=head1 名前

perlcall - C からの Perl 呼び出し規約

=head1 説明

このドキュメントは, 
Cから直接Perlの関数を呼び出す方法を示すのが目的です. 
(例えばI<コールバック>の様に)

コールバックを書くためにPerlから提供されているCインターフェースの話題は
ひとまずおいておいて, このドキュメントはどのようにインターフェースが
実際に昨日するかを見るために一連の例を使います. 
加えてコールバックをコーディングするためのいくつかのテクニックも
カバーします.

コールバックが含まれる必要のある場所の例

=over 5

=item エラーハンドラ

アプリケーションのC APIのXSUBインターフェースをつくります.

適切に共通なアプリケーションの機能は, 
なにかひどいコトが起きたときにはいつでも呼ばれるC関数の定義を許すことです. 
代わりに呼ばれるPerl関数を指定できることが望まれます.

=item イベント駆動プログラム

コールバックが使用される場所の基本的な例は, Xウィンドウアプリケーションの
様なイベント駆動プログラムを書いているときでしょう. 
マウスが押された, カーソルがウィンドウの中に移動した, 
メニューアイテムが選択された等, 指定したイベントが起きたときに
呼ばれる関数を登録します.

=back

CプログラムにPerlを埋め込むときここに書かれたテクニックは適切ではありますが, 
これはこのドキュメントの最大の目標ではありません. 
ほかに注目しなければならない点や, Perlを埋め込むための特殊なコトが
あります. CへPerlを埋め込む詳細については L<perlembed> を参照してください.

このドキュメントの残りの部分に目を向ける前に, 次の２つのドキュメントを
呼んでおくことをオススメします: 
L<perlxs>, L<perlguts>.

=head1 CALL_ 関数群

サンプルの使い方を説明するのは簡単ですが, 
まずいくつかの重要な定義を知っておく必要があります.

Perl関数を呼ぶためのC関数は次のようにいくつかあります:

    I32 call_sv(SV* sv, I32 flags) ;
    I32 call_pv(char *subname, I32 flags) ;
    I32 call_method(char *methname, I32 flags) ;
    I32 call_argv(char *subname, I32 flags, register char **argv) ;

鍵になるのはI<call_sv>関数です. 
他の関数はそれぞれのケースにおいて簡単にPerl関数を呼ぶための
ラッパーです.
Perl関数の呼び出しは, 最終的にはすべて I<call_sv> に
なります.

全てのI<call_*>関数は C<flags> パラメータを持っています. 
これは オプション設定のビットマスクをPerlに渡します. 
このビットマスクは各関数で共通です. 
ビットマスクの有効な設定については L</FLAG VALUES> 
を参照してください.

次は各関数について説明します.

=over 5

=item call_sv

I<call_sv> は２つのパラメータをとり, 
１つめ, C<sv> には SV* を必要とします. 
これをつかって呼び出すPerl関数をC文字列(まずSVに変換されます)もしくは
関数へのリファレンスを指定します. 
I<call_sv> の使用の節に説明があります.

=item call_pv

I<call_pv> 関数は I<call_sv> と１つめのパラメータが
呼び出したいPerl関数を表すC char*である点を除いてよく似ています. 
例えば, C<call_pv("fred", 0)> となります. 
もし呼び出したい関数が他のパッケージにあれば, 文字列にパッケージ名も
含めるようにします. 例, C<"pkg::fred">.

=item call_method

I<call_method> 関数はPerlクラスからメソッド呼び出しに使います. 
C<methname> は呼び出すメソッド名に対応します. 
メソッドの属しているクラスは, パラメータリストではなく Perl スタック
で渡されます. このクラスは, クラス名(staticメソッド)もしくは
オブジェクトへのリファレンス(virtualメソッド) になります. 
staticメソッドとvirtualメソッドの詳細な情報は, 
L<perlobj> を, I<call_method> の例については L</call_methodの使用> を
照してください.

=item call_argv

I<call_argv> は C<subname> パラメータに格納されたC文字列で
指定されたPerl関数を呼び出します. 
たいていは, C<flags> パラメータもとリます. 
最後のパラメータ C<argv> はPerl関数への引数として
NULLで閉じられているC文字列の配列を渡します. 
I<call_argv の使用>を参照してください.

=back

どの関数もintegerを返します. 
これはPerl関数から返されたアイテムの数です. 
関数から返されたアイテムは, Perlスタック上に格納されています.

一般的なルールとして, これらの関数の復帰値を I<常に>
確認するべきです. 
たとえPerl関数から特定個の復帰値しか返されないと考えていても, 
予期しないことが起こることを考慮外にしてはいけません.

=head1 FLAG 値

全ての I<call_*> 関数に使われる C<flags> パラメータは
以下に定義するシンボルの和集合からなるビットマスクです.

=head2 G_VOID

Perl 関数を void コンテキストで呼び出します.

このフラグには以下の２つの効果があります:

=over 5

=item 1.

呼び出される関数に対して void コンテキストで実行されることの通知(
I<wantarray> は未定義を返します).

=item 2.

関数から実際に何も返されないことの保証.

=back

Perl 関数が何個の要素を返したのかを示す I<call_*> 関数の復帰値は
0 になります.

=head2 G_SCALAR

Perl 関数をスカラーコンテキストで呼び出します. これは全ての I<call_*> 
関数のデフォルトです.

このフラグには以下の２つの効果があります:

=over 5

=item 1.

呼び出される関数に対してスカラーコンテキストで実行されることの通知(
I<wantarray> は偽を返します).

=item 2.

関数からスカラー１つのみが実際に返されることの保証.
関数は I<wantarray> を無視してリストを返すこともできますが, 
そのときはリストの最後の要素のみが返されます.

=back

Perl 関数が何個の要素を返したのかを示す I<call_*> 関数の復帰値は
0 もしくは 1 になります.

0 を返すときは G_DISCARD フラグを指定していることを示します.

1 のときは Perl 関数が実際に返した要素は Perl スタック上に格納されています. 
I<Returning a Scalar> 節にスタック上のこの値にアクセスする方法が記述されて
います. Perl 関数がいくつの復帰値を返したのかに関わらず, 最後の１つの
要素のみが返されることに注意してください. 返された他の値に関しては
I<call_*> 関数から制御が返った時点で既に存在していません. I<Returning 
a list in a scalar context> 節でこの振る舞いの例を示します.

=head2 G_ARRAY

Perl 関数を void コンテキストで呼び出します.

G_SCALAR の様にこのフラグには以下の２つの効果があります:

=over 5

=item 1.

呼び出される関数に対してリストコンテキストで実行されることの通知(
I<wantarray> は真を返します).

=item 2.

I<call_*> 関数から制御が返ったときに関数の返した全ての要素に
アクセス可能なことの保証.

=back

I<call_*> 関数から返される値は Perl 関数から何個の値が返された
のかを示します.

