perlhack.pod

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.

the top C<docatch>. This then starts another third-level runops level,
which executes the nextstate, pushmark and die ops on line 4. At the point
that the second C<pp_die> is called, the C call stack looks exactly like
that above, even though we are no longer within an inner eval; this is
because of the optimization mentioned earlier. However, the context stack
now looks like this, ie with the top CxEVAL popped:

    STACK 0: MAIN
      CX 0: BLOCK  =>
      CX 1: EVAL   => AV()  PV("A"\0)
      retop=leave
    STACK 1: MAGIC
      CX 0: SUB    =>
      retop=(null)

The die on line 4 pops the context stack back down to the CxEVAL, leaving
it as:

    STACK 0: MAIN
      CX 0: BLOCK  =>

As usual, C<PL_restartop> is extracted from the C<CxEVAL>, and a
C<JMPENV_JUMP(3)> done, which pops the C stack back to the docatch:

    S_docatch
    Perl_pp_entertry
    Perl_runops      # second loop
    S_call_body
    Perl_call_sv
    Perl_pp_tie
    Perl_runops      # first loop
    S_run_body
    perl_run
    main

In  this case, because the C<JMPENV> level recorded in the C<CxEVAL>
differs from the current one, C<docatch> just does a C<JMPENV_JUMP(3)>
and the C stack unwinds to:

    perl_run
    main

Because C<PL_restartop> is non-null, C<run_body> starts a new runops loop
and execution continues.

=back

=head2 Internal Variable Types

You should by now have had a look at L<perlguts>, which tells you about
Perl's internal variable types: SVs, HVs, AVs and the rest. If not, do
that now.

These variables are used not only to represent Perl-space variables, but
also any constants in the code, as well as some structures completely
internal to Perl. The symbol table, for instance, is an ordinary Perl
hash. Your code is represented by an SV as it's read into the parser;
any program files you call are opened via ordinary Perl filehandles, and
so on.

The core L<Devel::Peek|Devel::Peek> module lets us examine SVs from a
Perl program. Let's see, for instance, how Perl treats the constant
C<"hello">.

      % perl -MDevel::Peek -e 'Dump("hello")'
    1 SV = PV(0xa041450) at 0xa04ecbc
    2   REFCNT = 1
    3   FLAGS = (POK,READONLY,pPOK)
    4   PV = 0xa0484e0 "hello"\0
    5   CUR = 5
    6   LEN = 6

Reading C<Devel::Peek> output takes a bit of practise, so let's go
through it line by line.

Line 1 tells us we're looking at an SV which lives at C<0xa04ecbc> in
memory. SVs themselves are very simple structures, but they contain a
pointer to a more complex structure. In this case, it's a PV, a
structure which holds a string value, at location C<0xa041450>.  Line 2
is the reference count; there are no other references to this data, so
it's 1.

Line 3 are the flags for this SV - it's OK to use it as a PV, it's a
read-only SV (because it's a constant) and the data is a PV internally.
Next we've got the contents of the string, starting at location
C<0xa0484e0>.

Line 5 gives us the current length of the string - note that this does
B<not> include the null terminator. Line 6 is not the length of the
string, but the length of the currently allocated buffer; as the string
grows, Perl automatically extends the available storage via a routine
called C<SvGROW>.

You can get at any of these quantities from C very easily; just add
C<Sv> to the name of the field shown in the snippet, and you've got a
macro which will return the value: C<SvCUR(sv)> returns the current
length of the string, C<SvREFCOUNT(sv)> returns the reference count,
C<SvPV(sv, len)> returns the string itself with its length, and so on.
More macros to manipulate these properties can be found in L<perlguts>.

Let's take an example of manipulating a PV, from C<sv_catpvn>, in F<sv.c>

     1  void
     2  Perl_sv_catpvn(pTHX_ register SV *sv, register const char *ptr, register STRLEN len)
     3  {
     4      STRLEN tlen;
     5      char *junk;

     6      junk = SvPV_force(sv, tlen);
     7      SvGROW(sv, tlen + len + 1);
     8      if (ptr == junk)
     9          ptr = SvPVX(sv);
    10      Move(ptr,SvPVX(sv)+tlen,len,char);
    11      SvCUR(sv) += len;
    12      *SvEND(sv) = '\0';
    13      (void)SvPOK_only_UTF8(sv);          /* validate pointer */
    14      SvTAINT(sv);
    15  }

This is a function which adds a string, C<ptr>, of length C<len> onto
the end of the PV stored in C<sv>. The first thing we do in line 6 is
make sure that the SV B<has> a valid PV, by calling the C<SvPV_force>
macro to force a PV. As a side effect, C<tlen> gets set to the current
value of the PV, and the PV itself is returned to C<junk>.

