How to prevent program from crashing

How to prevent program from crashing

[Angela Zhu]

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Dear all,

I am trying to develop a new language with GUI using OCaml.
However, the language seems to be not very stable.

If a small syntax error in the program(written in this new language)  
happens,
it can through an error message.
If the syntax error is kind of critical,
(e.g., a semi-colon is missed at the end of a line,
  or a keyword is mis-spelled)
Then the whole GUI of the language crashes.

Does anyone else have ever encountered similar problem?
Or can anyone give me a hint on how to prevent this?

Thanks a lot!
Angela
------------------------------------------
Dept. of CS, Rice Unitersity
http://www.cs.rice.edu/~yz2/
------------------------------------------






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Re: [Caml-list] How to prevent program from crashing

[skaller]

On Tue, 2007-10-30 at 23:29 -0500, Angela Zhu wrote:

> 
> If a small syntax error in the program(written in this new language)
> happens,
[]
> Then the whole GUI of the language crashes.

You need to isolate the GUI part of your program from the
part that handles errors. In particular you should be careful
to undo the unfortunate tendency of various Ocaml functions to
throw exceptions, and thereby 'crash' your program if you fail
to handle them. For example write:

	let hfind msg table key = 
		try Hashtbl.find table key 
		with Not_found -> 
			print_endline ("Hashtbl.find failed in " ^ msg);
			abort()
	;;
			

	let maybe_hfind table key =
		try Some (Hashtbl.find table key)
		with Not_found -> None
	;;

and use one or other of these functions instead of the raw Hashtbl.find
function, which can throw an exception you forget to catch and crash
your program without a clear reason. Here 'hfind' will still crash your
program, but hopefully help find your programming error, whereas
the maybe_hfind function will never fail and uses the type system
to force you to check for errors and handle them.



-- 
John Skaller <skaller at users dot sf dot net>
Felix, successor to C++: http://felix.sf.net

Re: Re:Re: [Caml-list] How to prevent program from crashing

[skaller]

On Wed, 2007-10-31 at 00:52 -0500, Angela Zhu wrote:

> And I am sure the whole program and GUI crashes when function  
> "resetState2_4openFile"
> is called.
> I tried to catch the exception, but seems that it doesn't work.
> 
> Following is the message I got after the crashing:
> 
> 
> ************************************************************************ 
> ************************
> (acumen:4063): Gtk-CRITICAL **: _gtk_text_line_char_locate: assertion  
> `char_offs
> et >= 0' failed
> 
> Gtk-ERROR **: Char offset -1 is off the end of the line
> aborting...
> Abort trap

Yeah, this is an abort from a failed assertion in C,
so it isn't an Ocaml exception.

I don't know how GTK handles this, that is, whether there is
a way to register an error handler. You should ask
Jacques Garrigue that.. hmmm. no I'll post this to the list
because this is quite an interesting issue.

It would appear that this line in your code, which calls the
GTK binding:

 let appp = program2#buffer#get_text()

is the source of the problem, and that would be a bug in the Ocaml
library, not your code. Even if the buffer is empty, at least Ocaml
the library should handle this.


-- 
John Skaller <skaller at users dot sf dot net>
Felix, successor to C++: http://felix.sf.net

Re: Re: Re:Re: [Caml-list] How to prevent program from crashing

[Angela Zhu]

>
> Yeah, this is an abort from a failed assertion in C,
> so it isn't an Ocaml exception.
>
> I don't know how GTK handles this, that is, whether there is
> a way to register an error handler. You should ask
> Jacques Garrigue that.. hmmm. no I'll post this to the list
> because this is quite an interesting issue.
>
Thank you very much for your attention.
I wish this can be solved any way. :(

Best regards,
Angela
------------------------------------------
Dept. of CS, Rice Unitersity
http://www.cs.rice.edu/~yz2/
------------------------------------------



>
> It would appear that this line in your code, which calls the
> GTK binding:
>
>  let appp = program2#buffer#get_text()
>
> is the source of the problem, and that would be a bug in the Ocaml
> library, not your code. Even if the buffer is empty, at least Ocaml
> the library should handle this.
>
>
> -- 
> John Skaller <skaller at users dot sf dot net>
> Felix, successor to C++: http://felix.sf.net
>
>

Re: [Caml-list] How to prevent program from crashing

[Jacques Garrigue]

From: skaller <skaller@users.sourceforge.net>
> > Following is the message I got after the crashing:
> > 
> > 
> > ************************************************************************ 
> > ************************
> > (acumen:4063): Gtk-CRITICAL **: _gtk_text_line_char_locate: assertion  
> > `char_offs
> > et >= 0' failed
> > 
> > Gtk-ERROR **: Char offset -1 is off the end of the line
> > aborting...
> > Abort trap
> 
> Yeah, this is an abort from a failed assertion in C,
> so it isn't an Ocaml exception.
> 
> I don't know how GTK handles this, that is, whether there is
> a way to register an error handler. You should ask
> Jacques Garrigue that.. hmmm. no I'll post this to the list
> because this is quite an interesting issue.
> 
> It would appear that this line in your code, which calls the
> GTK binding:
> 
>  let appp = program2#buffer#get_text()
> 
> is the source of the problem, and that would be a bug in the Ocaml
> library, not your code. Even if the buffer is empty, at least Ocaml
> the library should handle this.

If this is intended as a bug report, this is a poor one indeed.
We need at least the version numbers for both Gtk and LablGTK2, and
the actual code...

As it is, buffer#get_text () seems to work perfectly with empty
buffers, with the latest version of LablGTK2, and Gtk-2.6.10.

Independently of that, text iterators can be very tricky to use
manually. LablGTK attempts at catching errors, but Gtk sometimes
crashes after the exception is caught... So better to be sure to make
no error.

Jacques Garrigue

Re: [Caml-list] How to prevent program from crashing

[skaller]

On Wed, 2007-10-31 at 16:47 +0900, Jacques Garrigue wrote:
> From: skaller <skaller@users.sourceforge.net>

> > It would appear that this line in your code, which calls the
> > GTK binding:
> > 
> >  let appp = program2#buffer#get_text()

> If this is intended as a bug report, this is a poor one indeed.

No, it isn't a bug report! It's a general request for more
information, maybe some help figuring out how this could
happen? Could you cause this with Ocaml code?


-- 
John Skaller <skaller at users dot sf dot net>
Felix, successor to C++: http://felix.sf.net