0 のときは G_DISCARD フラグを指定したことを示します.

0 でないときは関数が返した要素の数です. これらの要素は Perl スタック上に
格納されています. I<Returning a list of values> 節は G_ARRAY フラグと
Perl スタックに返された要素へのアクセス機構の使い方の例を示します.

=head2 G_DISCARD

デフォルトでは I<call_*> 関数は Perl 関数が返した値をスタック上に
配置します. しかしこれらの要素を使用しないのであればこのフラグを設定
することで自動的に取り除きます. このフラグを使用しているときでも 
G_SCALAR や G_ARRAY でコンテキストを指定することは可能です.

このフラグを使用しないときはテンポラリ(例えばPerl関数に渡したパラメータ
や関数から返された値)を自分自身で処理することが I<非常に> 重要です. 
I<Returning a Scalar> 節で明示的に店舗らるを処理する方法の詳細を
記述します. I<Using Perl to dispose of temporaries> 節で問題を
無視できる状況やPerlが注意を振り向ける状況を議論します.

=head2 G_NOARGS

I<call_*> 関数のいずれかで Perl 関数を呼び出すときは基本的にパラメータが
関数に渡される物と考えます. もし何もパラメータを渡さないのであれば
このフラグを設定することでいくらか時間を短縮できます. このフラグには
Perl 関数用の C<@_> 配列を作らないという効果があります.

このフラグによって提供される機能は簡単ではありますが, それを行う理由が
あるときのみ使われるべきです. G_NOARGS フラグが指定されても呼び出される
Perl 関数ではパラメータが渡されてきているかもしれないと考えることを
できるため, このフラグの使用には警告を告げておきます.

実際に何が起こるかというと, 呼び出した Perl 関数は, 前の Perl 関数の
C<@_> 配列にアクセスする事になります. これは I<call_*> 関数を実行している
コードそれ自身が別の Perl 関数から呼び出されているときに起こります. 
次のコードでこれを説明します.

    sub fred
      { print "@_\n"  }

    sub joe
      { &fred }

    &joe(1,2,3) ;

これは次の様に出力します.

    1 2 3

このとき, C<fred> は C<joe> の持っている C<@_> にアクセスしています.

=head2 G_EVAL

Perl 関数では I<die> の明示的な呼び出しやその他の方法で異常終了呼び出しが
行えます. デフォルトではこれらが起こったおきにはプロセスは直ぐに終了します. 
これらをトラップしたいときには G_EVAL フラグを指定します. 
これは関数呼び出しの周りに I<eval { }>を置きます.

I<call_*> 関数から制御が戻ったときには通常の Perl スクリプトと
同じように C<$@> を調べる必要があります.

I<call_*> 関数の復帰値は他にどんなフラグを設定したのか, そして
エラーが発生したのかどうかに応じて異なります. 以下に全てのケースを
説明します:

=over 5

=item *



I<call_*> 関数が通常に戻ったときは, その復帰値は前の節で説明
している通りです.

=item *



G_DISCARDが指定されていたときは復帰値は常に0です.

=item *



G_ARRAYが指定されていてI<かつ>エラーが発生した場合は, 復帰値は常に0になります.

=item *



G_SCALARが指定されていてI<かつ>エラーが発生した場合は, 復帰値は常に1になり, 
スタックに I<undef> が積まれます. このことから, C<$@>でエラーを認識してかつ
プログラムを継続するときには  I<undef> をスタックから取り除かなければ
なりません.

=back

G_EVAL の使い方に関しては I<Using G_EVAL> を参照してください.

=head2 G_KEEPERR

前に説明した G_EVAL フラグを使うと常に C<$@> 変数がクリアされ
呼び出したコードでエラーが発生したときにはエラーを説明する文字列が
設定されることに気付くでしょう. この強制的なC<$@>のリセットは 
C<eval {}> メカニズムを用いたときの確実なエラー認識に問題が生じます. 
なぜなら C<eval {}>の中でエラーが発生しC<$@>に設定されてから
ユーザのスクリプトで実行される C<$@> の値を調べようとする次の文との間で, 
perl が他のコード(例えばブロック終了処理コード)を実行する可能性があるためです.

この現象はデストラクタ, 非同期コールバック, シグナルハンドラ, 
C<__DIE__> 若しくは C<__WARN__> フック, そして C<tie> 関数から呼ばれる
コードにも多く適用できます. このような状況では C<$@> を常に消される
ことは望ましくなく, 新しいエラーをC<$@>に追加する程度が欲しいでしょう.

G_KEEPERR フラグはその様なコードを実装する I<call_*> 関数で
G_EVAL と共に使用します. このフラグは G_EVAL がないときにはなにも
行いません.

G_KEEPERR が使用されたときは呼び出されたコードでのエラーは "\t(in cleanup)"
という文字列を前置し, 現在の C<$@> の値に追加されます. もし既に同じエラー
文字列が C<$@> の末尾にあるときには追加されません.

加えて, 追加される文字列を使おうとすると警告が発生します. 
この警告は C<no warnings 'misc'> を使うことで無効にできます.

G_KEEPERR フラグは Perl バージョン 5.002 から導入されました.

このフラグを使用が正当な状況の例は I<Using G_EEPER> を参照してください.

=head2 コンテキストの決定

上に述べられているように, Perl ではそのとき実行中のサブルーティンの
コンテキストを I<wantarray> で調べることができます.
Cでのこれと同じ確認は C<GIMME_V> マクロを使って
行うことができます. 
リストコンテキストであれば C<G_ARRAY>を, 
スカラーコンテキストであれば C<G_SCALAR> を, 
voidコンテキスト(復帰値は使用されないコンテキスト)であれば 
C<G_VOID> を返します. 
古いバージョンでは C<GIMME> でした. 
これは, voidコンテキストでも C<G_VOID> の代わりに 
C<G_SCALAR> を返します. 
C<GIMME_V> マクロを使った例は, I<GIMME_Vの使用>節にあります.

=head1 既知の問題点

This section outlines all known problems that exist in the
I<call_*> functions.


=over 5

=item 1.

If you are intending to make use of both the G_EVAL and G_SCALAR flags
in your code, use a version of Perl greater than 5.000.  There is a bug
in version 5.000 of Perl which means that the combination of these two
flags will not work as described in the section I<FLAG VALUES>.


Specifically, if the two flags are used when calling a subroutine and
that subroutine does not call I<die>, the value returned by
I<call_*> will be wrong.


=item 2.

In Perl 5.000 and 5.001 there is a problem with using I<call_*> if
the Perl sub you are calling attempts to trap a I<die>.


The symptom of this problem is that the called Perl sub will continue
to completion, but whenever it attempts to pass control back to the
XSUB, the program will immediately terminate.


