perltrap.1

视频监控网络部分的协议ddns,的模块的实现代码,请大家大胆指正.

\&    $string =~ s\`^\`hostname\`;
\&    print $string, "\en";
\&
\&    # perl4 prints: <the local hostname>
\&    # perl5 prints: hostname
.Ve
.IP "\(bu" 5
Stricter parsing of variables in regular expressions
.Sp
Stricter parsing of variables used in regular expressions
.Sp
.Vb 1
\&    s/^([^$grpc]*$grpc[$opt$plus$rep]?)//o;
\&
\&    # perl4: compiles w/o error
\&    # perl5: with Scalar found where operator expected ..., near "$opt$plus"
.Ve
.Sp
an added component of this example, apparently from the same script, is
the actual value of the s'd string after the substitution.
\&\f(CW\*(C`[$opt]\*(C'\fR is a character class in perl4 and an array subscript in perl5
.Sp
.Vb 5
\&    $grpc = \*(Aqa\*(Aq;
\&    $opt  = \*(Aqr\*(Aq;
\&    $_ = \*(Aqbar\*(Aq;
\&    s/^([^$grpc]*$grpc[$opt]?)/foo/;
\&    print;
\&
\&    # perl4 prints: foo
\&    # perl5 prints: foobar
.Ve
.IP "\(bu" 5
\&\f(CW\*(C`m?x?\*(C'\fR matches only once
.Sp
Under perl5, \f(CW\*(C`m?x?\*(C'\fR matches only once, like \f(CW\*(C`?x?\*(C'\fR. Under perl4, it matched
repeatedly, like \f(CW\*(C`/x/\*(C'\fR or \f(CW\*(C`m!x!\*(C'\fR.
.Sp
.Vb 10
\&    $test = "once";
\&    sub match { $test =~ m?once?; }
\&    &match();
\&    if( &match() ) {
\&        # m?x? matches more then once
\&        print "perl4\en";
\&    } else {
\&        # m?x? matches only once
\&        print "perl5\en";
\&    }
\&
\&    # perl4 prints: perl4
\&    # perl5 prints: perl5
.Ve
.IP "\(bu" 5
Failed matches don't reset the match variables
.Sp
Unlike in Ruby, failed matches in Perl do not reset the match variables
($1, \f(CW$2\fR, ..., \f(CW\*(C`$\`\*(C'\fR, ...).
.Sh "Subroutine, Signal, Sorting Traps"
.IX Subsection "Subroutine, Signal, Sorting Traps"
The general group of Perl4\-to\-Perl5 traps having to do with
Signals, Sorting, and their related subroutines, as well as
general subroutine traps.  Includes some OS-Specific traps.
.IP "\(bu" 5
Barewords that used to look like strings look like subroutine calls
.Sp
Barewords that used to look like strings to Perl will now look like subroutine
calls if a subroutine by that name is defined before the compiler sees them.
.Sp
.Vb 3
\&    sub SeeYa { warn"Hasta la vista, baby!" }
\&    $SIG{\*(AqTERM\*(Aq} = SeeYa;
\&    print "SIGTERM is now $SIG{\*(AqTERM\*(Aq}\en";
\&
\&    # perl4 prints: SIGTERM is now main\*(AqSeeYa
\&    # perl5 prints: SIGTERM is now main::1 (and warns "Hasta la vista, baby!")
.Ve
.Sp
Use \fB\-w\fR to catch this one
.IP "\(bu" 5
Reverse is no longer allowed as the name of a sort subroutine
.Sp
reverse is no longer allowed as the name of a sort subroutine.
.Sp
.Vb 2
\&    sub reverse{ print "yup "; $a <=> $b }
\&    print sort reverse (2,1,3);
\&
\&    # perl4 prints: yup yup 123
\&    # perl5 prints: 123
\&    # perl5 warns (if using \-w): Ambiguous call resolved as CORE::reverse()
.Ve
.IP "\(bu" 5
\&\f(CW\*(C`warn()\*(C'\fR won't let you specify a filehandle.
.Sp
Although it _always_ printed to \s-1STDERR\s0, \fIwarn()\fR would let you specify a
filehandle in perl4.  With perl5 it does not.