In line 7, we make sure that the SV will have enough room to accommodate
the old string, the new string and the null terminator. If C<LEN> isn't
big enough, C<SvGROW> will reallocate space for us.

Now, if C<junk> is the same as the string we're trying to add, we can
grab the string directly from the SV; C<SvPVX> is the address of the PV
in the SV.

Line 10 does the actual catenation: the C<Move> macro moves a chunk of
memory around: we move the string C<ptr> to the end of the PV - that's
the start of the PV plus its current length. We're moving C<len> bytes
of type C<char>. After doing so, we need to tell Perl we've extended the
string, by altering C<CUR> to reflect the new length. C<SvEND> is a
macro which gives us the end of the string, so that needs to be a
C<"\0">.

Line 13 manipulates the flags; since we've changed the PV, any IV or NV
values will no longer be valid: if we have C<$a=10; $a.="6";> we don't
want to use the old IV of 10. C<SvPOK_only_utf8> is a special UTF-8-aware
version of C<SvPOK_only>, a macro which turns off the IOK and NOK flags
and turns on POK. The final C<SvTAINT> is a macro which launders tainted
data if taint mode is turned on.

AVs and HVs are more complicated, but SVs are by far the most common
variable type being thrown around. Having seen something of how we
manipulate these, let's go on and look at how the op tree is
constructed.

=head2 Op Trees

First, what is the op tree, anyway? The op tree is the parsed
representation of your program, as we saw in our section on parsing, and
it's the sequence of operations that Perl goes through to execute your
program, as we saw in L</Running>.

An op is a fundamental operation that Perl can perform: all the built-in
functions and operators are ops, and there are a series of ops which
deal with concepts the interpreter needs internally - entering and
leaving a block, ending a statement, fetching a variable, and so on.

The op tree is connected in two ways: you can imagine that there are two
"routes" through it, two orders in which you can traverse the tree.
First, parse order reflects how the parser understood the code, and
secondly, execution order tells perl what order to perform the
operations in.

The easiest way to examine the op tree is to stop Perl after it has
finished parsing, and get it to dump out the tree. This is exactly what
the compiler backends L<B::Terse|B::Terse>, L<B::Concise|B::Concise>
and L<B::Debug|B::Debug> do.

Let's have a look at how Perl sees C<$a = $b + $c>:

     % perl -MO=Terse -e '$a=$b+$c'
     1  LISTOP (0x8179888) leave
     2      OP (0x81798b0) enter
     3      COP (0x8179850) nextstate
     4      BINOP (0x8179828) sassign
     5          BINOP (0x8179800) add [1]
     6              UNOP (0x81796e0) null [15]
     7                  SVOP (0x80fafe0) gvsv  GV (0x80fa4cc) *b
     8              UNOP (0x81797e0) null [15]
     9                  SVOP (0x8179700) gvsv  GV (0x80efeb0) *c
    10          UNOP (0x816b4f0) null [15]
    11              SVOP (0x816dcf0) gvsv  GV (0x80fa460) *a

Let's start in the middle, at line 4. This is a BINOP, a binary
operator, which is at location C<0x8179828>. The specific operator in
question is C<sassign> - scalar assignment - and you can find the code
which implements it in the function C<pp_sassign> in F<pp_hot.c>. As a
binary operator, it has two children: the add operator, providing the
result of C<$b+$c>, is uppermost on line 5, and the left hand side is on
line 10.

Line 10 is the null op: this does exactly nothing. What is that doing
there? If you see the null op, it's a sign that something has been
optimized away after parsing. As we mentioned in L</Optimization>,
the optimization stage sometimes converts two operations into one, for
example when fetching a scalar variable. When this happens, instead of
rewriting the op tree and cleaning up the dangling pointers, it's easier
just to replace the redundant operation with the null op. Originally,
the tree would have looked like this:

    10          SVOP (0x816b4f0) rv2sv [15]
    11              SVOP (0x816dcf0) gv  GV (0x80fa460) *a

That is, fetch the C<a> entry from the main symbol table, and then look
at the scalar component of it: C<gvsv> (C<pp_gvsv> into F<pp_hot.c>)
happens to do both these things.

The right hand side, starting at line 5 is similar to what we've just
seen: we have the C<add> op (C<pp_add> also in F<pp_hot.c>) add together
two C<gvsv>s.