For example, say you want to call this Perl sub


    sub fred
    {
        eval { die "Fatal Error" ; }
        print "Trapped error: $@\n"
            if $@ ;
    }

via this XSUB


    void
    Call_fred()
        CODE:
        PUSHMARK(SP) ;
        call_pv("fred", G_DISCARD|G_NOARGS) ;
        fprintf(stderr, "back in Call_fred\n") ;

When C<Call_fred> is executed it will print


    Trapped error: Fatal Error

As control never returns to C<Call_fred>, the C<"back in Call_fred">
string will not get printed.


To work around this problem, you can either upgrade to Perl 5.002 or
higher, or use the G_EVAL flag with I<call_*> as shown below


    void
    Call_fred()
        CODE:
        PUSHMARK(SP) ;
        call_pv("fred", G_EVAL|G_DISCARD|G_NOARGS) ;
        fprintf(stderr, "back in Call_fred\n") ;

=back

=head1 例

定義は十分話したのでいくつか例をだしてみます.

Perl は Perlスタックにアクセスするのを助ける多くのマクロを提供しています.
必要なときはいつでも, Perl内部との対話のためにこれらのマクロが常に
使われるべきです. 
今後Perlに変更があってもこれが弱点のより少ないコードであるべき
ことを祈ります.

他に注目する価値がある点として, 例の最初では I<call_pv>関数 だけ
使用したことです. これはコードを単純に保ち, トピックに容易にします.
可能であればどこでも, I<call_pv>かI<call_sv>かで選ぶなら
I<call_sv>を試すべきでしょう. 
I<call_svの使用>に詳細があります.

=head2 パラメータなし, 復帰値なし

この最初の単純極まりない例は, Perl関数 I<PrintUID> を呼び出し, 
プロセスの UID を表示します.

    sub PrintUID
    {
        print "UID is $<\n" ;
    }

これを呼び出すC関数は次のようになります:

    static void
    call_PrintUID()
    {
        dSP ;

        PUSHMARK(SP) ;
        call_pv("PrintUID", G_DISCARD|G_NOARGS) ;
    }

単純ですね.

この例のポイントです.

=over 5

=item 1.

C<dSP> と C<PUSHMARK(SP)> は, 今のところ無視してください. 
次の例で説明します.

=item 2.

I<PrintUID> に何のパラメータも渡してないので G_NOARGS を
指定しています.

=item 3.

I<PrintUID> からの復帰値に興味はないので G_DISCARD を指定しています. 
たとえ I<PrintUID> が何らかの値を返すように修正されたとしても, 
G_DISCARD を指定しているためにI<call_pv>から制御が変える前に
除去されます.

=item 4.

I<call_pv>が使われているため, Perl関数はC文字列として指定されます.
このケースでは, 関数名はハードコーディングされています.

=item 5.

G_DISCARD が指定されているために, I<call_pv> の復帰値を
確認する必要はありません. 
常に 0 となります.

=back

=head2 パラメータを渡す

では少しずつ進めてみましょう. 今回は Perl 関数, C<LeftString> に
２つのパラメータ, 文字列 ($s) と数値 ($n) を渡して呼び出してみましょう. 
この関数は単純に文字列の初めの $n 文字を表示します.

Perl 関数は次のようになるでしょう.

    sub LeftString
    {
        my($s, $n) = @_ ;
        print substr($s, 0, $n), "\n" ;
    }

I<LeftString> を呼び出そうとするC関数は次のようになります.

    static void
    call_LeftString(a, b)
    char * a ;
    int b ;
    {
        dSP ;

	ENTER ;
        SAVETMPS ;

        PUSHMARK(SP) ;
        XPUSHs(sv_2mortal(newSVpv(a, 0)));
        XPUSHs(sv_2mortal(newSViv(b)));
        PUTBACK ;

        call_pv("LeftString", G_DISCARD);

        FREETMPS ;
        LEAVE ;
    }

C関数 I<call_LeftString> の説明をしましょう.

=over 5

=item 1.

パラメータは Perl スタックを使って Perl 関数に渡されます. 
これがコードの始まりに C<dSP> と書き, 終わりに C<PUTBACK> と書いた
理由です. C<dSP> はスタックポインタのローカルコピーを宣言します. 
このローカルコピーはB<常に> C<SP> としてアクセスされるべきです.

=item 2.

もし Perl スタックに何か積むのなら, どこに積めばいいのかを知る必要が
あります. これが C<dSP> マクロの目的です. これが Perl スタックポインタ
のI<ローカル>コピーを初期化します.

この例で使われている全ての他のマクロはこのマクロが使われていることを
前提にしています.

このルールの例外は Perl 関数を XSUB 関数から直接呼び出すときです. 
このときは C<dSP> マクロを明示的に使用する必要はありません. それは
すでにあなたのために自動的に宣言されているからです.

=item 3.

Any parameters to be pushed onto the stack should be bracketed by the
C<PUSHMARK> and C<PUTBACK> macros.  The purpose of these two macros, in
this context, is to count the number of parameters you are
pushing automatically.  Then whenever Perl is creating the C<@_> array for the
subroutine, it knows how big to make it.


The C<PUSHMARK> macro tells Perl to make a mental note of the current
stack pointer. Even if you aren't passing any parameters (like the
example shown in the section I<No Parameters, Nothing returned>) you
must still call the C<PUSHMARK> macro before you can call any of the
I<call_*> functions--Perl still needs to know that there are no
parameters.


The C<PUTBACK> macro sets the global copy of the stack pointer to be
the same as our local copy. If we didn't do this I<call_pv>
wouldn't know where the two parameters we pushed were--remember that
up to now all the stack pointer manipulation we have done is with our
local copy, I<not> the global copy.


=item 4.

Next, we come to XPUSHs. This is where the parameters actually get
pushed onto the stack. In this case we are pushing a string and an
integer.


See L<perlguts/XSUBs and the Argument Stack> for details
on how the XPUSH macros work.


=item 5.

Because we created temporary values (by means of sv_2mortal() calls)
we will have to tidy up the Perl stack and dispose of mortal SVs.


This is the purpose of


    ENTER ;
    SAVETMPS ;

at the start of the function, and


    FREETMPS ;
    LEAVE ;

at the end. The C<ENTER>/C<SAVETMPS> pair creates a boundary for any
temporaries we create.  This means that the temporaries we get rid of
will be limited to those which were created after these calls.


The C<FREETMPS>/C<LEAVE> pair will get rid of any values returned by
the Perl subroutine (see next example), plus it will also dump the
mortal SVs we have created.  Having C<ENTER>/C<SAVETMPS> at the
beginning of the code makes sure that no other mortals are destroyed.


Think of these macros as working a bit like using C<{> and C<}> in Perl
to limit the scope of local variables.


See the section I<Using Perl to dispose of temporaries> for details of
an alternative to using these macros.


=item 6.

Finally, I<LeftString> can now be called via the I<call_pv> function.
The only flag specified this time is G_DISCARD. Because we are passing
2 parameters to the Perl subroutine this time, we have not specified
G_NOARGS.


