perlcall - C からの Perl 呼び出し規約
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
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.
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.
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 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: perlxs, perlguts.
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 call_sv に なります.
全てのcall_*関数は 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 "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.
call_sv は２つのパラメータをとり,
１つめ, 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.
call_sv の使用の節に説明があります.
call_pv 関数は 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, 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, "pkg::fred".
call_method 関数はPerlクラスからメソッド呼び出しに使います.
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,
perlobj を, call_method の例については "call_methodの使用" を
照してください.
call_argv は 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, flags パラメータもとリます.
最後のパラメータ 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.
call_argv の使用を参照してください.
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 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.
全ての call_* 関数に使われる 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.
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:
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( wantarray は未定義を返します).
関数から実際に何も返されないことの保証.
Perl 関数が何個の要素を返したのかを示す call_* 関数の復帰値は 0 になります.
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 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:
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( wantarray は偽を返します).
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 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.
Perl 関数が何個の要素を返したのかを示す 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. 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 call_* 関数から制御が返った時点で既に存在していません. Returning a list in a scalar context 節でこの振る舞いの例を示します.
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:
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( wantarray は真を返します).
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.
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. 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.
デフォルトでは 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 非常に 重要です. 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. 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.
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 @_ 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
@_ 配列にアクセスする事になります. これは 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, fred は joe の持っている @_ にアクセスしています.
Perl 関数では 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 eval { }を置きます.
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 $@ を調べる必要があります.
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:
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.
G_DISCARDが指定されていたときは復帰値は常に0です.
G_ARRAYが指定されていて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.
G_SCALARが指定されていて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 undef が積まれます. このことから, $@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 undef をスタックから取り除かなければ
なりません.
G_EVAL の使い方に関しては Using G_EVAL を参照してください.
前に説明した G_EVAL フラグを使うと常に $@ 変数がクリアされ
呼び出したコードでエラーが発生したときにはエラーを説明する文字列が
設定されることに気付くでしょう. この強制的な$@のリセットは
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 eval {}の中でエラーが発生し$@に設定されてから
ユーザのスクリプトで実行される $@ 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,
__DIE__ 若しくは __WARN__ フック, そして 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 $@ を常に消される
ことは望ましくなく, 新しいエラーを$@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 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 $@ の値に追加されます. もし既に同じエラー
文字列が $@ 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 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 Using G_EEPER を参照してください.
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 wantarray で調べることができます.
Cでのこれと同じ確認は 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 G_ARRAYを,
スカラーコンテキストであれば 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
G_VOID を返します.
古いバージョンでは GIMME でした.
これは, voidコンテキストでも G_VOID の代わりに
G_SCALAR を返します.
GIMME_V マクロを使った例は, GIMME_Vの使用節にあります.
This section outlines all known problems that exist in the call_* functions.
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 FLAG VALUES.
Specifically, if the two flags are used when calling a subroutine and that subroutine does not call die, the value returned by call_* will be wrong.
In Perl 5.000 and 5.001 there is a problem with using call_* if the Perl sub you are calling attempts to trap a 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 Call_fred is executed it will print
Trapped error: Fatal Error
As control never returns to Call_fred, the "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 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") ;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 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, call_pvかcall_svかで選ぶなら call_svを試すべきでしょう. call_svの使用に詳細があります.
この最初の単純極まりない例は, Perl関数 PrintUID を呼び出し, プロセスの UID を表示します.
sub PrintUID
{
print "UID is $<\n" ;
}これを呼び出すC関数は次のようになります:
static void
call_PrintUID()
{
dSP ;
PUSHMARK(SP) ;
call_pv("PrintUID", G_DISCARD|G_NOARGS) ;
}単純ですね.
この例のポイントです.
dSP と PUSHMARK(SP) は, 今のところ無視してください.
次の例で説明します.
PrintUID に何のパラメータも渡してないので G_NOARGS を 指定しています.
PrintUID からの復帰値に興味はないので G_DISCARD を指定しています. たとえ PrintUID が何らかの値を返すように修正されたとしても, G_DISCARD を指定しているためにcall_pvから制御が変える前に 除去されます.
call_pvが使われているため, Perl関数はC文字列として指定されます. このケースでは, 関数名はハードコーディングされています.
G_DISCARD が指定されているために, call_pv の復帰値を 確認する必要はありません. 常に 0 となります.
では少しずつ進めてみましょう. 今回は Perl 関数, LeftString に
２つのパラメータ, 文字列 ($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" ;
}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 call_LeftString の説明をしましょう.
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 dSP と書き, 終わりに PUTBACK と書いた
理由です. 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常に SP としてアクセスされるべきです.
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 dSP マクロの目的です. これが Perl スタックポインタ
の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 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.
