wrong dereference for returned value?

Hello NG,

i'm new to this group, but already did some research on this topic here in
perl.beginners and of course via google, but haven't found a solution so
far, so i try to get some help here.

I started to write some scripts and in one of them a return value of a
function is being returned as a reference ($answer) to the hash (by
Frontier::Client).
This response has to be processed by the function printhash in order to get
the result printed.

#######################################################
### subfunctions.pl ###
[...]
sub QueryEventList(){
my %queryhash = %inithash; ## predefined basic values ##
$queryhash{'COMMAND'} = "QueryEventList"; ## more additions to the hash in
other subs
my $answer = $client->call("function.call",\%queryhash );
return $answer;
}
[...]
sub printhash(){
my %h = %{$_[0]}; ## this line contains the fault ##
my $o = $_[1] ? " => $_[1]" : "";

foreach my $k (keys %h) {
if ($h{$k} =~ /HASH/) {
&printhash($h{$k}, "$o$k");
} elsif ($h{$k} =~ /ARRAY/) {
for(@{$h{$k}}) {
print "$k => $_\n";
}
} else {
print "$k => $h{$k}\n";
}
}
}
[...]
#######################################################
### main ###
[...]
{
if ($arguments >= 1){&usage() && &dmax();}
print "EreignisListe abfragen: \n";
&QueryEventList(); ## from subfunctions.pl
}
[...]
&printhash($answer);
#######################################################

But as i call &printhash($answer) via the main program, i get the error "Use
of uninitialized value in hash dereference at subfunctions.pl
line 'faultline'".
I don't know what's wrong there.
The returned value contains the server response in the reference to the
response hash ($answer), so i dereference the value returned by the
subfunction as an argument to the printhash-function (by %{$_[0]}) in order
to get the hash, but this code seems to be buggy...

Has anyone an idea what went wrong?

thanks in advance.
Niko

Re: wrong dereference for returned value?

Nikolai Försterling wrote:
>
> i'm new to this group, but already did some research on this topic here in
> perl.beginners and of course via google, but haven't found a solution so
> far, so i try to get some help here.
>
> I started to write some scripts and in one of them a return value of a
> function is being returned as a reference ($answer) to the hash (by
> Frontier::Client).
> This response has to be processed by the function printhash in order to get
> the result printed.
>
> #######################################################
> ### subfunctions.pl ###
> [...]
> sub QueryEventList(){

You are using prototypes. Read:
http://library.n0i.net/programming/perl/articles/fm_prototyp es/ to find out
why you should *not* use prototypes.


> my %queryhash = %inithash; ## predefined basic values ##
> $queryhash{'COMMAND'} = "QueryEventList"; ## more additions to the hash in
> other subs
> my $answer = $client->call("function.call",\%queryhash );
> return $answer;
> }
> [...]
> sub printhash(){

Your prototype says that this subroutine accepts *NO* arguments. But you are
probably not getting a warning message about this because you apparently
always prepend '&' to all sub calls which bypasses the prototype. So why are
you using prototypes if you are just bypassing them anyways?

perldoc perlsub


> my %h = %{$_[0]}; ## this line contains the fault ##
> my $o = $_[1] ? " => $_[1]" : "";
>
> foreach my $k (keys %h) {
> if ($h{$k} =~ /HASH/) {
> &printhash($h{$k}, "$o$k");
> } elsif ($h{$k} =~ /ARRAY/) {
> for(@{$h{$k}}) {
> print "$k => $_\n";
> }
> } else {
> print "$k => $h{$k}\n";
> }
> }
> }
> [...]
> #######################################################
> ### main ###
> [...]
> {
> if ($arguments >= 1){&usage() && &dmax();}
> print "EreignisListe abfragen: \n";
> &QueryEventList(); ## from subfunctions.pl
> }
> [...]
> &printhash($answer);
> #######################################################
>
> But as i call &printhash($answer) via the main program, i get the error "Use
> of uninitialized value in hash dereference at subfunctions.pl
> line 'faultline'".