=back

=head2 スカラーを返す

Now for an example of dealing with the items returned from a Perl
subroutine.


Here is a Perl subroutine, I<Adder>, that takes 2 integer parameters
and simply returns their sum.


    sub Adder
    {
        my($a, $b) = @_ ;
        $a + $b ;
    }

Because we are now concerned with the return value from I<Adder>, the C
function required to call it is now a bit more complex.


    static void
    call_Adder(a, b)
    int a ;
    int b ;
    {
        dSP ;
        int count ;

        ENTER ;
        SAVETMPS;

        PUSHMARK(SP) ;
        XPUSHs(sv_2mortal(newSViv(a)));
        XPUSHs(sv_2mortal(newSViv(b)));
        PUTBACK ;

        count = call_pv("Adder", G_SCALAR);

        SPAGAIN ;

        if (count != 1)
            croak("Big trouble\n") ;

        printf ("The sum of %d and %d is %d\n", a, b, POPi) ;

        PUTBACK ;
        FREETMPS ;
        LEAVE ;
    }

Points to note this time are


=over 5

=item 1.

The only flag specified this time was G_SCALAR. That means the C<@_>
array will be created and that the value returned by I<Adder> will
still exist after the call to I<call_pv>.


=item 2.

The purpose of the macro C<SPAGAIN> is to refresh the local copy of the
stack pointer. This is necessary because it is possible that the memory
allocated to the Perl stack has been reallocated whilst in the
I<call_pv> call.


If you are making use of the Perl stack pointer in your code you must
always refresh the local copy using SPAGAIN whenever you make use
of the I<call_*> functions or any other Perl internal function.


=item 3.

Although only a single value was expected to be returned from I<Adder>,
it is still good practice to check the return code from I<call_pv>
anyway.


Expecting a single value is not quite the same as knowing that there
will be one. If someone modified I<Adder> to return a list and we
didn't check for that possibility and take appropriate action the Perl
stack would end up in an inconsistent state. That is something you
I<really> don't want to happen ever.


=item 4.

The C<POPi> macro is used here to pop the return value from the stack.
In this case we wanted an integer, so C<POPi> was used.


Here is the complete list of POP macros available, along with the types
they return.


    POPs	SV
    POPp	pointer
    POPn	double
    POPi	integer
    POPl	long

=item 5.

The final C<PUTBACK> is used to leave the Perl stack in a consistent
state before exiting the function.  This is necessary because when we
popped the return value from the stack with C<POPi> it updated only our
local copy of the stack pointer.  Remember, C<PUTBACK> sets the global
stack pointer to be the same as our local copy.


=back

=head2 値のリストを返す

Now, let's extend the previous example to return both the sum of the
parameters and the difference.


Here is the Perl subroutine


    sub AddSubtract
    {
       my($a, $b) = @_ ;
       ($a+$b, $a-$b) ;
    }

and this is the C function


    static void
    call_AddSubtract(a, b)
    int a ;
    int b ;
    {
        dSP ;
        int count ;

        ENTER ;
        SAVETMPS;

        PUSHMARK(SP) ;
        XPUSHs(sv_2mortal(newSViv(a)));
        XPUSHs(sv_2mortal(newSViv(b)));
        PUTBACK ;

        count = call_pv("AddSubtract", G_ARRAY);

        SPAGAIN ;

        if (count != 2)
            croak("Big trouble\n") ;

        printf ("%d - %d = %d\n", a, b, POPi) ;
        printf ("%d + %d = %d\n", a, b, POPi) ;

        PUTBACK ;
        FREETMPS ;
        LEAVE ;
    }

If I<call_AddSubtract> is called like this


    call_AddSubtract(7, 4) ;

then here is the output


    7 - 4 = 3
    7 + 4 = 11

Notes


=over 5

=item 1.

We wanted list context, so G_ARRAY was used.


=item 2.

Not surprisingly C<POPi> is used twice this time because we were
retrieving 2 values from the stack. The important thing to note is that
when using the C<POP*> macros they come off the stack in I<reverse>
order.


=back

=head2 スカラーコンテキストでリストを返す

Say the Perl subroutine in the previous section was called in a scalar
context, like this


    static void
    call_AddSubScalar(a, b)
    int a ;
    int b ;
    {
        dSP ;
        int count ;
        int i ;

        ENTER ;
        SAVETMPS;

        PUSHMARK(SP) ;
        XPUSHs(sv_2mortal(newSViv(a)));
        XPUSHs(sv_2mortal(newSViv(b)));
        PUTBACK ;

        count = call_pv("AddSubtract", G_SCALAR);

        SPAGAIN ;

        printf ("Items Returned = %d\n", count) ;

        for (i = 1 ; i <= count ; ++i)
            printf ("Value %d = %d\n", i, POPi) ;

        PUTBACK ;
        FREETMPS ;
        LEAVE ;
    }

The other modification made is that I<call_AddSubScalar> will print the
number of items returned from the Perl subroutine and their value (for
simplicity it assumes that they are integer).  So if
I<call_AddSubScalar> is called


    call_AddSubScalar(7, 4) ;

then the output will be


    Items Returned = 1
    Value 1 = 3

In this case the main point to note is that only the last item in the
list is returned from the subroutine, I<AddSubtract> actually made it back to
I<call_AddSubScalar>.


=head2 パラメータリストを通してデータを返す

It is also possible to return values directly via the parameter list -
whether it is actually desirable to do it is another matter entirely.


The Perl subroutine, I<Inc>, below takes 2 parameters and increments
each directly.


    sub Inc
    {
        ++ $_[0] ;
        ++ $_[1] ;
    }

and here is a C function to call it.


    static void
    call_Inc(a, b)
    int a ;
    int b ;
    {
        dSP ;
        int count ;
        SV * sva ;
        SV * svb ;

        ENTER ;
        SAVETMPS;

        sva = sv_2mortal(newSViv(a)) ;
        svb = sv_2mortal(newSViv(b)) ;

        PUSHMARK(SP) ;
        XPUSHs(sva);
        XPUSHs(svb);
        PUTBACK ;

        count = call_pv("Inc", G_DISCARD);

        if (count != 0)
            croak ("call_Inc: expected 0 values from 'Inc', got %d\n",
                   count) ;

        printf ("%d + 1 = %d\n", a, SvIV(sva)) ;
        printf ("%d + 1 = %d\n", b, SvIV(svb)) ;

    	FREETMPS ;
        LEAVE ;
    }

To be able to access the two parameters that were pushed onto the stack
after they return from I<call_pv> it is necessary to make a note
of their addresses--thus the two variables C<sva> and C<svb>.


The reason this is necessary is that the area of the Perl stack which
held them will very likely have been overwritten by something else by
the time control returns from I<call_pv>.


