perlvms.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.

.IP "Any other string" 4
.IX Item "Any other string"
If an element of \fI\s-1PERL_ENV_TABLES\s0\fR translates to any other string,
that string is used as the name of a logical name table, which is
consulted using \fIname\fR as the logical name.  The normal search
order of access modes is used.
.RE
.RS 4
.Sp
\&\fI\s-1PERL_ENV_TABLES\s0\fR is translated once when Perl starts up; any changes
you make while Perl is running do not affect the behavior of \f(CW%ENV\fR.
If \fI\s-1PERL_ENV_TABLES\s0\fR is not defined, then Perl defaults to consulting
first the logical name tables specified by \fI\s-1LNM$FILE_DEV\s0\fR, and then
the \s-1CRTL\s0 \f(CW\*(C`environ\*(C'\fR array.
.Sp
In all operations on \f(CW%ENV\fR, the key string is treated as if it 
were entirely uppercase, regardless of the case actually 
specified in the Perl expression.
.Sp
When an element of \f(CW%ENV\fR is read, the locations to which
\&\fI\s-1PERL_ENV_TABLES\s0\fR points are checked in order, and the value
obtained from the first successful lookup is returned.  If the
name of the \f(CW%ENV\fR element contains a semi-colon, it and
any characters after it are removed.  These are ignored when
the \s-1CRTL\s0 \f(CW\*(C`environ\*(C'\fR array or a \s-1CLI\s0 symbol table is consulted.
However, the name is looked up in a logical name table, the
suffix after the semi-colon is treated as the translation index
to be used for the lookup.   This lets you look up successive values
for search list logical names.  For instance, if you say
.Sp
.Vb 3
\&   $  Define STORY  once,upon,a,time,there,was
\&   $  perl \-e "for ($i = 0; $i <= 6; $i++) " \-
\&   _$ \-e "{ print $ENV{\*(Aqstory;\*(Aq.$i},\*(Aq \*(Aq}"
.Ve
.Sp
Perl will print \f(CW\*(C`ONCE UPON A TIME THERE WAS\*(C'\fR, assuming, of course,
that \fI\s-1PERL_ENV_TABLES\s0\fR is set up so that the logical name \f(CW\*(C`story\*(C'\fR
is found, rather than a \s-1CLI\s0 symbol or \s-1CRTL\s0 \f(CW\*(C`environ\*(C'\fR element with
the same name.
.Sp
When an element of \f(CW%ENV\fR is set to a defined string, the
corresponding definition is made in the location to which the
first translation of \fI\s-1PERL_ENV_TABLES\s0\fR points.  If this causes a
logical name to be created, it is defined in supervisor mode.
(The same is done if an existing logical name was defined in
executive or kernel mode; an existing user or supervisor mode
logical name is reset to the new value.)  If the value is an empty
string, the logical name's translation is defined as a single \s-1NUL\s0
(\s-1ASCII\s0 00) character, since a logical name cannot translate to a
zero-length string.  (This restriction does not apply to \s-1CLI\s0 symbols
or \s-1CRTL\s0 \f(CW\*(C`environ\*(C'\fR values; they are set to the empty string.)
An element of the \s-1CRTL\s0 \f(CW\*(C`environ\*(C'\fR array can be set only if your
copy of Perl knows about the \s-1CRTL\s0's \f(CW\*(C`setenv()\*(C'\fR function.  (This is
present only in some versions of the \s-1DECCRTL\s0; check \f(CW$Config{d_setenv}\fR
to see whether your copy of Perl was built with a \s-1CRTL\s0 that has this
function.)
.Sp
When an element of \f(CW%ENV\fR is set to \f(CW\*(C`undef\*(C'\fR,
the element is looked up as if it were being read, and if it is
found, it is deleted.  (An item \*(L"deleted\*(R" from the \s-1CRTL\s0 \f(CW\*(C`environ\*(C'\fR
array is set to the empty string; this can only be done if your
copy of Perl knows about the \s-1CRTL\s0 \f(CW\*(C`setenv()\*(C'\fR function.)  Using
\&\f(CW\*(C`delete\*(C'\fR to remove an element from \f(CW%ENV\fR has a similar effect,
but after the element is deleted, another attempt is made to
look up the element, so an inner-mode logical name or a name in
another location will replace the logical name just deleted.
In either case, only the first value found searching \s-1PERL_ENV_TABLES\s0
is altered.  It is not possible at present to define a search list
logical name via \f(CW%ENV\fR.
.Sp
The element \f(CW$ENV{DEFAULT}\fR is special: when read, it returns
Perl's current default device and directory, and when set, it
resets them, regardless of the definition of \fI\s-1PERL_ENV_TABLES\s0\fR.
It cannot be cleared or deleted; attempts to do so are silently
ignored.
