why erlang:get_stacktrace/0 get different outputs in shell? - erlang

Question

consider the code below:
-module(except).
-compile(export_all).
divone(X)-> erlang:error({badnews,erlang:get_stacktrace()}),1/X-1.
tryreturn(X)->
try divone(X) of
Val->{result,2/Val}
catch
exit:Reason->{exit,Reason};
throw:Throw->{throw,Throw};
error:Error->{error,Error}
end.
test the code in shell:
Eshell V5.9.1 (abort with ^G)
1> c(except).
{ok,except}
2> except:tryreturn(10). **%why cant't get stack trace info here?**
{error,{badnews,[]}}
3> except:tryreturn(10). **%why can get here?**
{error,{badnews,[{except,divone,1,
[{file,"except.erl"},{line,4}]},
{except,tryreturn,1,[{file,"except.erl"},{line,7}]},
{erl_eval,do_apply,6,[{file,"erl_eval.erl"},{line,576}]},
{shell,exprs,7,[{file,"shell.erl"},{line,668}]},
{shell,eval_exprs,7,[{file,"shell.erl"},{line,623}]},
{shell,eval_loop,3,[{file,"shell.erl"},{line,608}]}]}}

Answer 1

If there hasn't been any exceptions, the stacktrace returned by erlang:backtrace() is empty.
Something more helpful than erlang:backtrace() for you would be something that includes the instruction pointer and the process's callstack, irrespective of if you've caught an exception.
This should be the ticket:
io:format("~s~n", [element(2, process_info(self(), backtrace))]).
(Naturally, self can be freely exchanged with another Pid)

Answer 2

According to documentation:
Gets the call stack back-trace (stacktrace) of the last exception in the calling process as a list of {Module,Function,Arity,Location} tuples. Field Arity in the first tuple can be the argument list of that function call instead of an arity integer, depending on the exception.
Note the last exception in above text. During your first call, there was not any exception yet, so there is not any stacktrace to return. Then you make error class[1] exception using erlang:error/1 so next call returns stacktrace of the previous call!
There is idiom how to get stacktrace of the current code:
try throw(foo) catch foo -> erlang:get_stacktrace() end.
or using older (obsolete) way:
catch throw(foo), erlang:get_stacktrace().
[1]: There are three classes of exception: throw, error, exit.