=head2 G_EVAL の使用

Now an example using G_EVAL. Below is a Perl subroutine which computes
the difference of its 2 parameters. If this would result in a negative
result, the subroutine calls I<die>.


    sub Subtract
    {
        my ($a, $b) = @_ ;

        die "death can be fatal\n" if $a < $b ;

        $a - $b ;
    }

and some C to call it


    static void
    call_Subtract(a, b)
    int a ;
    int b ;
    {
        dSP ;
        int count ;

        ENTER ;
        SAVETMPS;

        PUSHMARK(SP) ;
        XPUSHs(sv_2mortal(newSViv(a)));
        XPUSHs(sv_2mortal(newSViv(b)));
        PUTBACK ;

        count = call_pv("Subtract", G_EVAL|G_SCALAR);

        SPAGAIN ;

        /* Check the eval first */
        if (SvTRUE(ERRSV))
        {
	    STRLEN n_a;
            printf ("Uh oh - %s\n", SvPV(ERRSV, n_a)) ;
            POPs ;
        }
        else
        {
            if (count != 1)
               croak("call_Subtract: wanted 1 value from 'Subtract', got %d\n",
                        count) ;

            printf ("%d - %d = %d\n", a, b, POPi) ;
        }

        PUTBACK ;
        FREETMPS ;
        LEAVE ;
    }

If I<call_Subtract> is called thus


    call_Subtract(4, 5)

the following will be printed


    Uh oh - death can be fatal

Notes


=over 5

=item 1.

We want to be able to catch the I<die> so we have used the G_EVAL
flag.  Not specifying this flag would mean that the program would
terminate immediately at the I<die> statement in the subroutine
I<Subtract>.


=item 2.

The code


    if (SvTRUE(ERRSV))
    {
	STRLEN n_a;
        printf ("Uh oh - %s\n", SvPV(ERRSV, n_a)) ;
        POPs ;
    }

is the direct equivalent of this bit of Perl


    print "Uh oh - $@\n" if $@ ;

C<PL_errgv> is a perl global of type C<GV *> that points to the
symbol table entry containing the error.  C<ERRSV> therefore
refers to the C equivalent of C<$@>.


=item 3.

Note that the stack is popped using C<POPs> in the block where
C<SvTRUE(ERRSV)> is true.  This is necessary because whenever a
I<call_*> function invoked with G_EVAL|G_SCALAR returns an error,
the top of the stack holds the value I<undef>. Because we want the
program to continue after detecting this error, it is essential that
the stack is tidied up by removing the I<undef>.


=back

=head2 G_KEEPERR の使用

Consider this rather facetious example, where we have used an XS
version of the call_Subtract example above inside a destructor:


    package Foo;
    sub new { bless {}, $_[0] }
    sub Subtract {
        my($a,$b) = @_;
        die "death can be fatal" if $a < $b ;
        $a - $b;
    }
    sub DESTROY { call_Subtract(5, 4); }
    sub foo { die "foo dies"; }

    package main;
    eval { Foo->new->foo };
    print "Saw: $@" if $@;             # should be, but isn't

This example will fail to recognize that an error occurred inside the
C<eval {}>.  Here's why: the call_Subtract code got executed while perl
was cleaning up temporaries when exiting the eval block, and because
call_Subtract is implemented with I<call_pv> using the G_EVAL
flag, it promptly reset C<$@>.  This results in the failure of the
outermost test for C<$@>, and thereby the failure of the error trap.


Appending the G_KEEPERR flag, so that the I<call_pv> call in
call_Subtract reads:


        count = call_pv("Subtract", G_EVAL|G_SCALAR|G_KEEPERR);

will preserve the error and restore reliable error handling.


=head2 call_sv の使用

In all the previous examples I have 'hard-wired' the name of the Perl
subroutine to be called from C.  Most of the time though, it is more
convenient to be able to specify the name of the Perl subroutine from
within the Perl script.


Consider the Perl code below


    sub fred
    {
        print "Hello there\n" ;
    }

    CallSubPV("fred") ;

Here is a snippet of XSUB which defines I<CallSubPV>.


    void
    CallSubPV(name)
    	char *	name
    	CODE:
    	PUSHMARK(SP) ;
    	call_pv(name, G_DISCARD|G_NOARGS) ;

That is fine as far as it goes. The thing is, the Perl subroutine
can be specified as only a string.  For Perl 4 this was adequate,
but Perl 5 allows references to subroutines and anonymous subroutines.
This is where I<call_sv> is useful.


The code below for I<CallSubSV> is identical to I<CallSubPV> except
that the C<name> parameter is now defined as an SV* and we use
I<call_sv> instead of I<call_pv>.


    void
    CallSubSV(name)
    	SV *	name
    	CODE:
    	PUSHMARK(SP) ;
    	call_sv(name, G_DISCARD|G_NOARGS) ;

Because we are using an SV to call I<fred> the following can all be used


    CallSubSV("fred") ;
    CallSubSV(\&fred) ;
    $ref = \&fred ;
    CallSubSV($ref) ;
    CallSubSV( sub { print "Hello there\n" } ) ;

As you can see, I<call_sv> gives you much greater flexibility in
how you can specify the Perl subroutine.


You should note that if it is necessary to store the SV (C<name> in the
example above) which corresponds to the Perl subroutine so that it can
be used later in the program, it not enough just to store a copy of the
pointer to the SV. Say the code above had been like this


    static SV * rememberSub ;

    void
    SaveSub1(name)
    	SV *	name
    	CODE:
    	rememberSub = name ;

    void
    CallSavedSub1()
    	CODE:
    	PUSHMARK(SP) ;
    	call_sv(rememberSub, G_DISCARD|G_NOARGS) ;

The reason this is wrong is that by the time you come to use the
pointer C<rememberSub> in C<CallSavedSub1>, it may or may not still refer
to the Perl subroutine that was recorded in C<SaveSub1>.  This is
particularly true for these cases


    SaveSub1(\&fred) ;
    CallSavedSub1() ;

    SaveSub1( sub { print "Hello there\n" } ) ;
    CallSavedSub1() ;

By the time each of the C<SaveSub1> statements above have been executed,
the SV*s which corresponded to the parameters will no longer exist.
Expect an error message from Perl of the form


    Can't use an undefined value as a subroutine reference at ...

for each of the C<CallSavedSub1> lines.


Similarly, with this code


    $ref = \&fred ;
    SaveSub1($ref) ;
    $ref = 47 ;
    CallSavedSub1() ;

you can expect one of these messages (which you actually get is dependent on
the version of Perl you are using)


    Not a CODE reference at ...
    Undefined subroutine &main::47 called ...

