Kentaro Shirakata
argra****@users*****
2007年 8月 4日 (土) 03:31:48 JST
Index: docs/perl/5.8.8/perlpacktut.pod
diff -u /dev/null docs/perl/5.8.8/perlpacktut.pod:1.1
--- /dev/null Sat Aug 4 03:31:48 2007
+++ docs/perl/5.8.8/perlpacktut.pod Sat Aug 4 03:31:48 2007
@@ -0,0 +1,2011 @@
+=head1 NAME
+
+=begin original
+
+perlpacktut - tutorial on C<pack> and C<unpack>
+
+=end original
+
+perlpacktut - C<pack> と C<unpack> のチュートリアル
+
+=head1 DESCRIPTION
+
+=begin original
+
+C<pack> and C<unpack> are two functions for transforming data according
+to a user-defined template, between the guarded way Perl stores values
+and some well-defined representation as might be required in the
+environment of a Perl program. Unfortunately, they're also two of
+the most misunderstood and most often overlooked functions that Perl
+provides. This tutorial will demystify them for you.
+
+=end original
+
+C<pack> は C<unpack> は、ユーザーが定義したテンプレートに従って、
+between the guarded way Perl stores values
+and some well-defined representation as might be required in the
+environment of a Perl program.
+残念ながら、これらは Perl が提供する関数の中でもっとも誤解され、
+もっとも見落とされやすい関数でもあります。
+このチュートリアルではこれらを分かりやすく説明します。
+
+=head1 The Basic Principle
+
+(基本原理)
+
+=begin original
+
+Most programming languages don't shelter the memory where variables are
+stored. In C, for instance, you can take the address of some variable,
+and the C<sizeof> operator tells you how many bytes are allocated to
+the variable. Using the address and the size, you may access the storage
+to your heart's content.
+
+=end original
+
+多くのプログラミング言語は変数が格納されているメモリを保護していません。
+例えば、C では、ある変数のアドレスを取得できますし、
+C<sizeof> 演算子は変数に何バイト割り当てられているかを返します。
+アドレスとサイズを使って、心臓部にあるストレージにアクセスできます。
+
+=begin original
+
+In Perl, you just can't access memory at random, but the structural and
+representational conversion provided by C<pack> and C<unpack> is an
+excellent alternative. The C<pack> function converts values to a byte
+sequence containing representations according to a given specification,
+the so-called "template" argument. C<unpack> is the reverse process,
+deriving some values from the contents of a string of bytes. (Be cautioned,
+however, that not all that has been packed together can be neatly unpacked -
+a very common experience as seasoned travellers are likely to confirm.)
+
+=end original
+
+
+=begin original
+
+Why, you may ask, would you need a chunk of memory containing some values
+in binary representation? One good reason is input and output accessing
+some file, a device, or a network connection, whereby this binary
+representation is either forced on you or will give you some benefit
+in processing. Another cause is passing data to some system call that
+is not available as a Perl function: C<syscall> requires you to provide
+parameters stored in the way it happens in a C program. Even text processing
+(as shown in the next section) may be simplified with judicious usage
+of these two functions.
+
+=end original
+
+
+=begin original
+
+To see how (un)packing works, we'll start with a simple template
+code where the conversion is in low gear: between the contents of a byte
+sequence and a string of hexadecimal digits. Let's use C<unpack>, since
+this is likely to remind you of a dump program, or some desperate last
+message unfortunate programs are wont to throw at you before they expire
+into the wild blue yonder. Assuming that the variable C<$mem> holds a
+sequence of bytes that we'd like to inspect without assuming anything
+about its meaning, we can write
+
+=end original
+
+
+ my( $hex ) = unpack( 'H*', $mem );
+ print "$hex\n";
+
+=begin original
+
+whereupon we might see something like this, with each pair of hex digits
+corresponding to a byte:
+
+=end original
+
+
+ 41204d414e204120504c414e20412043414e414c2050414e414d41
+
+=begin original
+
+What was in this chunk of memory? Numbers, characters, or a mixture of
+both? Assuming that we're on a computer where ASCII (or some similar)
+encoding is used: hexadecimal values in the range C<0x40> - C<0x5A>
+indicate an uppercase letter, and C<0x20> encodes a space. So we might
+assume it is a piece of text, which some are able to read like a tabloid;
+but others will have to get hold of an ASCII table and relive that
+firstgrader feeling. Not caring too much about which way to read this,
+we note that C<unpack> with the template code C<H> converts the contents
+of a sequence of bytes into the customary hexadecimal notation. Since
+"a sequence of" is a pretty vague indication of quantity, C<H> has been
+defined to convert just a single hexadecimal digit unless it is followed
+by a repeat count. An asterisk for the repeat count means to use whatever
+remains.