.Sp
.Vb 1
\&    warn STDERR "Foo!";
\&
\&    # perl4 prints: Foo!
\&    # perl5 prints: String found where operator expected
.Ve
.Sh "\s-1OS\s0 Traps"
.IX Subsection "OS Traps"
.IP "\(bu" 5
SysV resets signal handler correctly
.Sp
Under \s-1HPUX\s0, and some other SysV OSes, one had to reset any signal handler,
within  the signal handler function, each time a signal was handled with
perl4.  With perl5, the reset is now done correctly.  Any code relying
on the handler _not_ being reset will have to be reworked.
.Sp
Since version 5.002, Perl uses \fIsigaction()\fR under SysV.
.Sp
.Vb 4
\&    sub gotit {
\&        print "Got @_... ";
\&    }
\&    $SIG{\*(AqINT\*(Aq} = \*(Aqgotit\*(Aq;
\&
\&    $| = 1;
\&    $pid = fork;
\&    if ($pid) {
\&        kill(\*(AqINT\*(Aq, $pid);
\&        sleep(1);
\&        kill(\*(AqINT\*(Aq, $pid);
\&    } else {
\&        while (1) {sleep(10);}
\&    }
\&
\&    # perl4 (HPUX) prints: Got INT...
\&    # perl5 (HPUX) prints: Got INT... Got INT...
.Ve
.IP "\(bu" 5
SysV \f(CW\*(C`seek()\*(C'\fR appends correctly
.Sp
Under SysV OSes, \f(CW\*(C`seek()\*(C'\fR on a file opened to append \f(CW\*(C`>>\*(C'\fR now does
the right thing w.r.t. the \fIfopen()\fR manpage. e.g., \- When a file is opened
for append,  it  is  impossible to overwrite information already in
the file.
.Sp
.Vb 8
\&    open(TEST,">>seek.test");
\&    $start = tell TEST;
\&    foreach(1 .. 9){
\&        print TEST "$_ ";
\&    }
\&    $end = tell TEST;
\&    seek(TEST,$start,0);
\&    print TEST "18 characters here";
\&
\&    # perl4 (solaris) seek.test has: 18 characters here
\&    # perl5 (solaris) seek.test has: 1 2 3 4 5 6 7 8 9 18 characters here
.Ve
.Sh "Interpolation Traps"
.IX Subsection "Interpolation Traps"
Perl4\-to\-Perl5 traps having to do with how things get interpolated
within certain expressions, statements, contexts, or whatever.
.IP "\(bu" 5
\&\f(CW\*(C`@\*(C'\fR always interpolates an array in double-quotish strings
.Sp
@ now always interpolates an array in double-quotish strings.
.Sp
.Vb 1
\&    print "To: [e-mail]\en";
\&
\&    # perl4 prints: To:[e-mail]
\&    # perl < 5.6.1, error : In string, @somewhere now must be written as \e@somewhere
\&    # perl >= 5.6.1, warning : Possible unintended interpolation of @somewhere in string
.Ve
.IP "\(bu" 5
Double-quoted strings may no longer end with an unescaped $
.Sp
Double-quoted strings may no longer end with an unescaped $.
.Sp
.Vb 2
\&    $foo = "foo$";
\&    print "foo is $foo\en";
\&
\&    # perl4 prints: foo is foo$
\&    # perl5 errors: Final $ should be \e$ or $name
.Ve
.Sp
Note: perl5 \s-1DOES\s0 \s-1NOT\s0 error on the terminating @ in \f(CW$bar\fR
.IP "\(bu" 5
Arbitrary expressions are evaluated inside braces within double quotes
.Sp
Perl now sometimes evaluates arbitrary expressions inside braces that occur
within double quotes (usually when the opening brace is preceded by \f(CW\*(C`$\*(C'\fR
or \f(CW\*(C`@\*(C'\fR).