The variable $ref may have referred to the subroutine C<fred>
whenever the call to C<SaveSub1> was made but by the time
C<CallSavedSub1> gets called it now holds the number C<47>. Because we
saved only a pointer to the original SV in C<SaveSub1>, any changes to
$ref will be tracked by the pointer C<rememberSub>. This means that
whenever C<CallSavedSub1> gets called, it will attempt to execute the
code which is referenced by the SV* C<rememberSub>.  In this case
though, it now refers to the integer C<47>, so expect Perl to complain
loudly.


A similar but more subtle problem is illustrated with this code


    $ref = \&fred ;
    SaveSub1($ref) ;
    $ref = \&joe ;
    CallSavedSub1() ;

This time whenever C<CallSavedSub1> get called it will execute the Perl
subroutine C<joe> (assuming it exists) rather than C<fred> as was
originally requested in the call to C<SaveSub1>.


To get around these problems it is necessary to take a full copy of the
SV.  The code below shows C<SaveSub2> modified to do that


    static SV * keepSub = (SV*)NULL ;

    void
    SaveSub2(name)
        SV *	name
    	CODE:
     	/* Take a copy of the callback */
    	if (keepSub == (SV*)NULL)
    	    /* First time, so create a new SV */
    	    keepSub = newSVsv(name) ;
    	else
    	    /* Been here before, so overwrite */
    	    SvSetSV(keepSub, name) ;

    void
    CallSavedSub2()
    	CODE:
    	PUSHMARK(SP) ;
    	call_sv(keepSub, G_DISCARD|G_NOARGS) ;

To avoid creating a new SV every time C<SaveSub2> is called,
the function first checks to see if it has been called before.  If not,
then space for a new SV is allocated and the reference to the Perl
subroutine, C<name> is copied to the variable C<keepSub> in one
operation using C<newSVsv>.  Thereafter, whenever C<SaveSub2> is called
the existing SV, C<keepSub>, is overwritten with the new value using
C<SvSetSV>.


=head2 call_argv の使用

Here is a Perl subroutine which prints whatever parameters are passed
to it.


    sub PrintList
    {
        my(@list) = @_ ;

        foreach (@list) { print "$_\n" }
    }

and here is an example of I<call_argv> which will call
I<PrintList>.


    static char * words[] = {"alpha", "beta", "gamma", "delta", NULL} ;

    static void
    call_PrintList()
    {
        dSP ;

        call_argv("PrintList", G_DISCARD, words) ;
    }

Note that it is not necessary to call C<PUSHMARK> in this instance.
This is because I<call_argv> will do it for you.


=head2 call_method の使用

Consider the following Perl code


    {
        package Mine ;

        sub new
        {
            my($type) = shift ;
            bless [@_]
        }

        sub Display
        {
            my ($self, $index) = @_ ;
            print "$index: $$self[$index]\n" ;
        }

        sub PrintID
        {
            my($class) = @_ ;
            print "This is Class $class version 1.0\n" ;
        }
    }

It implements just a very simple class to manage an array.  Apart from
the constructor, C<new>, it declares methods, one static and one
virtual. The static method, C<PrintID>, prints out simply the class
name and a version number. The virtual method, C<Display>, prints out a
single element of the array.  Here is an all Perl example of using it.


    $a = new Mine ('red', 'green', 'blue') ;
    $a->Display(1) ;
    PrintID Mine;

will print


    1: green
    This is Class Mine version 1.0

Calling a Perl method from C is fairly straightforward. The following
things are required


=over 5

=item *



a reference to the object for a virtual method or the name of the class
for a static method.


=item *



the name of the method.


=item *



any other parameters specific to the method.


=back

Here is a simple XSUB which illustrates the mechanics of calling both
the C<PrintID> and C<Display> methods from C.


    void
    call_Method(ref, method, index)
        SV *	ref
        char *	method
        int		index
        CODE:
        PUSHMARK(SP);
        XPUSHs(ref);
        XPUSHs(sv_2mortal(newSViv(index))) ;
        PUTBACK;

        call_method(method, G_DISCARD) ;

    void
    call_PrintID(class, method)
        char *	class
        char *	method
        CODE:
        PUSHMARK(SP);
        XPUSHs(sv_2mortal(newSVpv(class, 0))) ;
        PUTBACK;

        call_method(method, G_DISCARD) ;

So the methods C<PrintID> and C<Display> can be invoked like this


    $a = new Mine ('red', 'green', 'blue') ;
    call_Method($a, 'Display', 1) ;
    call_PrintID('Mine', 'PrintID') ;

The only thing to note is that in both the static and virtual methods,
the method name is not passed via the stack--it is used as the first
parameter to I<call_method>.


=head2 GIMME_V の使用

Here is a trivial XSUB which prints the context in which it is
currently executing.


    void
    PrintContext()
        CODE:
        I32 gimme = GIMME_V;
        if (gimme == G_VOID)
            printf ("Context is Void\n") ;
        else if (gimme == G_SCALAR)
            printf ("Context is Scalar\n") ;
        else
            printf ("Context is Array\n") ;

and here is some Perl to test it


    PrintContext ;
    $a = PrintContext ;
    @a = PrintContext ;

The output from that will be


    Context is Void
    Context is Scalar
    Context is Array

=head2 テンポラリ処理のための Perl の使用

In the examples given to date, any temporaries created in the callback
(i.e., parameters passed on the stack to the I<call_*> function or
values returned via the stack) have been freed by one of these methods


=over 5

=item *



specifying the G_DISCARD flag with I<call_*>.


=item *



explicitly disposed of using the C<ENTER>/C<SAVETMPS> -
C<FREETMPS>/C<LEAVE> pairing.


=back

There is another method which can be used, namely letting Perl do it
for you automatically whenever it regains control after the callback
has terminated.  This is done by simply not using the


    ENTER ;
    SAVETMPS ;
    ...
    FREETMPS ;
    LEAVE ;

sequence in the callback (and not, of course, specifying the G_DISCARD
flag).


If you are going to use this method you have to be aware of a possible
memory leak which can arise under very specific circumstances.  To
explain these circumstances you need to know a bit about the flow of
control between Perl and the callback routine.


The examples given at the start of the document (an error handler and
an event driven program) are typical of the two main sorts of flow
control that you are likely to encounter with callbacks.  There is a
very important distinction between them, so pay attention.


In the first example, an error handler, the flow of control could be as
follows.  You have created an interface to an external library.
Control can reach the external library like this


    perl --> XSUB --> external library

Whilst control is in the library, an error condition occurs. You have
previously set up a Perl callback to handle this situation, so it will
get executed. Once the callback has finished, control will drop back to
Perl again.  Here is what the flow of control will be like in that
situation


    perl --> XSUB --> external library
                      ...
                      error occurs
                      ...
                      external library --> call_* --> perl
                                                          |
    perl <-- XSUB <-- external library <-- call_* <----+

After processing of the error using I<call_*> is completed,
control reverts back to Perl more or less immediately.


In the diagram, the further right you go the more deeply nested the
scope is.  It is only when control is back with perl on the extreme
left of the diagram that you will have dropped back to the enclosing
scope and any temporaries you have left hanging around will be freed.


