I have a server that also counts the number of distinct IPs connected in a minute. I get a "Modification of a read-only value attempted" in the marked line in the code below:
$lsock=new IO::Socket::INET(LocalPort=>$port,Proto=>'tcp',Listen=>1,Reuse=>1);
$clients={};
while (1) {
$sock=$lsock->accept();
if ($sock) {
$clients->{$sock->peerhost()}=1; # THIS LINE !!!
...
}
}
# then later from an alarm signal I do:
sub save_stats {
my $cnt=scalar keys %$clients;
$clients={};
...
}
The error appears very rarely, once a month or less, but it's driving me insane, can someone please explain it to me why and what can I do?
After a lot of investigation I found the answer. It can happen that the connection is lost between accept() and peerhost(). In this case the peeerhost() function's return value is not suitable as a hash key. The solution is very simple, if peerhost() returns a "false" ignore the connection alltogether.
Related
I have a monthly Script running to archive files from past month. I'm using Archive::Tar to generate the archive. How can I check if calls to the ->write method are successful?
Does the following work? I didn't manage to fail $tar->write() yet.
unless ($tar->write( $name . '.tar.xz', COMPRESS_XZ )) {
die("cant write tar"); # or any other doings instead
}
Yes, the code in your question will catch any error within ->write.
The documentation of Archive::Tar does not specify what the return value of write is (except when no argument is provided, which isn't the case here). However, looking at the code of the module, write returns undef in case of an error and a true value in case of success:
1 if it was writing to a file (this is the case for the code in your question)
the written string if it was writing in a string.
Note that if something goes wrong and Archive::Tar returns undef, then it will also print an error message (unless you set $Archive::Tar::WARN to 0 by doing $Archive::Tar::WARN = 0). If you want to do something specific depending on the error, you can access the error message using the ->error method.
When I asked a question last year about promises, my echo server was working (see this link: perl6 how to get specific identity of promises? ). However, with the new versions of perl6, my echo server is no longer working.
I guess I can try the example from the perl6 documentation site ( https://docs.perl6.org/type/IO::Socket::INET ), but I want to find out what mistake I have made in my code. My current level has precluded me from seeing the difference between my codes and the codes on the perl6 documentation site. Please give me a hint; thanks !
my #result;
for 0 .. 2 -> $index {
#result[$index] = start {
my $myPromiseID = $index;
say "======> $myPromiseID\n";
my $rsSocket = IO::Socket::INET.new:
localhost => 'localhost',
localport => 1234 + $index,
listen => 1;
while $rsSocket.accept -> $rsConnection {
say "Promise $myPromiseID accepted connection";
while $rsConnection.recv -> $stuff {
say "Promise $myPromiseID Echoing $stuff";
$rsConnection.print($stuff);
}
$rsConnection.close;
}
}
}
await #result;
And the error messages are:
Tried to get the result of a broken Promise
in block <unit> at p6EchoMulti.pl line 24
Original exception:
Nothing given for new socket to connect or bind to
in block at p6EchoMulti.pl line 8
Actually thrown at:
in block at p6EchoMulti.pl line 13
This commit, which was announced in the Jan 2017 section of Rakudo's changelog as "Fixed bug where IPv6 URIs were not parsed correctly" did a lot more that just fix a URI parsing bug. It also completely redid the parameter binding/validation of an IO::Socket::INET.new call, and one consequence is it broke your code because the updated code requires that listen be an actual Bool, not merely coerce to one.
The old code (the code on the left of the commit link above) had a simple method new (*%args is copy). This matched your call. The error (fail "Nothing given for new socket to connect or bind to") did not trigger because 1 evaluates to True in a boolean context so %args<host> || %args<listen> was also True. So the rest of the code ran with listen set to 1 and it all worked out fine.
Rakudos from 2017.01 have the code on the right at the commit link above. Note how there are now multiple new methods (i.e. multiple multi method new ... declarations).
The multi(s) intended to handle a call that specifies a listen argument is/are of the form multi method new (..., Bool:D :$listen!, ...). Note the Bool:D.
A call to new, with the listen parameter set to True, matches this multi and works as expected.
