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Perl IO vs GLOB symbols

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Old 07-03-2008
Error Perl IO vs GLOB symbols


Can someone please clarify how we are able to use both IO and GLOB symbols of a package variable interchangeably?

Please consider the following code:
open(FH,"myfile") || die "Unable to open file myfile:$@";
my $glob_var = *main::FH{GLOB};
my $io_var = *main::FH{IO};
print $glob_var "From glob\n";
print $io_var "From IO\n";
close FH;

Now 'myfile' contains,
From glob
From IO


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Devel::Symdump(3)					User Contributed Perl Documentation					 Devel::Symdump(3)

Devel::Symdump - dump symbol names or the symbol table SYNOPSIS
# Constructor require Devel::Symdump; @packs = qw(some_package another_package); $obj = Devel::Symdump->new(@packs); # no recursion $obj = Devel::Symdump->rnew(@packs); # with recursion # Methods @array = $obj->packages; @array = $obj->scalars; @array = $obj->arrays; @array = $obj->hashes; @array = $obj->functions; @array = $obj->filehandles; # deprecated, use ios instead @array = $obj->dirhandles; # deprecated, use ios instead @array = $obj->ios; @array = $obj->unknowns; $string = $obj->as_string; $string = $obj->as_HTML; $string = $obj1->diff($obj2); $string = Devel::Symdump->isa_tree; # or $obj->isa_tree $string = Devel::Symdump->inh_tree; # or $obj->inh_tree # Methods with autogenerated objects # all of those call new(@packs) internally @array = Devel::Symdump->packages(@packs); @array = Devel::Symdump->scalars(@packs); @array = Devel::Symdump->arrays(@packs); @array = Devel::Symdump->hashes(@packs); @array = Devel::Symdump->functions(@packs); @array = Devel::Symdump->ios(@packs); @array = Devel::Symdump->unknowns(@packs); Incompatibility with versions before 2.00 Perl 5.003 already offered the opportunity to test for the individual slots of a GLOB with the *GLOB{XXX} notation. Devel::Symdump version 2.00 uses this method internally which means that the type of undefined values is recognized in general. Previous versions couldn't deter- mine the type of undefined values, so the slot unknowns was invented. From version 2.00 this slot is still present but will usually not contain any elements. The interface has changed slightly between the perl versions 5.003 and 5.004. To be precise, from perl5.003_11 the names of the members of a GLOB have changed. "IO" is the internal name for all kinds of input-output handles while "FILEHANDLE" and "DIRHANDLE" are deprecated. "Devel::Symdump" accordingly introduces the new method ios() which returns filehandles and directory handles. The old methods filehandles() and dirhandles() are still supported for a transitional period. They will probably have to go in future versions. DESCRIPTION
This little package serves to access the symbol table of perl. "Devel::Symdump->rnew(@packages)" returns a symbol table object for all subtrees below @packages. Nested Modules are analyzed recursively. If no package is given as argument, it defaults to "main". That means to get the whole symbol table, just do a "rnew" without arguments. "Devel::Symdump->new(@packages)" does not go into recursion and only analyzes the packages that are given as arguments. The methods packages(), scalars(), arrays(), hashes(), functions(), ios(), and unknowns() each return an array of fully qualified symbols of the specified type in all packages that are held within a Devel::Symdump object, but without the leading "$", "@" or "%". In a scalar context, they will return the number of such symbols. Unknown symbols are usually either formats or variables that haven't yet got a defined value. As_string() and as_HTML() return a simple string/HTML representations of the object. Diff() prints the difference between two Devel::Symdump objects in human readable form. The format is similar to the one used by the as_string method. Isa_tree() and inh_tree() both return a simple string representation of the current inheritance tree. The difference between the two meth- ods is the direction from which the tree is viewed: top-down or bottom-up. As I'm sure, many users will have different expectation about what is top and what is bottom, I'll provide an example what happens when the Socket module is loaded: % print Devel::Symdump->inh_tree AutoLoader DynaLoader Socket DynaLoader Socket Exporter Carp Config Socket The inh_tree method shows on the left hand side a package name and indented to the right the packages that use the former. % print Devel::Symdump->isa_tree Carp Exporter Config Exporter DynaLoader AutoLoader Socket Exporter DynaLoader AutoLoader The isa_tree method displays from left to right ISA relationships, so Socket IS A DynaLoader and DynaLoader IS A AutoLoader. (Actually, they were at the time this manpage was written) You may call both methods, isa_tree() and inh_tree(), with an object. If you do that, the object will store the output and retrieve it when you call the same method again later. The typical usage would be to use them as class methods directly though. SUBCLASSING
The design of this package is intentionally primitive and allows it to be subclassed easily. An example of a (maybe) useful subclass is Devel::Symdump::Export, a package which exports all methods of the Devel::Symdump package and turns them into functions. AUTHORS
Andreas Koenig <> and Tom Christiansen <>. Based on the old by Larry Wall. perl v5.8.0 2002-04-18 Devel::Symdump(3)

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