Now, what's this about?

     1  LISTOP (0x8179888) leave
     2      OP (0x81798b0) enter
     3      COP (0x8179850) nextstate

C<enter> and C<leave> are scoping ops, and their job is to perform any
housekeeping every time you enter and leave a block: lexical variables
are tidied up, unreferenced variables are destroyed, and so on. Every
program will have those first three lines: C<leave> is a list, and its
children are all the statements in the block. Statements are delimited
by C<nextstate>, so a block is a collection of C<nextstate> ops, with
the ops to be performed for each statement being the children of
C<nextstate>. C<enter> is a single op which functions as a marker.

That's how Perl parsed the program, from top to bottom:

                        Program
                           |
                       Statement
                           |
                           =
                          / \
                         /   \
                        $a   +
                            / \
                          $b   $c

However, it's impossible to B<perform> the operations in this order:
you have to find the values of C<$b> and C<$c> before you add them
together, for instance. So, the other thread that runs through the op
tree is the execution order: each op has a field C<op_next> which points
to the next op to be run, so following these pointers tells us how perl
executes the code. We can traverse the tree in this order using
the C<exec> option to C<B::Terse>:

     % perl -MO=Terse,exec -e '$a=$b+$c'
     1  OP (0x8179928) enter
     2  COP (0x81798c8) nextstate
     3  SVOP (0x81796c8) gvsv  GV (0x80fa4d4) *b
     4  SVOP (0x8179798) gvsv  GV (0x80efeb0) *c
     5  BINOP (0x8179878) add [1]
     6  SVOP (0x816dd38) gvsv  GV (0x80fa468) *a
     7  BINOP (0x81798a0) sassign
     8  LISTOP (0x8179900) leave

This probably makes more sense for a human: enter a block, start a
statement. Get the values of C<$b> and C<$c>, and add them together.
Find C<$a>, and assign one to the other. Then leave.

The way Perl builds up these op trees in the parsing process can be
unravelled by examining F<perly.y>, the YACC grammar. Let's take the
piece we need to construct the tree for C<$a = $b + $c>

    1 term    :   term ASSIGNOP term
    2                { $$ = newASSIGNOP(OPf_STACKED, $1, $2, $3); }
    3         |   term ADDOP term
    4                { $$ = newBINOP($2, 0, scalar($1), scalar($3)); }

If you're not used to reading BNF grammars, this is how it works: You're
fed certain things by the tokeniser, which generally end up in upper
case. Here, C<ADDOP>, is provided when the tokeniser sees C<+> in your
code. C<ASSIGNOP> is provided when C<=> is used for assigning. These are
"terminal symbols", because you can't get any simpler than them.

The grammar, lines one and three of the snippet above, tells you how to
build up more complex forms. These complex forms, "non-terminal symbols"
are generally placed in lower case. C<term> here is a non-terminal
symbol, representing a single expression.

The grammar gives you the following rule: you can make the thing on the
left of the colon if you see all the things on the right in sequence.
This is called a "reduction", and the aim of parsing is to completely
reduce the input. There are several different ways you can perform a
reduction, separated by vertical bars: so, C<term> followed by C<=>
followed by C<term> makes a C<term>, and C<term> followed by C<+>
followed by C<term> can also make a C<term>.

So, if you see two terms with an C<=> or C<+>, between them, you can
turn them into a single expression. When you do this, you execute the
code in the block on the next line: if you see C<=>, you'll do the code
in line 2. If you see C<+>, you'll do the code in line 4. It's this code
which contributes to the op tree.

            |   term ADDOP term
            { $$ = newBINOP($2, 0, scalar($1), scalar($3)); }

What this does is creates a new binary op, and feeds it a number of
variables. The variables refer to the tokens: C<$1> is the first token in
the input, C<$2> the second, and so on - think regular expression
backreferences. C<$$> is the op returned from this reduction. So, we
call C<newBINOP> to create a new binary operator. The first parameter to
C<newBINOP>, a function in F<op.c>, is the op type. It's an addition
operator, so we want the type to be C<ADDOP>. We could specify this
directly, but it's right there as the second token in the input, so we
use C<$2>. The second parameter is the op's flags: 0 means "nothing
special". Then the things to add: the left and right hand side of our
expression, in scalar context.

=head2 Stacks

When perl executes something like C<addop>, how does it pass on its
results to the next op? The answer is, through the use of stacks. Perl
has a number of stacks to store things it's currently working on, and
we'll look at the three most important ones here.

=over 3

=item Argument stack