But a call with :listen(1) will just match the generic multi method new (*%args) signature instead. This latter does an unconditional fail "Nothing given for new socket to connect or bind to";.
Okay, after some struggling, it seems to have improved if I changed listen=>1 to listen=>True.
Can anyone care to explain why 1 was not evaluated to True, and why it worked before?
Thanks.
I wrote a procedure which imports data from an xml-file into a MariaDB using library DBI. The procedure works but I don't understand why the following code gives me the message:
use of uninitialized value $DBI::err in concatenation (.) or string at ...
Here the code (abbreviated):
my $insert_art = $dbh->prepare(
"INSERT INTO odbb_articles (creation_dt,ref_data,comp_no)".
"VALUES (?,?,?)"
);
....
my $comp_no = $xpc->findvalue('./sr:ARTCOMP/sr:COMPNO',$node1);
....
$insert_art->execute($creation_dt,$ref_data,$comp_no)
or die "Fehler bei der Ausfuehrung: ".
"$DBI::err -> $DBI::errstr (odbb_articles $DBI::state)\n";
If I insert the code
if ($comp_no eq "") { $comp_no = undef; }
just before $insert_art->execute the procedure works. This error happens when there is no entry in the xml-file for element COMPNO. I can avoid it if I define it as undef. I just wonder
why $comp_no cause this problem and
is there another solution than to control if $comp_no is "" and define it as undef?
The reason for the second question is to avoid the if statement if there are a lot of variables/columns which may have empty entries.
Thanks for help.
use of uninitialized value $DBI::err in concatenation (.) or string at ...
The error message you are seeing is Perl telling you that $DBI::err is undef. That is not because of the value of your $comp_no. It's just a result of what your program is doing.
So when you pass an empty string to the comp_no column, the database doesn't like that. It throws an error. DBI catches that error and passes it on. The $insert_art->execute returns a false value and the right-hand-side of the or gets called. That's your die.
Now in the string that you pass to die you put three variables:
$DBI::err
$DBI::errstr
$DBI::state
According to the DBI documentation, those are equivalent to the functions $h->err, $h->errstr and $h->state with $h being the last handle used. Let's look at the docs for those.
$h->err
Returns the native database engine error code from the last driver method called. The code is typically an integer but you should not assume that.
The DBI resets $h->err to undef before almost all DBI method calls, so the value only has a short lifespan. Also, for most drivers, the statement handles share the same error variable as the parent database handle, so calling a method on one handle may reset the error on the related handles. [...]
This does not explain when it can be undef.
$h->errstr
Returns the native database engine error message from the last DBI method called. This has the same lifespan issues as the "err" method described above.
The returned string may contain multiple messages separated by newline characters.
The errstr() method should not be used to test for errors, use err() for that, because drivers may return 'success with information' or warning messages via errstr() for methods that have not 'failed'.
Ok, so this is text. Don't use it to test for specific errors. You're not doing that. You just want to give debug output when the program fails.
$h->state
Returns a state code in the standard SQLSTATE five character format. Note that the specific success code 00000 is translated to any empty string (false). If the driver does not support SQLSTATE (and most don't), then state() will return S1000 (General Error) for all errors.
The driver is free to return any value via state, e.g., warning codes, even if it has not declared an error by returning a true value via the "err" method described above.
The state() method should not be used to test for errors, use err() for that, because drivers may return a 'success with information' or warning state code via state() for methods that have not 'failed'.
Again, this is not very clear about how useful it is.
My advice is to get rid of $DBI::err and $DBI::state. You don't need those to figure out what the problem is. Just output $DBI::errstr.
$insert_art->execute($creation_dt,$ref_data,$comp_no)
or die "Fehler bei der Ausfuehrung: " . $dbh->errstr;
Now your program will still fail, but at least you will have a meaningful error message that will explain what your database didn't like about the statement. That's better than being told how there is a bug in your error handling code.
Afterwards, the other answers will probably apply to fix the reason this is happening in the first case.
On another note, a word on die: If you provide a \n at the end of your arguments, it will not print your current script, line number and input handle line number. But those might be useful in your case. You can include them.
In a SQL database an empty string is very different to null.
If comp_no has a foreign key pointing to a record in another table, then the value "" is an accettable one only if there is a record with "" as primary key, very improbable.