That *warning* (not error) message means that the value of the first argument
passed to printhash() is undefined. Use the defined() function to test
whether a value is defined or not.

perldoc -f defined



John
--
Perl isn't a toolbox, but a small machine shop where you can special-order
certain sorts of tools at low cost and in short order. -- Larry Wall

Re: wrong dereference for returned value?

Thanks for your answer.

John W. Krahn wrote:

[...]
>> sub QueryEventList(){
>
> You are using prototypes. Read:
> http://library.n0i.net/programming/perl/articles/fm_prototyp es/ to find
> out why you should *not* use prototypes.

[x]planned this evening :-)

[...]
>> return $answer;
>> }
>> [...]
>> sub printhash(){
>
> Your prototype says that this subroutine accepts *NO* arguments. But you
> are probably not getting a warning message about this because you
> apparently
> always prepend '&' to all sub calls which bypasses the prototype. So why
> are you using prototypes if you are just bypassing them anyways?

I'm not that experienced in Perl since i started to write my first scripts
about two months ago and don't understand every detail so far.

> perldoc perlsub

[x]noticed and planned to study, too

>> my %h = %{$_[0]}; ## this line contains the fault ##
>> my $o = $_[1] ? " => $_[1]" : "";
[...]
>> But as i call &printhash($answer) via the main program, i get the error
>> "Use of uninitialized value in hash dereference at subfunctions.pl
>> line 'faultline'".
>
> That *warning* (not error) message means that the value of the first
> argument
> passed to printhash() is undefined. Use the defined() function to test
> whether a value is defined or not.

That value should be defined in every case as every sub gets a response
i already tried the functions without (), especially printhash.

> perldoc -f defined

If i got some more questions, i'll be back.
But a lot to read til then ;-)

> John

Thanks

Re: wrong dereference for returned value?

John W. Krahn wrote:

>> [...]
>> sub QueryEventList(){
>
> You are using prototypes. Read:
> http://library.n0i.net/programming/perl/articles/fm_prototyp es/ to find
> out why you should *not* use prototypes.

First, I tried to just change the sub entries to the correct(?) form, e.g.
sub test($$$) {...}, while sub QueryEventList() is correct as this function
doesn't need parameters.


[...]
>> sub printhash(){

changed to:
sub printhash(\[$%]){ ## should accept these variables as parameters
if (!defined %{$_[0]}) {die "keine Uebergabe des Hashes!\n";}
my %h = %{$_[0]};

> Your prototype says that this subroutine accepts *NO* arguments. But you
> are probably not getting a warning message about this because you
> apparently
> always prepend '&' to all sub calls which bypasses the prototype. So why
> are you using prototypes if you are just bypassing them anyways?

Additionally, i removed all "&" from the function calls in the main script,
but %{$_[0]} still seems to be dead...

>> [...]
>> #######################################################
>> ### main ###
>> [...]
>> {
>> if ($arguments >= 1){&usage() && &dmax();}
>> print "EreignisListe abfragen: \n";
>> &QueryEventList(); ## from subfunctions.pl

changed to:
QueryEventList(); ## from subfunctions.pl
while those which need Parameters now look like:
NewEvent($$); ## when two Params are expected

> That *warning* (not error) message means that the value of the first
> argument
> passed to printhash() is undefined. Use the defined() function to test
> whether a value is defined or not.

As mentioned above, printhash now always dies as there is no %{$_[0]}, but
the subfunctions should return it in $answer, which is the reference to the
response hash.
But e.g.
sub QueryEventList{
my %queryhash = %inithash;
$queryhash{'COMMAND'} = "QueryEventList";
my $answer = $client->call("function.call",\%queryhash );
return $answer;
}
should return the expected value when i call
&printhash($answer);

lg Niko

Re: wrong dereference for returned value?