.Sp
.Vb 5
\&    @www = "buz";
\&    $foo = "foo";
\&    $bar = "bar";
\&    sub foo { return "bar" };
\&    print "|@{w.w.w}|${main\*(Aqfoo}|";
\&
\&    # perl4 prints: |@{w.w.w}|foo|
\&    # perl5 prints: |buz|bar|
.Ve
.Sp
Note that you can \f(CW\*(C`use strict;\*(C'\fR to ward off such trappiness under perl5.
.IP "\(bu" 5
\&\f(CW$$x\fR now tries to dereference \f(CW$x\fR
.Sp
The construct \*(L"this is $$x\*(R" used to interpolate the pid at that point, but
now tries to dereference \f(CW$x\fR.  \f(CW$$\fR by itself still works fine, however.
.Sp
.Vb 3
\&    $s = "a reference";
\&    $x = *s;
\&    print "this is $$x\en";
\&
\&    # perl4 prints: this is XXXx   (XXX is the current pid)
\&    # perl5 prints: this is a reference
.Ve
.IP "\(bu" 5
Creation of hashes on the fly with \f(CW\*(C`eval "EXPR"\*(C'\fR requires protection
.Sp
Creation of hashes on the fly with \f(CW\*(C`eval "EXPR"\*(C'\fR now requires either both
\&\f(CW\*(C`$\*(C'\fR's to be protected in the specification of the hash name, or both curlies
to be protected.  If both curlies are protected, the result will be compatible
with perl4 and perl5.  This is a very common practice, and should be changed
to use the block form of \f(CW\*(C`eval{}\*(C'\fR  if possible.
.Sp
.Vb 5
\&    $hashname = "foobar";
\&    $key = "baz";
\&    $value = 1234;
\&    eval "\e$$hashname{\*(Aq$key\*(Aq} = q|$value|";
\&    (defined($foobar{\*(Aqbaz\*(Aq})) ?  (print "Yup") : (print "Nope");
\&
\&    # perl4 prints: Yup
\&    # perl5 prints: Nope
.Ve
.Sp
Changing
.Sp
.Vb 1
\&    eval "\e$$hashname{\*(Aq$key\*(Aq} = q|$value|";
.Ve
.Sp
to
.Sp
.Vb 1
\&    eval "\e$\e$hashname{\*(Aq$key\*(Aq} = q|$value|";
.Ve
.Sp
causes the following result:
.Sp
.Vb 2
\&    # perl4 prints: Nope
\&    # perl5 prints: Yup
.Ve
.Sp
or, changing to
.Sp
.Vb 1
\&    eval "\e$$hashname\e{\*(Aq$key\*(Aq\e} = q|$value|";
.Ve
.Sp
causes the following result:
.Sp
.Vb 3
\&    # perl4 prints: Yup
\&    # perl5 prints: Yup
\&    # and is compatible for both versions
.Ve
.IP "\(bu" 5
Bugs in earlier perl versions
.Sp
perl4 programs which unconsciously rely on the bugs in earlier perl versions.
.Sp
.Vb 1
\&    perl \-e \*(Aq$bar=q/not/; print "This is $foo{$bar} perl5"\*(Aq
\&
\&    # perl4 prints: This is not perl5
\&    # perl5 prints: This is perl5
.Ve
.IP "\(bu" 5
Array and hash brackets during interpolation
.Sp
You also have to be careful about array and hash brackets during
interpolation.
.Sp
.Vb 1
\&    print "$foo["
\&
\&    perl 4 prints: [
\&    perl 5 prints: syntax error
\&
\&    print "$foo{"
\&
\&    perl 4 prints: {
\&    perl 5 prints: syntax error
.Ve
.Sp
Perl 5 is expecting to find an index or key name following the respective
brackets, as well as an ending bracket of the appropriate type.  In order
to mimic the behavior of Perl 4, you must escape the bracket like so.
.Sp
.Vb 2
\&    print "$foo\e[";
\&    print "$foo\e{";
.Ve
.IP "\(bu" 5
Interpolation of \f(CW\*(C`\e$$foo{bar}\*(C'\fR
.Sp
Similarly, watch out for: \f(CW\*(C`\e$$foo{bar}\*(C'\fR
.Sp
.Vb 2
\&    $foo = "baz";
\&    print "\e$$foo{bar}\en";
\&
\&    # perl4 prints: $baz{bar}
\&    # perl5 prints: $
.Ve
.Sp
Perl 5 is looking for \f(CW$foo{bar}\fR which doesn't exist, but perl 4 is
happy just to expand \f(CW$foo\fR to \*(L"baz\*(R" by itself.  Watch out for this
especially in \f(CW\*(C`eval\*(C'\fR's.