.Sp
Note that if you want to pass on any elements of the
C\-local environ array to a subprocess which isn't
started by fork/exec, or isn't running a C program, you
can \*(L"promote\*(R" them to logical names in the current
process, which will then be inherited by all subprocesses,
by saying
.Sp
.Vb 4
\&    foreach my $key (qw[C\-local keys you want promoted]) {
\&        my $temp = $ENV{$key}; # read from C\-local array
\&        $ENV{$key} = $temp;    # and define as logical name
\&    }
.Ve
.Sp
(You can't just say \f(CW$ENV{$key} = $ENV{$key}\fR, since the
Perl optimizer is smart enough to elide the expression.)
.Sp
Don't try to clear \f(CW%ENV\fR by saying \f(CW\*(C`%ENV = ();\*(C'\fR, it will throw
a fatal error.  This is equivalent to doing the following from \s-1DCL:\s0
.Sp
.Vb 1
\&    DELETE/LOGICAL *
.Ve
.Sp
You can imagine how bad things would be if, for example, the \s-1SYS$MANAGER\s0
or \s-1SYS$SYSTEM\s0 logical names were deleted.
.Sp
At present, the first time you iterate over \f(CW%ENV\fR using
\&\f(CW\*(C`keys\*(C'\fR, or \f(CW\*(C`values\*(C'\fR,  you will incur a time penalty as all
logical names are read, in order to fully populate \f(CW%ENV\fR.
Subsequent iterations will not reread logical names, so they
won't be as slow, but they also won't reflect any changes
to logical name tables caused by other programs.
.Sp
You do need to be careful with the logical names representing
process-permanent files, such as \f(CW\*(C`SYS$INPUT\*(C'\fR and \f(CW\*(C`SYS$OUTPUT\*(C'\fR.
The translations for these logical names are prepended with a
two-byte binary value (0x1B 0x00) that needs to be stripped off
if you wantto use it. (In previous versions of Perl it wasn't
possible to get the values of these logical names, as the null
byte acted as an end-of-string marker)
.RE
.IP "$!" 4
The string value of \f(CW$!\fR is that returned by the \s-1CRTL\s0's
\&\fIstrerror()\fR function, so it will include the \s-1VMS\s0 message for
VMS-specific errors.  The numeric value of \f(CW$!\fR is the
value of \f(CW\*(C`errno\*(C'\fR, except if errno is \s-1EVMSERR\s0, in which
case \f(CW$!\fR contains the value of vaxc$errno.  Setting \f(CW$!\fR
always sets errno to the value specified.  If this value is
\&\s-1EVMSERR\s0, it also sets vaxc$errno to 4 (NONAME-F-NOMSG), so
that the string value of \f(CW$!\fR won't reflect the \s-1VMS\s0 error
message from before \f(CW$!\fR was set.
.IP "$^E" 4
.IX Item "$^E"
This variable provides direct access to \s-1VMS\s0 status values
in vaxc$errno, which are often more specific than the
generic Unix-style error messages in \f(CW$!\fR.  Its numeric value
is the value of vaxc$errno, and its string value is the
corresponding \s-1VMS\s0 message string, as retrieved by sys$\fIgetmsg()\fR.
Setting \f(CW$^E\fR sets vaxc$errno to the value specified.
.Sp
While Perl attempts to keep the vaxc$errno value to be current, if
errno is not \s-1EVMSERR\s0, it may not be from the current operation.
.IP "$?" 4
The \*(L"status value\*(R" returned in \f(CW$?\fR is synthesized from the
actual exit status of the subprocess in a way that approximates
\&\s-1POSIX\s0 \fIwait\fR\|(5) semantics, in order to allow Perl programs to
portably test for successful completion of subprocesses.  The
low order 8 bits of \f(CW$?\fR are always 0 under \s-1VMS\s0, since the
termination status of a process may or may not have been
generated by an exception.
.Sp
The next 8 bits contain the termination status of the program.
.Sp
If the child process follows the convention of C programs
compiled with the _POSIX_EXIT macro set, the status value will
contain the actual value of 0 to 255 returned by that program
on a normal exit.
.Sp
With the _POSIX_EXIT macro set, the \s-1UNIX\s0 exit value of zero is
represented as a \s-1VMS\s0 native status of 1, and the \s-1UNIX\s0 values
from 2 to 255 are encoded by the equation:
.Sp
.Vb 1
\&   VMS_status = 0x35a000 + (unix_value * 8) + 1.
.Ve
.Sp
And in the special case of unix value 1 the encoding is:
.Sp
.Vb 1
\&   VMS_status = 0x35a000 + 8 + 2 + 0x10000000.
.Ve
.Sp
For other termination statuses, the severity portion of the
subprocess' exit status is used: if the severity was success or
informational, these bits are all 0; if the severity was
warning, they contain a value of 1; if the severity was
error or fatal error, they contain the actual severity bits,
which turns out to be a value of 2 for error and 4 for severe_error.
Fatal is another term for the severe_error status.