Any parameters to be pushed onto the stack should be bracketed by the
PUSHMARK and 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 @_ array for the
subroutine, it knows how big to make it.
The 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 No Parameters, Nothing returned) you
must still call the PUSHMARK macro before you can call any of the
call_* functions--Perl still needs to know that there are no
parameters.
The PUTBACK macro sets the global copy of the stack pointer to be
the same as our local copy. If we didn't do this 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, not the global copy.
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 "XSUBs and the Argument Stack" in perlguts for details on how the XPUSH macros work.
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 ENTER/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 FREETMPS/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 ENTER/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 { and } in Perl
to limit the scope of local variables.
See the section Using Perl to dispose of temporaries for details of an alternative to using these macros.
Finally, LeftString can now be called via the 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.
Now for an example of dealing with the items returned from a Perl subroutine.
Here is a Perl subroutine, 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 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
The only flag specified this time was G_SCALAR. That means the @_
array will be created and that the value returned by Adder will
still exist after the call to call_pv.
The purpose of the macro 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
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 call_* functions or any other Perl internal function.
Although only a single value was expected to be returned from Adder, it is still good practice to check the return code from call_pv anyway.
Expecting a single value is not quite the same as knowing that there will be one. If someone modified 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 really don't want to happen ever.
The POPi macro is used here to pop the return value from the stack.
In this case we wanted an integer, so 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
The final 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 POPi it updated only our
local copy of the stack pointer. Remember, PUTBACK sets the global
stack pointer to be the same as our local copy.
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 call_AddSubtract is called like this
call_AddSubtract(7, 4) ;then here is the output
7 - 4 = 3
7 + 4 = 11Notes
We wanted list context, so G_ARRAY was used.
Not surprisingly 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 POP* macros they come off the stack in reverse
order.
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 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 call_AddSubScalar is called
call_AddSubScalar(7, 4) ;then the output will be
Items Returned = 1
Value 1 = 3In this case the main point to note is that only the last item in the list is returned from the subroutine, AddSubtract actually made it back to call_AddSubScalar.
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, Inc, below takes 2 parameters and increments each directly.
sub Inc
{
++ $_[0] ;
++ $_[1] ;
}and here is a C function to call it.
static void
call_Inc(a, b)
int a ;
int b ;
{
dSP ;
int count ;
SV * sva ;
SV * svb ;
ENTER ;
SAVETMPS;
sva = sv_2mortal(newSViv(a)) ;
svb = sv_2mortal(newSViv(b)) ;
PUSHMARK(SP) ;
XPUSHs(sva);
XPUSHs(svb);
PUTBACK ;
count = call_pv("Inc", G_DISCARD);
if (count != 0)
croak ("call_Inc: expected 0 values from 'Inc', got %d\n",
count) ;
printf ("%d + 1 = %d\n", a, SvIV(sva)) ;
printf ("%d + 1 = %d\n", b, SvIV(svb)) ;
FREETMPS ;
LEAVE ;
}
To be able to access the two parameters that were pushed onto the stack
after they return from call_pv it is necessary to make a note
of their addresses--thus the two variables sva and svb.
The reason this is necessary is that the area of the Perl stack which held them will very likely have been overwritten by something else by the time control returns from call_pv.
Now an example using G_EVAL. Below is a Perl subroutine which computes the difference of its 2 parameters. If this would result in a negative result, the subroutine calls die.
sub Subtract
{
my ($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 call_Subtract is called thus
call_Subtract(4, 5)the following will be printed
Uh oh - death can be fatalNotes
We want to be able to catch the die so we have used the G_EVAL flag. Not specifying this flag would mean that the program would terminate immediately at the die statement in the subroutine Subtract.
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 $@ ;
PL_errgv is a perl global of type GV * that points to the
symbol table entry containing the error. ERRSV therefore
refers to the C equivalent of $@.
Note that the stack is popped using POPs in the block where
SvTRUE(ERRSV) is true. This is necessary because whenever a
call_* function invoked with G_EVAL|G_SCALAR returns an error,
the top of the stack holds the value undef. Because we want the
program to continue after detecting this error, it is essential that
the stack is tidied up by removing the undef.
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
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 call_pv using the G_EVAL
flag, it promptly reset $@. This results in the failure of the
outermost test for $@, and thereby the failure of the error trap.
Appending the G_KEEPERR flag, so that the 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.
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 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 call_sv is useful.
