perlxstut.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.

\&        foo(a,b,c)
\&                int             a
\&                long            b
\&                const char *    c
.Ve
.PP
Can we do the same with an \s-1XSUB\s0
.PP
.Vb 7
\&        int
\&        is_even(input)
\&                int     input
\&            CODE:
\&                RETVAL = (input % 2 == 0);
\&            OUTPUT:
\&                RETVAL
.Ve
.PP
of \*(L"\s-1EXAMPLE\s0 2\*(R"?  To do this, one needs to define a C function \f(CW\*(C`int
is_even(int input)\*(C'\fR.  As we saw in \*(L"Anatomy of .xs file\*(R", a proper place
for this definition is in the first part of .xs file.  In fact a C function
.PP
.Vb 5
\&        int
\&        is_even(int arg)
\&        {
\&                return (arg % 2 == 0);
\&        }
.Ve
.PP
is probably overkill for this.  Something as simple as a \f(CW\*(C`#define\*(C'\fR will
do too:
.PP
.Vb 1
\&        #define is_even(arg)    ((arg) % 2 == 0)
.Ve
.PP
After having this in the first part of .xs file, the \*(L"Perl glue\*(R" part becomes
as simple as
.PP
.Vb 3
\&        int
\&        is_even(input)
\&                int     input
.Ve
.PP
This technique of separation of the glue part from the workhorse part has
obvious tradeoffs: if you want to change a Perl interface, you need to
change two places in your code.  However, it removes a lot of clutter,
and makes the workhorse part independent from idiosyncrasies of Perl calling
convention.  (In fact, there is nothing Perl-specific in the above description,
a different version of \fBxsubpp\fR might have translated this to \s-1TCL\s0 glue or
Python glue as well.)
.Sh "More about \s-1XSUB\s0 arguments"
.IX Subsection "More about XSUB arguments"
With the completion of Example 4, we now have an easy way to simulate some
real-life libraries whose interfaces may not be the cleanest in the world.
We shall now continue with a discussion of the arguments passed to the
\&\fBxsubpp\fR compiler.
.PP
When you specify arguments to routines in the .xs file, you are really
passing three pieces of information for each argument listed.  The first
piece is the order of that argument relative to the others (first, second,
etc).  The second is the type of argument, and consists of the type
declaration of the argument (e.g., int, char*, etc).  The third piece is
the calling convention for the argument in the call to the library function.
.PP
While Perl passes arguments to functions by reference,
C passes arguments by value; to implement a C function which modifies data
of one of the \*(L"arguments\*(R", the actual argument of this C function would be
a pointer to the data.  Thus two C functions with declarations
.PP
.Vb 2
\&        int string_length(char *s);
\&        int upper_case_char(char *cp);
.Ve
.PP
may have completely different semantics: the first one may inspect an array
of chars pointed by s, and the second one may immediately dereference \f(CW\*(C`cp\*(C'\fR
and manipulate \f(CW*cp\fR only (using the return value as, say, a success
indicator).  From Perl one would use these functions in
a completely different manner.
.PP
One conveys this info to \fBxsubpp\fR by replacing \f(CW\*(C`*\*(C'\fR before the
argument by \f(CW\*(C`&\*(C'\fR.  \f(CW\*(C`&\*(C'\fR means that the argument should be passed to a library
function by its address.  The above two function may be XSUB-ified as
.PP
.Vb 3
\&        int
\&        string_length(s)
\&                char *  s
\&
\&        int
\&        upper_case_char(cp)
\&                char    &cp
.Ve
.PP
For example, consider:
.PP
.Vb 4
\&        int
\&        foo(a,b)
\&                char    &a
\&                char *  b
.Ve
.PP
The first Perl argument to this function would be treated as a char and assigned
to the variable a, and its address would be passed into the function foo.
The second Perl argument would be treated as a string pointer and assigned to the
variable b.  The \fIvalue\fR of b would be passed into the function foo.  The
actual call to the function foo that \fBxsubpp\fR generates would look like this:
.PP
.Vb 1
\&        foo(&a, b);
.Ve
.PP
\&\fBxsubpp\fR will parse the following function argument lists identically:
.PP
.Vb 3
\&        char    &a
\&        char&a
\&        char    & a
.Ve
.PP
However, to help ease understanding, it is suggested that you place a \*(L"&\*(R"
next to the variable name and away from the variable type), and place a
\&\*(L"*\*(R" near the variable type, but away from the variable name (as in the
call to foo above).  By doing so, it is easy to understand exactly what
will be passed to the C function \*(-- it will be whatever is in the \*(L"last
column\*(R".
