Re: [Openexr-devel] validity check on files

[Paul Schneider]

The EXR library in general uses C++ exceptions as the preferred means 
of letting you know that something went wrong, so you really should be 
prepared to catch a exception from the library at any time.

In this case, the InputFile's constructor causes the file to be opened 
and the header information to be read.  If the header isn't valid (not 
an EXR image, or an unsupported version), it will throw an 
Iex::InputExc.  Calling the what() method of this exception will let 
you know what went wrong.

Since Iex::InputExc inherits from Iex::BaseExc, which inherits from 
std::exception, you can catch any exeption that might be thrown like 
this:

try
{
        InputFile file(fileName);
}
catch (const std::exception& ex)
{
	cerr << "couldn't open input file " << fileName << ": " << ex.what() 
<< endl;
}

This will also let you handle exceptions such as std::bad_alloc (out of 
memory), and Iex::ErrnoExc exceptions, which mean that the file 
couldn't be opened for some reason (permissions, maybe).

Note that you can put the image reading code in the try block as well, 
and you only have to worry about error handling in one place.

If you need more fine-grained error handling, you can catch specific 
exceptions:

try
{
        InputFile file(fileName);
}
catch (const EpermExc& ex)
{
        cerr << "don't have permission to open that file" << endl;
}
catch (const Iex::InputExc& ex)
{
        cerr << "file doesn't appear to really be an EXR file" << endl;
}
catch (const std::bad_alloc& ex)
{
        cerr << "out of memory opening file" << endl;
}
catch (const std::exception& ex)
{
        cerr << "unkown error" << endl;
}

This could let you silently ignore invalid EXR files, if you were just 
filtering them in an open file dialog for example.  In general, you 
should probably pass the what() string along to the user in some way, 
as they tend to be pretty descriptive.

Hope that helps,
Paul


On Friday, October 3, 2003, at 12:36 PM, James McPhail wrote:

> I hope this hasn't been touched upon in a previous thread.
>
> Is there functionality in the toolkit for determining if an 
> "InputFile" is bogus? Specifically, can the toolkit determine, when 
> passed a file name, that the given file is not of the EXR format?
>
> I had initially wanted to try
>
> InputFile file(fileName);
>
> then had hoped there would be some flag or function call on this 
> 'file' variable or its associated header which would indicate that it 
> was not a valid file, however this InputFile constructor throws an 
> exception (which I couldn't manage to catch) and then crashes.
>
> Any help would be appreciated.
>
> James
>
>
>
>
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