.Sp
As a result, \f(CW$?\fR will always be zero if the subprocess' exit
status indicated successful completion, and non-zero if a
warning or error occurred or a program compliant with encoding
_POSIX_EXIT values was run and set a status.
.Sp
How can you tell the difference between a non-zero status that is
the result of a \s-1VMS\s0 native error status or an encoded \s-1UNIX\s0 status?
You can not unless you look at the ${^CHILD_ERROR_NATIVE} value.
The ${^CHILD_ERROR_NATIVE} value returns the actual \s-1VMS\s0 status value
and check the severity bits. If the severity bits are equal to 1,
then if the numeric value for \f(CW$?\fR is between 2 and 255 or 0, then
\&\f(CW$?\fR accurately reflects a value passed back from a \s-1UNIX\s0 application.
If \f(CW$?\fR is 1, and the severity bits indicate a \s-1VMS\s0 error (2), then
\&\f(CW$?\fR is from a \s-1UNIX\s0 application exit value.
.Sp
In practice, Perl scripts that call programs that return _POSIX_EXIT
type status values will be expecting those values, and programs that
call traditional \s-1VMS\s0 programs will either be expecting the previous
behavior or just checking for a non-zero status.
.Sp
And success is always the value 0 in all behaviors.
.Sp
When the actual \s-1VMS\s0 termination status of the child is an error,
internally the \f(CW$!\fR value will be set to the closest \s-1UNIX\s0 errno
value to that error so that Perl scripts that test for error
messages will see the expected \s-1UNIX\s0 style error message instead
of a \s-1VMS\s0 message.
.Sp
Conversely, when setting \f(CW$?\fR in an \s-1END\s0 block, an attempt is made
to convert the \s-1POSIX\s0 value into a native status intelligible to
the operating system upon exiting Perl.  What this boils down to
is that setting \f(CW$?\fR to zero results in the generic success value
\&\s-1SS$_NORMAL\s0, and setting \f(CW$?\fR to a non-zero value results in the
generic failure status \s-1SS$_ABORT\s0.  See also \*(L"exit\*(R" in perlport.
.Sp
With the future \s-1POSIX_EXIT\s0 mode set, setting \f(CW$?\fR will cause the
new value to also be encoded into \f(CW$^E\fR so that the either the
original parent or child exit status values of 0 to 255
can be automatically recovered by C programs expecting _POSIX_EXIT
behavior.  If both a parent and a child exit value are non-zero, then it
will be assumed that this is actually a \s-1VMS\s0 native status value to
be passed through.  The special value of 0xFFFF is almost a \s-1NOOP\s0 as
it will cause the current native \s-1VMS\s0 status in the C library to
become the current native Perl \s-1VMS\s0 status, and is handled this way
as consequence of it known to not be a valid native \s-1VMS\s0 status value.
It is recommend that only values in range of normal \s-1UNIX\s0 parent or
child status numbers, 0 to 255 are used.
.Sp
The pragma \f(CW\*(C`use vmsish \*(Aqstatus\*(Aq\*(C'\fR makes \f(CW$?\fR reflect the actual 
\&\s-1VMS\s0 exit status instead of the default emulation of \s-1POSIX\s0 status 
described above.  This pragma also disables the conversion of
non-zero values to \s-1SS$_ABORT\s0 when setting \f(CW$?\fR in an \s-1END\s0
block (but zero will still be converted to \s-1SS$_NORMAL\s0).
.Sp
Do not use the pragma \f(CW\*(C`use vmsish \*(Aqstatus\*(Aq\*(C'\fR with the future
\&\s-1POSIX_EXIT\s0 mode, as they are at times requesting conflicting
actions and the consequence of ignoring this advice will be
undefined to allow future improvements in the \s-1POSIX\s0 exit handling.
.IP "$|" 4
Setting \f(CW$|\fR for an I/O stream causes data to be flushed
all the way to disk on each write (\fIi.e.\fR not just to
the underlying \s-1RMS\s0 buffers for a file).  In other words,
it's equivalent to calling \fIfflush()\fR and \fIfsync()\fR from C.
.SH "Standard modules with VMS-specific differences"
.IX Header "Standard modules with VMS-specific differences"
.Sh "SDBM_File"
.IX Subsection "SDBM_File"
SDBM_File works properly on \s-1VMS\s0. It has, however, one minor
difference. The database directory file created has a \fI.sdbm_dir\fR
extension rather than a \fI.dir\fR extension. \fI.dir\fR files are \s-1VMS\s0 filesystem
directory files, and using them for other purposes could cause unacceptable
problems.
.SH "Revision date"
.IX Header "Revision date"
This document was last updated on 3\-Dec\-2007, for Perl 5,
patchlevel 10.
.SH "AUTHOR"
.IX Header "AUTHOR"
Charles Bailey  [e-mail]
Craig Berry  [e-mail]
Dan Sugalski  [e-mail]
John Malmberg [e-mail]