.PP
You should take great pains to try to pass the function the type of variable
it wants, when possible.  It will save you a lot of trouble in the long run.
.Sh "The Argument Stack"
.IX Subsection "The Argument Stack"
If we look at any of the C code generated by any of the examples except
example 1, you will notice a number of references to \s-1ST\s0(n), where n is
usually 0.  \*(L"\s-1ST\s0\*(R" is actually a macro that points to the n'th argument
on the argument stack.  \s-1\fIST\s0\fR\|(0) is thus the first argument on the stack and
therefore the first argument passed to the \s-1XSUB\s0, \s-1\fIST\s0\fR\|(1) is the second
argument, and so on.
.PP
When you list the arguments to the \s-1XSUB\s0 in the .xs file, that tells \fBxsubpp\fR
which argument corresponds to which of the argument stack (i.e., the first
one listed is the first argument, and so on).  You invite disaster if you
do not list them in the same order as the function expects them.
.PP
The actual values on the argument stack are pointers to the values passed
in.  When an argument is listed as being an \s-1OUTPUT\s0 value, its corresponding
value on the stack (i.e., \s-1\fIST\s0\fR\|(0) if it was the first argument) is changed.
You can verify this by looking at the C code generated for Example 3.
The code for the \fIround()\fR \s-1XSUB\s0 routine contains lines that look like this:
.PP
.Vb 3
\&        double  arg = (double)SvNV(ST(0));
\&        /* Round the contents of the variable arg */
\&        sv_setnv(ST(0), (double)arg);
.Ve
.PP
The arg variable is initially set by taking the value from \s-1\fIST\s0\fR\|(0), then is
stored back into \s-1\fIST\s0\fR\|(0) at the end of the routine.
.PP
XSUBs are also allowed to return lists, not just scalars.  This must be
done by manipulating stack values \s-1\fIST\s0\fR\|(0), \s-1\fIST\s0\fR\|(1), etc, in a subtly
different way.  See perlxs for details.
.PP
XSUBs are also allowed to avoid automatic conversion of Perl function arguments
to C function arguments.  See perlxs for details.  Some people prefer
manual conversion by inspecting \f(CWST(i)\fR even in the cases when automatic
conversion will do, arguing that this makes the logic of an \s-1XSUB\s0 call clearer.
Compare with \*(L"Getting the fat out of XSUBs\*(R" for a similar tradeoff of
a complete separation of \*(L"Perl glue\*(R" and \*(L"workhorse\*(R" parts of an \s-1XSUB\s0.
.PP
While experts may argue about these idioms, a novice to Perl guts may
prefer a way which is as little Perl-guts-specific as possible, meaning
automatic conversion and automatic call generation, as in
\&\*(L"Getting the fat out of XSUBs\*(R".  This approach has the additional
benefit of protecting the \s-1XSUB\s0 writer from future changes to the Perl \s-1API\s0.
.Sh "Extending your Extension"
.IX Subsection "Extending your Extension"
Sometimes you might want to provide some extra methods or subroutines
to assist in making the interface between Perl and your extension simpler
or easier to understand.  These routines should live in the .pm file.
Whether they are automatically loaded when the extension itself is loaded
or only loaded when called depends on where in the .pm file the subroutine
definition is placed.  You can also consult AutoLoader for an alternate
way to store and load your extra subroutines.
.Sh "Documenting your Extension"
.IX Subsection "Documenting your Extension"
There is absolutely no excuse for not documenting your extension.
Documentation belongs in the .pm file.  This file will be fed to pod2man,
and the embedded documentation will be converted to the manpage format,
then placed in the blib directory.  It will be copied to Perl's
manpage directory when the extension is installed.
.PP
You may intersperse documentation and Perl code within the .pm file.
In fact, if you want to use method autoloading, you must do this,
as the comment inside the .pm file explains.
.PP
See perlpod for more information about the pod format.
.Sh "Installing your Extension"
.IX Subsection "Installing your Extension"
Once your extension is complete and passes all its tests, installing it
is quite simple: you simply run \*(L"make install\*(R".  You will either need
to have write permission into the directories where Perl is installed,
or ask your system administrator to run the make for you.
.PP
Alternately, you can specify the exact directory to place the extension's
files by placing a \*(L"PREFIX=/destination/directory\*(R" after the make install.
