Re: [boost] gil::io "non-review" (was: [gil] Can not open test.jpg)

Subject: Re: [boost] gil::io "non-review" (was: [gil] Can not open test.jpg)
From: John Bytheway (jbytheway+boost_at_[hidden])
Date: 2010-03-23 15:56:05

Christian Henning wrote:
> Hi Phil, let's start at the beginning:
> 
>>> Next we clean up and issue an exception. Hope this makes sense.
>> No, it doesn't make sense.  Sorry if I am missing something important.
>>  Maybe someone else would like to have a look at this code?  It's here:
>>  http://gil-contributions.googlecode.com/svn/trunk/gil_2/boost/gil/extension/io_new/formats/jpeg/read.hpp
>> near the start of the file.
>>
> 
> My understanding setjmp/longjmp is this:
> 
> #include <iostream>
> #include <csetjmp>
> 
> static jmp_buf mark;
> 
> 
> void try_something()
> {
>     if( setjmp( mark ))
>     {
>         std::cout << "we are back" << std::endl;
>     }
> }
> 
> void error_exit()
> {
>     std::cout << "ups we had a problem" << std::endl;
>     longjmp( mark, 1 );
> }
> 
> int main( int argc, char* argv[] )
> {
>     try_something();
>     error_exit();
> 
>     return 0;
> }
> 
> When error_exit is called we are jumping back to where setjmp was called before.
> 
> Do you agree that's the correct behavior?

I agree with Phil; that's undefined behaviour.  I can't access the
official C standard here, but in the C99 draft N1256
(http://www.open-std.org/jtc1/sc22/WG14/www/docs/n1256.pdf), in
7.13.2.1p2, it says (of longjmp):

... if the function containing
the invocation of the setjmp macro has terminated execution in the
interim, ... the behavior is undefined.

So, once try_something has returned, the effect of calls to longjmp is
undefined.

John Bytheway