NAME

Astro::FITS::CFITSIO::CheckStatus - automatically catch CFITSIO status errors

SYNOPSIS

use Astro::FITS::CFITSIO::CheckStatus;

# call Carp::croak upon error
tie my $status, 'Astro::FITS::CFITSIO::CheckStatus';
$fptr = Astro::FITS::CFITSIO::create_file( $file, $status );

# call user specified function upon error:
tie my $status, 'Astro::FITS::CFITSIO::CheckStatus', $mycroak;
$fptr = Astro::FITS::CFITSIO::create_file( $file, $status );

# call Log::Log4perl->logcroak;
$logger = Log::Log4perl::get_logger();
tie my $status, 'Astro::FITS::CFITSIO::CheckStatus', $logger;
$fptr = Astro::FITS::CFITSIO::create_file( $file, $status );

DESCRIPTION

The CFITSIO library uses the concept of a status variable passed to each CFITSIO function call to return an error status. At present, the Astro::FITS::CFITSIO Perl interface mirrors the CFITSIO interfaces directly, and does not do anything special to handle error returns (e.g., by throwing an exception). It should be noted that CFITSIO routines will not perform their requested action if a non-zero status value is passed in, so as long as the same status variable is used throughout, CFITSIO routines won't do extra work after an error. However, this can lead to the situation where one does not know at which step the error occurred.

In order to immediately catch an error, the status error must be checked after each call to a CFITSIO routine. Littering one's code with status variable checks is ugly.

This module resolves the impasse by tieing the status variable to a class which will check the value every time it is set, and throw an exception (via Carp::croak) containing the CFITISO error message if the value is non-zero.

The drawback to this approach is that only the (sometimes) impenetrable CFITSIO error message is available. If the tied variable is set equal to a string (which should not pass the Scalar::Util::look_like_number() test) or a subroutine, the string or return value from the subroutine is prepended to the CFITSIO error message. For example

  Astro::FITS::CFITSIO::open_file( 'file_does_not_exist.fits',
	   Astro::FITS::CFITSIO::READONLY(), $status = "Bad Open:" );

will result in:

Bad Open: CFITSIO error: could not open the named file

The prefixing value may also be specified with the the utxt() method (see "Class Methods"). Whenever the status variable is set to zero, the prefixing value is forgotten. As Astro::FITS::CFITSIO sets the status variable (whether zero or not) after every call to a CFITSIO function, this implies that the prefix will not reset after every successful function call. This behavior may be modified by the reset_ustr() and reset_usub() class methods in "Class Methods". Alternatively, the prefix may be reset by assigning undef to the status variable.

The caller may provide an alternate means of throwing the exception, either by passing in a subroutine reference,

tie my $status, 'Astro::FITS::CFITSIO::CheckStatus', 
         sub { die "An awful thing happened: @_" };
$fptr = Astro::FITS::CFITSIO::create_file( $file, $status );

or a reference to a Log::Log4perl::Logger object.

$logger = Log::Log4perl::get_logger();
tie my $status, 'Astro::FITS::CFITSIO::CheckStatus', $logger;
$fptr = Astro::FITS::CFITSIO::create_file( $file, $status );

In the latter case, it will be equivalent to calling $logger->logcroak. An alternative interface is available via the set_croak() method (see "Class Methods").

Class Methods

The object to which the status variable is tied (accessible via the Perl tied function) has the following methods:

set_croak

$func = $obj->set_croak;
$old_value = $obj->set_croak( sub { ... } )

Get or set the function called if the status variable is set to a non-zero numerical value. The function takes a single argument.

utxt

$utxt = $obj->utxt;
$old_value = $obj->utxt( "user error message" );

This provides access to the "user text" which may prefix the output error message. It is another interface to the functionality provided by setting the status variable to either a string or a code reference.

etxt

print $obj->etxt, "\n";

This provides access to the CFITSIO error string if the status has been set to a non-zero numerical value.

reset_ustr

$old_value = $obj->reset_ustr( $boolean )

If the passed value is true, every time that the status variable is set to zero, and the "user text" is a string, it will be set to the undefined value. The default value is true

reset_usub

$old_value = $obj->reset_usub( $boolean )

If the passed value is true, every time that the status variable is set to zero, and the "user text" is a subroutine reference, it will be set to the undefined value. The default value is true.

EXPORT

None by default.

AUTHOR

Diab Jerius, <djerius@cpan.org>

COPYRIGHT AND LICENSE

Copyright (C) 2004 by The Smithsonian Astrophysical Observatory.

This software is released under the GNU General Public License. You may find a copy at http://www.fsf.org/copyleft/gpl.html.

SEE ALSO

Astro::FITS::CFITSIO, perl.