docs::api::APR::Error(3)                       User Contributed Perl Documentation                      docs::api::APR::Error(3)



NAME
       APR::Error - Perl API for APR/Apache/mod_perl exceptions

Synopsis
         eval { $obj->mp_method() };
         if ($@ && $ref $@ eq 'APR::Error' && $@ == $some_code) {
             # handle the exception
         }
         else {
             die $@; # rethrow it
         }

Description
       "APR::Error" handles APR/Apache/mod_perl exceptions for you, while leaving you in control.

       Apache and APR API return a status code for almost all methods, so if you didn't check the return code and handled any
       possible problems, you may have silent failures which may cause all kind of obscure problems. On the other hand checking
       the status code after each call is just too much of a kludge and makes quick prototyping/development almost impossible,
       not talking about the code readability. Having methods return status codes, also complicates the API if you need to
       return other values.

       Therefore to keep things nice and make the API readable we decided to not return status codes, but instead throw
       exceptions with "APR::Error" objects for each method that fails. If you don't catch those exceptions, everything works
       transparently - perl will intercept the exception object and "die()" with a proper error message. So you get all the
       errors logged without doing any work.

       Now, in certain cases you don't want to just die, but instead the error needs to be trapped and handled. For example if
       some IO operation times out, may be it is OK to trap that and try again. If we were to die with an error message, you
       would have had to match the error message, which is ugly, inefficient and may not work at all if locale error strings are
       involved. Therefore you need to be able to get the original status code that Apache or APR has generated. And the
       exception objects give you that if you want to. Moreover the objects contain additional information, such as the function
       name (in case you were eval'ing several commands in one block), file and line number where that function was invoked
       from. More attributes could be added in the future.

       "APR::Error" uses Perl operator overloading, such that in boolean and numerical contexts, the object returns the status
       code; in the string context the full error message is returned.

       When intercepting exceptions you need to check whether $@ is an object (reference). If your application uses other
       exception objects you additionally need to check whether this is a an "APR::Error" object. Therefore most of the time
       this is enough:

         eval { $obj->mp_method() };
         if ($@ && $ref $@ && $@ == $some_code)
             warn "handled exception: $@";
         }

       But with other, non-mod_perl, exception objects you need to do:

         eval { $obj->mp_method() };
         if ($@ && $ref $@ eq 'APR::Error' && $@ == $some_code)
             warn "handled exception: $@";
         }

       In theory you could even do:

         eval { $obj->mp_method() };
         if ($@ && $@ == $some_code)
             warn "handled exception: $@";
         }

       but it's possible that the method will die with a plain string and not an object, in which case "$@ == $some_code" won't
       quite work. Remember that mod_perl throws exception objects only when Apache and APR fail, and in a few other special
       cases of its own (like "exit").

         warn "handled exception: $@" if $@ && $ref $@;

       There are two ways to figure out whether an error fits your case. In most cases you just compare $@ with an the error
       constant. For example if a socket has a timeout set and the data wasn't read within the timeout limit a
       "APR::Const::TIMEUP")

         use APR::Const -compile => qw(TIMEUP);
         $sock->timeout_set(1_000_000); # 1 sec
         my $buff;
         eval { $sock->recv($buff, BUFF_LEN) };
         if ($@ && ref $@ && $@ == APR::Const::TIMEUP) {

         }

       However there are situations, where on different Operating Systems a different error code will be returned. In which case
       to simplify the code you should use the special subroutines provided by the "APR::Status" class. One such condition is
       socket "recv()" timeout, which on Unix throws the "EAGAIN" error, but on other system it throws a different error. In
       this case "APR::Status::is_EAGAIN" should be used.

       Let's look at a complete example. Here is a code that performs a socket read:

         my $rlen = $sock->recv(my $buff, 1024);
         warn "read $rlen bytes\n";

       and in certain cases it times out. The code will die and log the reason for the failure, which is fine, but later on you
       may decide that you want to have another attempt to read before dying and add some fine grained sleep time between
       attempts, which can be achieved with "select". Which gives us:

         use APR::Status ();
         # ....
         my $tries = 0;
         my $buffer;
         RETRY: my $rlen = eval { $sock->recv($buffer, SIZE) };
         if ($@)
             die $@ unless ref $@ && APR::Status::is_EAGAIN($@);
             if ($tries++ < 3) {
                 # sleep 250msec
                 select undef, undef, undef, 0.25;
                 goto RETRY;
             }
             else {
                 # do something else
             }
         }
         warn "read $rlen bytes\n"

       Notice that we handle non-object and non-"APR::Error" exceptions as well, by simply re-throwing them.

       Finally, the class is called "APR::Error" because it needs to be used outside mod_perl as well, when called from "APR"
       applications written in Perl.

API
   "cluck"
       "cluck" is an equivalent of "Carp::cluck" that works with "APR::Error" exception objects.

   "confess"
       "confess" is an equivalent of "Carp::confess" that works with "APR::Error" exception objects.

   "strerror"
       Convert APR error code to its string representation.

         $error_str = APR::Error::strerror($rc);

       ret: $rc ( "APR::Const status constant" )
           The numerical value for the return (error) code

       ret: $error_str ( string )
           The string error message corresponding to the numerical value inside $rc.  (Similar to the C function strerror(3))

       since: 2.0.00

       Example:

       Try to retrieve the bucket brigade, and if the return value doesn't indicate success or end of file (usually in protocol
       handlers) die, but give the user the human-readable version of the error and not just the code.

         my $rc = $c->input_filters->get_brigade($bb_in,
                                                 Apache2::Const::MODE_GETLINE);
         if ($rc != APR::Const::SUCCESS && $rc != APR::Const::EOF) {
             my $error = APR::Error::strerror($rc);
             die "get_brigade error: $rc: $error\n";
         }

       It's probably a good idea not to omit the numerical value in the error message, in case the error string is generated
       with non-English locale.

See Also
       mod_perl 2.0 documentation.

Copyright
       mod_perl 2.0 and its core modules are copyrighted under The Apache Software License, Version 2.0.

Authors
       The mod_perl development team and numerous contributors.



perl v5.12.0                                               2007-11-12                                   docs::api::APR::Error(3)