+
+=end original
+
+
+=begin original
+
+The inverse operation - packing byte contents from a string of hexadecimal
+digits - is just as easily written. For instance:
+
+=end original
+
+
+ my $s = pack( 'H2' x 10, map { "3$_" } ( 0..9 ) );
+ print "$s\n";
+
+=begin original
+
+Since we feed a list of ten 2-digit hexadecimal strings to C<pack>, the
+pack template should contain ten pack codes. If this is run on a computer
+with ASCII character coding, it will print C<0123456789>.
+
+=end original
+
+
+
+=head1 Packing Text
+
+(テキストをパックする)
+
+=begin original
+
+Let's suppose you've got to read in a data file like this:
+
+=end original
+
+以下のようなデータファイルを読み込むことを考えます:
+
+ Date |Description | Income|Expenditure
+ 01/24/2001 Ahmed's Camel Emporium 1147.99
+ 01/28/2001 Flea spray 24.99
+ 01/29/2001 Camel rides to tourists 235.00
+
+=begin original
+
+How do we do it? You might think first to use C<split>; however, since
+C<split> collapses blank fields, you'll never know whether a record was
+income or expenditure. Oops. Well, you could always use C<substr>:
+
+=end original
+
+どうすればいいでしょう?最初に思いつくのは C<split> かもしれません;
+しかし、C<split> は空白のフィールドを壊してしまうので、
+そのレコードが収入だったか支出だったが分かりません。あらら。
+では、C<substr> を使うとどうでしょう:
+
+ while (<>) {
+ my $date = substr($_, 0, 11);
+ my $desc = substr($_, 12, 27);
+ my $income = substr($_, 40, 7);
+ my $expend = substr($_, 52, 7);
+ ...
+ }
+
+=begin original
+
+It's not really a barrel of laughs, is it? In fact, it's worse than it
+may seem; the eagle-eyed may notice that the first field should only be
+10 characters wide, and the error has propagated right through the other
+numbers - which we've had to count by hand. So it's error-prone as well
+as horribly unfriendly.
+
+=end original
+
+これはあまり愉快ではないですよね?
+実際、これは思ったより悪いです;注意深い人は最初のフィールドが 10 文字分しか
+なく、エラーが他の数値に拡大してしまう - 手で数えなければなりません -
+ことに気付くでしょう。
+従って、これは恐ろしく不親切であると同様、間違いが発生しやすいです．
+
+=begin original
+
+Or maybe we could use regular expressions:
+
+=end original
+
+あるいは正規表現も使えます:
+
+ while (<>) {
+ my($date, $desc, $income, $expend) =
+ m|(\d\d/\d\d/\d{4}) (.{27}) (.{7})(.*)|;
+ ...
+ }
+
+=begin original
+
+Urgh. Well, it's a bit better, but - well, would you want to maintain
+that?
+
+=end original
+
+うわあ。えーと、少しましです。
+しかし - えーと、これを保守したいと思います?
+
+=begin original
+
+Hey, isn't Perl supposed to make this sort of thing easy? Well, it does,
+if you use the right tools. C<pack> and C<unpack> are designed to help
+you out when dealing with fixed-width data like the above. Let's have a
+look at a solution with C<unpack>:
+
+=end original
+
+ねえ、Perl はこの手のことを簡単にできないの?
+ええ、できます、正しい道具を使えば。
+C<pack> と C<unpack> は上記のような固定長データを扱う時の
+助けになるように設計されています。
+C<unpack> による開放を見てみましょう:
+
+ while (<>) {
+ my($date, $desc, $income, $expend) = unpack("A10xA27xA7A*", $_);
+ ...
+ }
+
+=begin original
+
+That looks a bit nicer; but we've got to take apart that weird template.
+Where did I pull that out of?
+
+=end original
+
+これはちょっとましに見えます;
+でも変なテンプレートを分析しなければなりません。
+これはどこから来たのでしょう?
+
+=begin original
+
+OK, let's have a look at some of our data again; in fact, we'll include
+the headers, and a handy ruler so we can keep track of where we are.