Yu can fix this converting empty values to undef:
for ($creation_dt,$ref_data,$comp_no ){
defined $_ and $_ eq '' and $_ = undef;
}
$insert_art->execute($creation_dt,$ref_data,$comp_no);
or also
$insert_art->execute(map {defined($_) && length($_) ? $_ : undef} ($creation_dt,$ref_data,$comp_no));
This is a possible shortcut:
$comp_no ||= undef;
With the caveat that this will work in any case where $comp_no evaluates as false, meaning a value of 0 will actually cause the result to go undef also, which may or may not matter to you. If your field is numeric, I'd say it matters a lot.
I am trying to extract data from website using perl API. The process is to use a list of uris as input. Then I extract related information for each uri from website. If the information for one uri is not present it dies. Some thing like the code below
my #tags = $c->posts_for(uri =>"$currentURI");
die "No candidate related articles\n" unless #tags;
Now, I don't want the program to stop if it doesn't get any tags. I want the program to skip that particular uri and go to the next available uri. How can i do it?
Thank you for your time and help.
Thank you,
Sammed
Well, assuming that you're inside a loop processing each of the URIs in turn, you should be able to do something like:
next unless #tags;
For example, the following program only prints lines that are numeric:
while (<STDIN>) {
next unless /^\d+$/;
print;
}
The loop processes every input line in turn but, when one is found that doesn't match that regular expression (all numeric), it restarts the loop (for the next input line) without printing.
The same method is used in that first code block above to restart the loop if there are no tags, moving to the next URI.
Besides the traditional flow control tools, i.e. next/last in a loop or return in a sub, one can use exceptions in perl:
eval {
die "Bad bad thing";
};
if ($#) {
# do something about it
};
Or just use Try::Tiny.
However, from the description of the task it seems next is enough (so I voted for #paxdiablo's answer).
The question is rather strange, but as near as I can tell, you are asking how to control the flow of your current loop. Of course, using die will cause your program to exit, so if you do not want that, you should not use die. Seems elementary to me, that's why it is a strange questions.
So, I assume you have a loop such as:
for my $currentURI (#uris) {
my #tags = $c->posts_for(uri =>"$currentURI");
die "No candidate related articles\n" unless #tags;
# do stuff with #tags here....
}
And if #tags is empty, you want to go to the next URI. Well, that's a simple thing to solve. There are many ways.
next unless #tags;
for my $tag (#tags) { ... stuff ... }
if (#tags) { .... }
Next is the simplest one. It skips to the end of the loop block and starts with the next iteration. However, using a for or if block causes the same behaviour, and so are equivalent. For example:
for my $currentURI (#uris) {
my #tags = $c->posts_for(uri =>"$currentURI");
for my $tag (#tags) {
do_something($tag);
}
}
Or even:
for my $currentURI (#uris) {
for my $tag ($c->posts_for(uri =>"$currentURI")) {
do_something($tag);
}
}
In this last example, we removed #tags all together, because it is not needed. The inner loop will run zero times if there are no "tags".
This is not really complex stuff, and if you feel unsure, I suggest you play around a little with loops and conditionals to learn how they work.
Like in Perl, if a hash key is uninitialized then if you perform the below code
$hash{$key} =~ $hash{$key}++
then the value for that particular key increases to 1 (cause, it's first undefined and then as per the context, here it's numaical ... it takes the value to 0 ... increases it to 1).
My question is, does the same concept follows in case of C# as well? I mean, if I perform the above code in c# what would be the result? Will it be 1 or what?
Any idea?
Thanks,
Rahul
That bit of code makes no sense.
If you want to know if the key exists in the hash:
if (exists $hash{$key}) { ... }
If you want to know if it has a value defined:
if (defined $hash{$key}) { ... }
If you want to increment the value,
$hash{$key}++
As it is, you're attempting to do a regex match in a rather nonsensical way.
OOPS!!! Sorry ... I got it; actually the Perl code mentioned is meant to check/confirm whether the particular "KEY" exist or not ... OR sort of that.
So, in C# I cna just check for "hashtable.containskey(key)" ... that will do the trick.
Thanks.