.IP "\(bu" 5
\&\f(CW\*(C`qq()\*(C'\fR string passed to \f(CW\*(C`eval\*(C'\fR will not find string terminator
.Sp
\&\f(CW\*(C`qq()\*(C'\fR string passed to \f(CW\*(C`eval\*(C'\fR
.Sp
.Vb 5
\&    eval qq(
\&        foreach \e$y (keys %\e$x\e) {
\&            \e$count++;
\&        }
\&    );
\&
\&    # perl4 runs this ok
\&    # perl5 prints: Can\*(Aqt find string terminator ")"
.Ve
.Sh "\s-1DBM\s0 Traps"
.IX Subsection "DBM Traps"
General \s-1DBM\s0 traps.
.IP "\(bu" 5
Perl5 must have been linked with same dbm/ndbm as the default for \f(CW\*(C`dbmopen()\*(C'\fR
.Sp
Existing dbm databases created under perl4 (or any other dbm/ndbm tool)
may cause the same script, run under perl5, to fail.  The build of perl5
must have been linked with the same dbm/ndbm as the default for \f(CW\*(C`dbmopen()\*(C'\fR
to function properly without \f(CW\*(C`tie\*(C'\fR'ing to an extension dbm implementation.
.Sp
.Vb 2
\&    dbmopen (%dbm, "file", undef);
\&    print "ok\en";
\&
\&    # perl4 prints: ok
\&    # perl5 prints: ok (IFF linked with \-ldbm or \-lndbm)
.Ve
.IP "\(bu" 5
\&\s-1DBM\s0 exceeding limit on the key/value size will cause perl5 to exit immediately
.Sp
Existing dbm databases created under perl4 (or any other dbm/ndbm tool)
may cause the same script, run under perl5, to fail.  The error generated
when exceeding the limit on the key/value size will cause perl5 to exit
immediately.
.Sp
.Vb 3
\&    dbmopen(DB, "testdb",0600) || die "couldn\*(Aqt open db! $!";
\&    $DB{\*(Aqtrap\*(Aq} = "x" x 1024;  # value too large for most dbm/ndbm
\&    print "YUP\en";
\&
\&    # perl4 prints:
\&    dbm store returned \-1, errno 28, key "trap" at \- line 3.
\&    YUP
\&
\&    # perl5 prints:
\&    dbm store returned \-1, errno 28, key "trap" at \- line 3.
.Ve
.Sh "Unclassified Traps"
.IX Subsection "Unclassified Traps"
Everything else.
.IP "\(bu" 5
\&\f(CW\*(C`require\*(C'\fR/\f(CW\*(C`do\*(C'\fR trap using returned value
.Sp
If the file doit.pl has:
.Sp
.Vb 5
\&    sub foo {
\&        $rc = do "./do.pl";
\&        return 8;
\&    }
\&    print &foo, "\en";
.Ve
.Sp
And the do.pl file has the following single line:
.Sp
.Vb 1
\&    return 3;
.Ve
.Sp
Running doit.pl gives the following:
.Sp
.Vb 2
\&    # perl 4 prints: 3 (aborts the subroutine early)
\&    # perl 5 prints: 8
.Ve
.Sp
Same behavior if you replace \f(CW\*(C`do\*(C'\fR with \f(CW\*(C`require\*(C'\fR.
.IP "\(bu" 5
\&\f(CW\*(C`split\*(C'\fR on empty string with \s-1LIMIT\s0 specified
.Sp
.Vb 2
\&    $string = \*(Aq\*(Aq;
\&    @list = split(/foo/, $string, 2)
.Ve
.Sp
Perl4 returns a one element list containing the empty string but Perl5
returns an empty list.
.PP
As always, if any of these are ever officially declared as bugs,
they'll be fixed and removed.