cover_internal_data_table,{bump,hello,start,0,1,8},1)
(<0.34.0>) returned from ets:update_counter/3 -> 1
(<0.34.0>) call ets:update_counter(cover_internal_data_table,{bump,hello,start,0,1,9},1)
(<0.34.0>) returned from ets:update_counter/3 -> 1
(<0.34.0>) call ets:update_counter(cover_internal_data_table,{bump,hello,start,0,1,10},1)
(<0.34.0>) returned from ets:update_counter/3 -> 1
(<0.34.0>) call ets:update_counter(cover_internal_data_table,{bump,hello,start,0,1,11},1)
(<0.34.0>) returned from ets:update_counter/3 -> 1
(<0.34.0>) call ets:update_counter(cover_internal_data_table,{bump,hello,test,1,1,16},1)
(<0.34.0>) returned from ets:update_counter/3 -> 1
(<0.34.0>) call ets:update_counter(cover_internal_data_table,{bump,hello,test,1,1,17},1)   %這裡我們看到出錯的行號
(<0.34.0>) returned from ets:update_counter/3 -> 1
3> 
BREAK: (a)bort (c)ontinue (p)roc info (i)nfo (l)oaded
       (v)ersion (k)ill (D)b-tables (d)istribution

我們可以看到最後一次執行hello模組的行數是17.

Bingo!

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