On 11/01/2006 07:28 AM, Nikolai Försterling wrote:
> Hello NG,
>
> i'm new to this group, but already did some research on this topic here in
> perl.beginners and of course via google, but haven't found a solution so
> far, so i try to get some help here.
>
> I started to write some scripts and in one of them a return value of a
> function is being returned as a reference ($answer) to the hash (by
> Frontier::Client).
> This response has to be processed by the function printhash in order to get
> the result printed.
>
> #######################################################
> ### subfunctions.pl ###
> [...]
> sub QueryEventList(){
> my %queryhash = %inithash; ## predefined basic values ##
> $queryhash{'COMMAND'} = "QueryEventList"; ## more additions to the hash in
> other subs
> my $answer = $client->call("function.call",\%queryhash );

"My $answer" makes $answer a lexical variable that is restricted to
QueryEventList. By default, $answer has no association with any other
$answer variable anywhere else in the program.

> return $answer;
> }
> [...]
> sub printhash(){
> my %h = %{$_[0]}; ## this line contains the fault ##
> my $o = $_[1] ? " => $_[1]" : "";
>
> foreach my $k (keys %h) {
> if ($h{$k} =~ /HASH/) {
> &printhash($h{$k}, "$o$k");
> } elsif ($h{$k} =~ /ARRAY/) {
> for(@{$h{$k}}) {
> print "$k => $_\n";
> }
> } else {
> print "$k => $h{$k}\n";
> }
> }
> }
> [...]
> #######################################################
> ### main ###
> [...]
> {
> if ($arguments >= 1){&usage() && &dmax();}
> print "EreignisListe abfragen: \n";
> &QueryEventList(); ## from subfunctions.pl

The value returned by QueryEventList is not stored anywhere.


> }
> [...]
> &printhash($answer);

Understandably, $answer is undefined because it hasn't been initialized
with the return value of QueryEventList.

> #######################################################
>
> But as i call &printhash($answer) via the main program, i get the error "Use
> of uninitialized value in hash dereference at subfunctions.pl
> line 'faultline'".
> I don't know what's wrong there.
> The returned value contains the server response in the reference to the
> response hash ($answer), so i dereference the value returned by the
> subfunction as an argument to the printhash-function (by %{$_[0]}) in order
> to get the hash, but this code seems to be buggy...
>
> Has anyone an idea what went wrong?
>
> thanks in advance.
> Niko

You're welcome.

--
paduille.4060.mumia.w@earthlink.net

Re: wrong dereference for returned value?

Mumia W. (reading news) wrote:

>> ### subfunctions.pl ###
>> [...]
>> sub QueryEventList(){
>> my %queryhash = %inithash; ## predefined basic values ##
>> $queryhash{'COMMAND'} = "QueryEventList"; ## more additions to the hash
>> in
>> other subs
>> my $answer = $client->call("function.call",\%queryhash );
>
> "My $answer" makes $answer a lexical variable that is restricted to
> QueryEventList. By default, $answer has no association with any other
> $answer variable anywhere else in the program.

But if this is the return code, shouldn't it be transfered as a parameter to
printhash?

>> return $answer;
>> }
>> [...]
>> sub printhash(){
>> my %h = %{$_[0]}; ## this line contains the fault ##
>> my $o = $_[1] ? " => $_[1]" : "";


>> [...]
>> if ($arguments >= 1){&usage() && &dmax();}
>> print "EreignisListe abfragen: \n";
>> &QueryEventList(); ## from subfunctions.pl
>
> The value returned by QueryEventList is not stored anywhere.

I thought that if i return $answer, this would become %{$_[0]} somewhat
automagically.

>> [...]
>> &printhash($answer);
>
> Understandably, $answer is undefined because it hasn't been initialized
> with the return value of QueryEventList.

Shouldn't this have happened with the declaration of "our $answer"?
Sorry, i haven't posted the whole code, so you cannot know that that script
includes "our (%inithash,%queryhash,$answer,$client);"

greetings Niko

Re: wrong dereference for returned value?