In the second example, an event driven program, the flow of control
will be more like this


    perl --> XSUB --> event handler
                      ...
                      event handler --> call_* --> perl
                                                       |
                      event handler <-- call_* <----+
                      ...
                      event handler --> call_* --> perl
                                                       |
                      event handler <-- call_* <----+
                      ...
                      event handler --> call_* --> perl
                                                       |
                      event handler <-- call_* <----+

In this case the flow of control can consist of only the repeated
sequence


    event handler --> call_* --> perl

for practically the complete duration of the program.  This means that
control may I<never> drop back to the surrounding scope in Perl at the
extreme left.


So what is the big problem? Well, if you are expecting Perl to tidy up
those temporaries for you, you might be in for a long wait.  For Perl
to dispose of your temporaries, control must drop back to the
enclosing scope at some stage.  In the event driven scenario that may
never happen.  This means that as time goes on, your program will
create more and more temporaries, none of which will ever be freed. As
each of these temporaries consumes some memory your program will
eventually consume all the available memory in your system--kapow!


So here is the bottom line--if you are sure that control will revert
back to the enclosing Perl scope fairly quickly after the end of your
callback, then it isn't absolutely necessary to dispose explicitly of
any temporaries you may have created. Mind you, if you are at all
uncertain about what to do, it doesn't do any harm to tidy up anyway.


=head2 コールバックコンテキスト情報の格納の選択

Potentially one of the trickiest problems to overcome when designing a
callback interface can be figuring out how to store the mapping between
the C callback function and the Perl equivalent.


To help understand why this can be a real problem first consider how a
callback is set up in an all C environment.  Typically a C API will
provide a function to register a callback.  This will expect a pointer
to a function as one of its parameters.  Below is a call to a
hypothetical function C<register_fatal> which registers the C function
to get called when a fatal error occurs.


    register_fatal(cb1) ;

The single parameter C<cb1> is a pointer to a function, so you must
have defined C<cb1> in your code, say something like this


    static void
    cb1()
    {
        printf ("Fatal Error\n") ;
        exit(1) ;
    }

Now change that to call a Perl subroutine instead


    static SV * callback = (SV*)NULL;

    static void
    cb1()
    {
        dSP ;

        PUSHMARK(SP) ;

        /* Call the Perl sub to process the callback */
        call_sv(callback, G_DISCARD) ;
    }

    void
    register_fatal(fn)
        SV *	fn
        CODE:
        /* Remember the Perl sub */
        if (callback == (SV*)NULL)
            callback = newSVsv(fn) ;
        else
            SvSetSV(callback, fn) ;

        /* register the callback with the external library */
        register_fatal(cb1) ;

where the Perl equivalent of C<register_fatal> and the callback it
registers, C<pcb1>, might look like this


    # Register the sub pcb1
    register_fatal(\&pcb1) ;

    sub pcb1
    {
        die "I'm dying...\n" ;
    }

The mapping between the C callback and the Perl equivalent is stored in
the global variable C<callback>.


This will be adequate if you ever need to have only one callback
registered at any time. An example could be an error handler like the
code sketched out above. Remember though, repeated calls to
C<register_fatal> will replace the previously registered callback
function with the new one.


Say for example you want to interface to a library which allows asynchronous
file i/o.  In this case you may be able to register a callback whenever
a read operation has completed. To be of any use we want to be able to
call separate Perl subroutines for each file that is opened.  As it
stands, the error handler example above would not be adequate as it
allows only a single callback to be defined at any time. What we
require is a means of storing the mapping between the opened file and
the Perl subroutine we want to be called for that file.


Say the i/o library has a function C<asynch_read> which associates a C
function C<ProcessRead> with a file handle C<fh>--this assumes that it
has also provided some routine to open the file and so obtain the file
handle.


    asynch_read(fh, ProcessRead)

This may expect the C I<ProcessRead> function of this form


    void
    ProcessRead(fh, buffer)
    int	fh ;
    char *	buffer ;
    {
         ...
    }

To provide a Perl interface to this library we need to be able to map
between the C<fh> parameter and the Perl subroutine we want called.  A
hash is a convenient mechanism for storing this mapping.  The code
below shows a possible implementation


    static HV * Mapping = (HV*)NULL ;

    void
    asynch_read(fh, callback)
        int	fh
        SV *	callback
        CODE:
        /* If the hash doesn't already exist, create it */
        if (Mapping == (HV*)NULL)
            Mapping = newHV() ;

        /* Save the fh -> callback mapping */
        hv_store(Mapping, (char*)&fh, sizeof(fh), newSVsv(callback), 0) ;

        /* Register with the C Library */
        asynch_read(fh, asynch_read_if) ;

and C<asynch_read_if> could look like this


    static void
    asynch_read_if(fh, buffer)
    int	fh ;
    char *	buffer ;
    {
        dSP ;
        SV ** sv ;

        /* Get the callback associated with fh */
        sv =  hv_fetch(Mapping, (char*)&fh , sizeof(fh), FALSE) ;
        if (sv == (SV**)NULL)
            croak("Internal error...\n") ;

        PUSHMARK(SP) ;
        XPUSHs(sv_2mortal(newSViv(fh))) ;
        XPUSHs(sv_2mortal(newSVpv(buffer, 0))) ;
        PUTBACK ;

        /* Call the Perl sub */
        call_sv(*sv, G_DISCARD) ;
    }

For completeness, here is C<asynch_close>.  This shows how to remove
the entry from the hash C<Mapping>.


    void
    asynch_close(fh)
        int	fh
        CODE:
        /* Remove the entry from the hash */
        (void) hv_delete(Mapping, (char*)&fh, sizeof(fh), G_DISCARD) ;

        /* Now call the real asynch_close */
        asynch_close(fh) ;

So the Perl interface would look like this


    sub callback1
    {
        my($handle, $buffer) = @_ ;
    }

    # Register the Perl callback
    asynch_read($fh, \&callback1) ;

    asynch_close($fh) ;

The mapping between the C callback and Perl is stored in the global
hash C<Mapping> this time. Using a hash has the distinct advantage that
it allows an unlimited number of callbacks to be registered.


What if the interface provided by the C callback doesn't contain a
parameter which allows the file handle to Perl subroutine mapping?  Say
in the asynchronous i/o package, the callback function gets passed only
the C<buffer> parameter like this


    void
    ProcessRead(buffer)
    char *	buffer ;
    {
        ...
    }

Without the file handle there is no straightforward way to map from the
C callback to the Perl subroutine.