The code below for CallSubSV is identical to CallSubPV except
that the name parameter is now defined as an SV* and we use
call_sv instead of 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 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, 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 (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 rememberSub in CallSavedSub1, it may or may not still refer
to the Perl subroutine that was recorded in SaveSub1. This is
particularly true for these cases
SaveSub1(\&fred) ;
CallSavedSub1() ;
SaveSub1( sub { print "Hello there\n" } ) ;
CallSavedSub1() ;
By the time each of the 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 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 fred
whenever the call to SaveSub1 was made but by the time
CallSavedSub1 gets called it now holds the number 47. Because we
saved only a pointer to the original SV in SaveSub1, any changes to
$ref will be tracked by the pointer rememberSub. This means that
whenever CallSavedSub1 gets called, it will attempt to execute the
code which is referenced by the SV* rememberSub. In this case
though, it now refers to the integer 47, so expect Perl to complain
loudly.
A similar but more subtle problem is illustrated with this code
$ref = \&fred ;
SaveSub1($ref) ;
$ref = \&joe ;
CallSavedSub1() ;
This time whenever CallSavedSub1 get called it will execute the Perl
subroutine joe (assuming it exists) rather than fred as was
originally requested in the call to SaveSub1.
To get around these problems it is necessary to take a full copy of the
SV. The code below shows SaveSub2 modified to do that
static SV * keepSub = (SV*)NULL ;
void
SaveSub2(name)
SV * name
CODE:
/* Take a copy of the callback */
if (keepSub == (SV*)NULL)
/* First time, so create a new SV */
keepSub = newSVsv(name) ;
else
/* Been here before, so overwrite */
SvSetSV(keepSub, name) ;
void
CallSavedSub2()
CODE:
PUSHMARK(SP) ;
call_sv(keepSub, G_DISCARD|G_NOARGS) ;
To avoid creating a new SV every time SaveSub2 is called,
the function first checks to see if it has been called before. If not,
then space for a new SV is allocated and the reference to the Perl
subroutine, name is copied to the variable keepSub in one
operation using newSVsv. Thereafter, whenever SaveSub2 is called
the existing SV, keepSub, is overwritten with the new value using
SvSetSV.
Here is a Perl subroutine which prints whatever parameters are passed to it.
sub PrintList
{
my(@list) = @_ ;
foreach (@list) { print "$_\n" }
}and here is an example of call_argv which will call 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 PUSHMARK in this instance.
This is because call_argv will do it for you.
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, new, it declares methods, one static and one
virtual. The static method, PrintID, prints out simply the class
name and a version number. The virtual method, 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 1.0Calling a Perl method from C is fairly straightforward. The following things are required
a reference to the object for a virtual method or the name of the class for a static method.
the name of the method.
any other parameters specific to the method.
Here is a simple XSUB which illustrates the mechanics of calling both
the PrintID and 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 PrintID and 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 call_method.
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 ArrayIn the examples given to date, any temporaries created in the callback (i.e., parameters passed on the stack to the call_* function or values returned via the stack) have been freed by one of these methods
specifying the G_DISCARD flag with call_*.
explicitly disposed of using the ENTER/SAVETMPS -
FREETMPS/LEAVE pairing.
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 libraryWhilst 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 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_* --> perlfor practically the complete duration of the program. This means that control may 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.
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 register_fatal which registers the C function
to get called when a fatal error occurs.
register_fatal(cb1) ;
The single parameter cb1 is a pointer to a function, so you must
have defined 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 register_fatal and the callback it
registers, 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 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
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 asynch_read which associates a C
function ProcessRead with a file handle 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 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 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 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 asynch_close. This shows how to remove
the entry from the hash 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 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 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 fn1, fn2, and 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 Pcb to
access the 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
For a lot of situations, like interfacing to an error handler, this may be a perfectly adequate solution.
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.
A hash is an ideal mechanism to store the mapping between C and Perl.
Although I have made use of only the POP* macros to access values
returned from Perl subroutines, it is also possible to bypass these
macros and read the stack using the ST macro (See perlxs for a
full description of the ST macro).
Most of the time the 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 ST macro as used when coding an XSUB is ideal
for this purpose.
The code below is the example given in the section Returning a list
of values recoded to use ST instead of 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
Notice that it was necessary to define the variable ax. This is
because the ST macro expects it to exist. If we were in an XSUB it
would not be necessary to define ax as it is already defined for
you.
The code
SPAGAIN ;
SP -= count ;
ax = (SP - PL_stack_base) + 1 ;
sets the stack up so that we can use the ST macro.
Unlike the original coding of this example, the returned
values are not accessed in reverse order. So ST(0) refers to the
first value returned by the Perl subroutine and ST(count-1)
refers to the last.
As we've already shown, 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);
eval_pv is used to compile the anonymous subroutine, which
will be the return value as well (read more about eval_pv in
"eval_pv" in perlapi). Once this code reference is in hand, it
can be mixed in with all the previous examples we've shown.
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.
Version 1.3, 14th Apr 1997