(or in between the make and install if you have a brain-dead version of make).
This can be very useful if you are building an extension that will eventually
be distributed to multiple systems.  You can then just archive the files in
the destination directory and distribute them to your destination systems.
.Sh "\s-1EXAMPLE\s0 5"
.IX Subsection "EXAMPLE 5"
In this example, we'll do some more work with the argument stack.  The
previous examples have all returned only a single value.  We'll now
create an extension that returns an array.
.PP
This extension is very Unix-oriented (struct statfs and the statfs system
call).  If you are not running on a Unix system, you can substitute for
statfs any other function that returns multiple values, you can hard-code
values to be returned to the caller (although this will be a bit harder
to test the error case), or you can simply not do this example.  If you
change the \s-1XSUB\s0, be sure to fix the test cases to match the changes.
.PP
Return to the Mytest directory and add the following code to the end of
Mytest.xs:
.PP
.Vb 6
\&        void
\&        statfs(path)
\&                char *  path
\&            INIT:
\&                int i;
\&                struct statfs buf;
\&
\&            PPCODE:
\&                i = statfs(path, &buf);
\&                if (i == 0) {
\&                        XPUSHs(sv_2mortal(newSVnv(buf.f_bavail)));
\&                        XPUSHs(sv_2mortal(newSVnv(buf.f_bfree)));
\&                        XPUSHs(sv_2mortal(newSVnv(buf.f_blocks)));
\&                        XPUSHs(sv_2mortal(newSVnv(buf.f_bsize)));
\&                        XPUSHs(sv_2mortal(newSVnv(buf.f_ffree)));
\&                        XPUSHs(sv_2mortal(newSVnv(buf.f_files)));
\&                        XPUSHs(sv_2mortal(newSVnv(buf.f_type)));
\&                } else {
\&                        XPUSHs(sv_2mortal(newSVnv(errno)));
\&                }
.Ve
.PP
You'll also need to add the following code to the top of the .xs file, just
after the include of \*(L"\s-1XSUB\s0.h\*(R":
.PP
.Vb 1
\&        #include <sys/vfs.h>
.Ve
.PP
Also add the following code segment to Mytest.t while incrementing the \*(L"9\*(R"
tests to \*(L"11\*(R":
.PP
.Vb 4
\&        @a = &Mytest::statfs("/blech");
\&        ok( scalar(@a) == 1 && $a[0] == 2 );
\&        @a = &Mytest::statfs("/");
\&        is( scalar(@a), 7 );
.Ve
.Sh "New Things in this Example"
.IX Subsection "New Things in this Example"
This example added quite a few new concepts.  We'll take them one at a time.
.IP "\(bu" 4
The \s-1INIT:\s0 directive contains code that will be placed immediately after
the argument stack is decoded.  C does not allow variable declarations at
arbitrary locations inside a function,
so this is usually the best way to declare local variables needed by the \s-1XSUB\s0.
(Alternatively, one could put the whole \f(CW\*(C`PPCODE:\*(C'\fR section into braces, and
put these declarations on top.)
.IP "\(bu" 4
This routine also returns a different number of arguments depending on the
success or failure of the call to statfs.  If there is an error, the error
number is returned as a single-element array.  If the call is successful,
then a 9\-element array is returned.  Since only one argument is passed into
this function, we need room on the stack to hold the 9 values which may be
returned.
.Sp
We do this by using the \s-1PPCODE:\s0 directive, rather than the \s-1CODE:\s0 directive.
This tells \fBxsubpp\fR that we will be managing the return values that will be
put on the argument stack by ourselves.
.IP "\(bu" 4
When we want to place values to be returned to the caller onto the stack,
we use the series of macros that begin with \*(L"\s-1XPUSH\s0\*(R".  There are five
different versions, for placing integers, unsigned integers, doubles,
strings, and Perl scalars on the stack.  In our example, we placed a
Perl scalar onto the stack.  (In fact this is the only macro which
can be used to return multiple values.)
.Sp
The XPUSH* macros will automatically extend the return stack to prevent
it from being overrun.  You push values onto the stack in the order you
want them seen by the calling program.
.IP "\(bu" 4
The values pushed onto the return stack of the \s-1XSUB\s0 are actually mortal \s-1SV\s0's.
They are made mortal so that once the values are copied by the calling
program, the \s-1SV\s0's that held the returned values can be deallocated.
If they were not mortal, then they would continue to exist after the \s-1XSUB\s0
routine returned, but would not be accessible.  This is a memory leak.