In this case a possible way around this problem is to predefine a
series of C functions to act as the interface to Perl, thus


    #define MAX_CB		3
    #define NULL_HANDLE	-1
    typedef void (*FnMap)() ;

    struct MapStruct {
        FnMap    Function ;
        SV *     PerlSub ;
        int      Handle ;
      } ;

    static void  fn1() ;
    static void  fn2() ;
    static void  fn3() ;

    static struct MapStruct Map [MAX_CB] =
        {
            { fn1, NULL, NULL_HANDLE },
            { fn2, NULL, NULL_HANDLE },
            { fn3, NULL, NULL_HANDLE }
        } ;

    static void
    Pcb(index, buffer)
    int index ;
    char * buffer ;
    {
        dSP ;

        PUSHMARK(SP) ;
        XPUSHs(sv_2mortal(newSVpv(buffer, 0))) ;
        PUTBACK ;

        /* Call the Perl sub */
        call_sv(Map[index].PerlSub, G_DISCARD) ;
    }

    static void
    fn1(buffer)
    char * buffer ;
    {
        Pcb(0, buffer) ;
    }

    static void
    fn2(buffer)
    char * buffer ;
    {
        Pcb(1, buffer) ;
    }

    static void
    fn3(buffer)
    char * buffer ;
    {
        Pcb(2, buffer) ;
    }

    void
    array_asynch_read(fh, callback)
        int		fh
        SV *	callback
        CODE:
        int index ;
        int null_index = MAX_CB ;

        /* Find the same handle or an empty entry */
        for (index = 0 ; index < MAX_CB ; ++index)
        {
            if (Map[index].Handle == fh)
                break ;

            if (Map[index].Handle == NULL_HANDLE)
                null_index = index ;
        }

        if (index == MAX_CB && null_index == MAX_CB)
            croak ("Too many callback functions registered\n") ;

        if (index == MAX_CB)
            index = null_index ;

        /* Save the file handle */
        Map[index].Handle = fh ;

        /* Remember the Perl sub */
        if (Map[index].PerlSub == (SV*)NULL)
            Map[index].PerlSub = newSVsv(callback) ;
        else
            SvSetSV(Map[index].PerlSub, callback) ;

        asynch_read(fh, Map[index].Function) ;

    void
    array_asynch_close(fh)
        int	fh
        CODE:
        int index ;

        /* Find the file handle */
        for (index = 0; index < MAX_CB ; ++ index)
            if (Map[index].Handle == fh)
                break ;

        if (index == MAX_CB)
            croak ("could not close fh %d\n", fh) ;

        Map[index].Handle = NULL_HANDLE ;
        SvREFCNT_dec(Map[index].PerlSub) ;
        Map[index].PerlSub = (SV*)NULL ;

        asynch_close(fh) ;

In this case the functions C<fn1>, C<fn2>, and C<fn3> are used to
remember the Perl subroutine to be called. Each of the functions holds
a separate hard-wired index which is used in the function C<Pcb> to
access the C<Map> array and actually call the Perl subroutine.


There are some obvious disadvantages with this technique.


Firstly, the code is considerably more complex than with the previous
example.


Secondly, there is a hard-wired limit (in this case 3) to the number of
callbacks that can exist simultaneously. The only way to increase the
limit is by modifying the code to add more functions and then
recompiling.  None the less, as long as the number of functions is
chosen with some care, it is still a workable solution and in some
cases is the only one available.


To summarize, here are a number of possible methods for you to consider
for storing the mapping between C and the Perl callback


=over 5

=item 1. Ignore the problem - Allow only 1 callback

For a lot of situations, like interfacing to an error handler, this may
be a perfectly adequate solution.


=item 2. Create a sequence of callbacks - hard wired limit

If it is impossible to tell from the parameters passed back from the C
callback what the context is, then you may need to create a sequence of C
callback interface functions, and store pointers to each in an array.


=item 3. Use a parameter to map to the Perl callback

A hash is an ideal mechanism to store the mapping between C and Perl.


=back

=head2 もう１つのスタック操作

Although I have made use of only the C<POP*> macros to access values
returned from Perl subroutines, it is also possible to bypass these
macros and read the stack using the C<ST> macro (See L<perlxs> for a
full description of the C<ST> macro).


Most of the time the C<POP*> macros should be adequate, the main
problem with them is that they force you to process the returned values
in sequence. This may not be the most suitable way to process the
values in some cases. What we want is to be able to access the stack in
a random order. The C<ST> macro as used when coding an XSUB is ideal
for this purpose.


The code below is the example given in the section I<Returning a list
of values> recoded to use C<ST> instead of C<POP*>.


    static void
    call_AddSubtract2(a, b)
    int a ;
    int b ;
    {
        dSP ;
        I32 ax ;
        int count ;

        ENTER ;
        SAVETMPS;

        PUSHMARK(SP) ;
        XPUSHs(sv_2mortal(newSViv(a)));
        XPUSHs(sv_2mortal(newSViv(b)));
        PUTBACK ;

        count = call_pv("AddSubtract", G_ARRAY);

        SPAGAIN ;
        SP -= count ;
        ax = (SP - PL_stack_base) + 1 ;

        if (count != 2)
            croak("Big trouble\n") ;

        printf ("%d + %d = %d\n", a, b, SvIV(ST(0))) ;
        printf ("%d - %d = %d\n", a, b, SvIV(ST(1))) ;

        PUTBACK ;
        FREETMPS ;
        LEAVE ;
    }

Notes


=over 5

=item 1.

Notice that it was necessary to define the variable C<ax>.  This is
because the C<ST> macro expects it to exist.  If we were in an XSUB it
would not be necessary to define C<ax> as it is already defined for
you.


=item 2.

The code


        SPAGAIN ;
        SP -= count ;
        ax = (SP - PL_stack_base) + 1 ;

sets the stack up so that we can use the C<ST> macro.


=item 3.

Unlike the original coding of this example, the returned
values are not accessed in reverse order.  So C<ST(0)> refers to the
first value returned by the Perl subroutine and C<ST(count-1)>
refers to the last.


=back

=head2 C での無名関数の作成と呼び出し

As we've already shown, C<call_sv> can be used to invoke an
anonymous subroutine.  However, our example showed a Perl script
invoking an XSUB to perform this operation.  Let's see how it can be
done inside our C code:


 ...

 SV *cvrv = eval_pv("sub { print 'You will not find me cluttering any namespace!' }", TRUE);

 ...

 call_sv(cvrv, G_VOID|G_NOARGS);

C<eval_pv> is used to compile the anonymous subroutine, which
will be the return value as well (read more about C<eval_pv> in
L<perlapi/eval_pv>).  Once this code reference is in hand, it
can be mixed in with all the previous examples we've shown.


=head1 関連項目

L<perlxs>, L<perlguts>, L<perlembed>


=head1 著者

Paul Marquess 


Special thanks to the following people who assisted in the creation of
the document.


Jeff Okamoto, Tim Bunce, Nick Gianniotis, Steve Kelem, Gurusamy Sarathy
and Larry Wall.


=head1 日時

Version 1.3, 14th Apr 1997