Bug

Joe Armstrong (AL/EAB) <>
Wed Dec 29 09:45:42 CET 2004



    I have found an interesting bug - to illustrate it here are two modules bug.erl and bug1.erl

    If you compile bug.erl and evaluate bug:module_info() you'll see what gets exported from bug.erl - so
far so good.

   Now compile and run bug1.erl - bug1.erl calls bug:c() but there is no routine called c/0 in bug.erl

   Then evauate bug:module_info() again - look at the exports - bug.erl now mysteriously exports c/0.

   So calling a routine which does not exist has the side effect of corrupting the module_info() data in
an already loaded module - ooops :-)

   Happy New Year

   /Joe




Here's a fun bug:

--- bug.erl

-module(bug).

-export([a/0]).

a() ->
    1.

---- bug1.erl

-module(bug1).

-export([b/0]).

b() ->
    bug:c().

----------------------------

Compile bug and llook at what is exported


Eshell V5.4  (abort with ^G)
1> c(bug).
{ok,bug}
2> bug:module_info().
[{exports,[{a,0},{module_info,0},{module_info,1}]},
 {imports,[]},
 {attributes,[{vsn,[275716032013154121462940958450039619938]}]},
 {compile,[{options,[]},
           {version,"4.3"},
           {time,{2004,12,23,18,35,59}},
           {source,"/home/joe/blogger/src/styles/bug.erl"}]}]

Compile and run bug1

3> c(bug1).
{ok,bug1}
4> bug1:b().
** exited: {undef,[{bug,c,[]},
                   {erl_eval,do_apply,5},
                   {shell,exprs,6},
                   {shell,eval_loop,3}]} **

Now look at what is exported from module bug

5> bug:module_info().
[{exports,[{c,0},{a,0},{module_info,0},{module_info,1}]},
           *****
 {imports,[]},
 {attributes,[{vsn,[275716032013154121462940958450039619938]}]},
 {compile,[{options,[]},
           {version,"4.3"},
           {time,{2004,12,23,18,35,59}},
           {source,"/home/joe/blogger/src/styles/bug.erl"}]}]

c/0 now appears to be exported from bug !!!

Happy Christmas

/Joe





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