On 11/02/2006 08:26 AM, Nikolai Försterling wrote:
> Mumia W. (reading news) wrote:
>
>>> ### subfunctions.pl ###
>>> [...]
>>> sub QueryEventList(){
>>> my %queryhash = %inithash; ## predefined basic values ##
>>> $queryhash{'COMMAND'} = "QueryEventList"; ## more additions to the hash
>>> in
>>> other subs
>>> my $answer = $client->call("function.call",\%queryhash );
>> "My $answer" makes $answer a lexical variable that is restricted to
>> QueryEventList. By default, $answer has no association with any other
>> $answer variable anywhere else in the program.
>
> But if this is the return code, shouldn't it be transfered as a parameter to
> printhash?
>
>>> return $answer;
>>> }
>>> [...]
>>> sub printhash(){
>>> my %h = %{$_[0]}; ## this line contains the fault ##
>>> my $o = $_[1] ? " => $_[1]" : "";
>
>
>>> [...]
>>> if ($arguments >= 1){&usage() && &dmax();}
>>> print "EreignisListe abfragen: \n";
>>> &QueryEventList(); ## from subfunctions.pl
>> The value returned by QueryEventList is not stored anywhere.
>
> I thought that if i return $answer, this would become %{$_[0]} somewhat
> automagically.
>
>>> [...]
>>> &printhash($answer);
>> Understandably, $answer is undefined because it hasn't been initialized
>> with the return value of QueryEventList.
>
> Shouldn't this have happened with the declaration of "our $answer"?

No, as I said above, the $answer variable in QueryEventList is trapped
within QueryEventList and has no association with any other
variable--even ones named $answer in other parts of the script.


> Sorry, i haven't posted the whole code, so you cannot know that that script
> includes "our (%inithash,%queryhash,$answer,$client);"
>
> greetings Niko

Since $answer was already declared "our," you shouldn't have declared it
with "my" in QueryEventList.

Use debugging code (print statements) to help you find out what is
really in $answer before you call printhash, e.g.

print "The value of answer is $answer\n";
printhash($answer);

In fact, you should put a print statement in QueryEventList too. Put in
the debugging statements and test, then remove the "my" before $answer
in QueryEventList and test.


--
paduille.4060.mumia.w@earthlink.net

Re: wrong dereference for returned value?

Mumia W. (reading news) wrote:

>> Shouldn't this have happened with the declaration of "our $answer"?
>
> No, as I said above, the $answer variable in QueryEventList is trapped
> within QueryEventList and has no association with any other
> variable--even ones named $answer in other parts of the script.

I thought that the return of $answer automatically gets read in printhash's
input variable %{$_[0]}...

>> Sorry, i haven't posted the whole code, so you cannot know that that
>> script includes "our (%inithash,%queryhash,$answer,$client);"
>
> Since $answer was already declared "our," you shouldn't have declared it
> with "my" in QueryEventList.

Huh, I had to d o something like this, cause the program is splitted into
four files and though I used "use 'vars'" and "require 'other_file'", the
interpreter couldn't find the values declared in the other files, so I
Thought I had to declare them in all other files, too.

> Use debugging code (print statements) to help you find out what is
> really in $answer before you call printhash, e.g.
>
> print "The value of answer is $answer\n";
> printhash($answer);
>
> In fact, you should put a print statement in QueryEventList too. Put in
> the debugging statements and test, then remove the "my" before $answer
> in QueryEventList and test.

I'm afraid you're right and the previous errors mentioned above have been
caused by other mistakes in the code and that the our-declaration of
$answer hasn't been necessary.

Though not really a solution, i bypassed the problem by doing the following
(in the subs file):
[...]
sub QueryEventList{
my %queryhash = %inithash;
$queryhash{'COMMAND'} = "QueryEventList";
my $answer = $client->call("api.call",\%queryhash );
if (defined $answer) {&printhash($answer);} ## <--
else {&noanswer;}
[...]
The return code ($answer) is no longer needed and now it works as intended.
Next thing to is a frontend for it :-/
Some sleepless nights again...

Thanks for your help and patience :-)