+
+=end original
+
+よろしい、ここでデータをもう一度見てみましょう;
+実際、ヘッダも含めて、何をしているかを追いかけるために
+手書きの目盛りも付けています。
+
+ 1 2 3 4 5
+ 1234567890123456789012345678901234567890123456789012345678
+ Date |Description | Income|Expenditure
+ 01/28/2001 Flea spray 24.99
+ 01/29/2001 Camel rides to tourists 235.00
+
+=begin original
+
+From this, we can see that the date column stretches from column 1 to
+column 10 - ten characters wide. The C<pack>-ese for "character" is
+C<A>, and ten of them are C<A10>. So if we just wanted to extract the
+dates, we could say this:
+
+=end original
+
+ここから、日付の桁は 1 桁目から 10 桁目まで - 10 文字の幅があることが
+わかります。
+「文字」のパックは C<A> で、10 文字の場合は C<A10> です。
+それで、もし単に日付を展開したいだけなら、以下のように書けます:
+
+ my($date) = unpack("A10", $_);
+
+=begin original
+
+OK, what's next? Between the date and the description is a blank column;
+we want to skip over that. The C<x> template means "skip forward", so we
+want one of those. Next, we have another batch of characters, from 12 to
+38. That's 27 more characters, hence C<A27>. (Don't make the fencepost
+error - there are 27 characters between 12 and 38, not 26. Count 'em!)
+
+=end original
+
+よろしい、次は?
+日付と説明の間には空白の桁があります;これは読み飛ばしたいです。
+C<x> テンプレートは「読み飛ばす」ことを意味し、
+これで 1 文字読み飛ばせます。
+次に、別の文字の塊が 12 桁から 38 桁まであります。
+これは 27 文字あるので、C<A27> です。
+(数え間違えないように - 12 から 38 の間には 26 ではなく 27 文字あります。)
+
+=begin original
+
+Now we skip another character and pick up the next 7 characters:
+
+=end original
+
+次の文字は読み飛ばして、次の 7 文字を取り出します:
+
+ my($date,$description,$income) = unpack("A10xA27xA7", $_);
+
+=begin original
+
+Now comes the clever bit. Lines in our ledger which are just income and
+not expenditure might end at column 46. Hence, we don't want to tell our
+C<unpack> pattern that we B<need> to find another 12 characters; we'll
+just say "if there's anything left, take it". As you might guess from
+regular expressions, that's what the C<*> means: "use everything
+remaining".
+
+=end original
+
+ここで少し賢くやりましょう。
+Lines in our ledger which are just income and
+not expenditure might end at column 46. Hence, we don't want to tell our
+C<unpack> pattern that we B<need> to find another 12 characters; we'll
+just say "if there's anything left, take it". As you might guess from
+regular expressions, that's what the C<*> means: "use everything
+remaining".
+
+=over 3
+
+=item *
+
+=begin original
+
+Be warned, though, that unlike regular expressions, if the C<unpack>
+template doesn't match the incoming data, Perl will scream and die.
+
+=end original
+
+但し、正規表現とは違うことに注意してください。
+もし C<unpack> テンプレートが入力データと一致しない場合、
+Perl は悲鳴をあげて die します。
+
+=back
+
+
+=begin original
+
+Hence, putting it all together:
+
+=end original
+
+従って、これを全部あわせると:
+
+ my($date,$description,$income,$expend) = unpack("A10xA27xA7xA*", $_);
+
+=begin original
+
+Now, that's our data parsed. I suppose what we might want to do now is
+total up our income and expenditure, and add another line to the end of
+our ledger - in the same format - saying how much we've brought in and
+how much we've spent:
+
+=end original
+
+これで、データがパースできます。
+今ほしいものが収入と支出をそれぞれ足し合わせて、台帳の最後に - 同じ形式で - 1 行
+付け加えることで、どれだけの収入と支出があったかを記すことだとします:
+
+ while (<>) {
+ my($date, $desc, $income, $expend) = unpack("A10xA27xA7xA*", $_);
+ $tot_income += $income;
+ $tot_expend += $expend;
+ }
+
+ $tot_income = sprintf("%.2f", $tot_income); # Get them into
+ $tot_expend = sprintf("%.2f", $tot_expend); # "financial" format
+
+ $date = POSIX::strftime("%m/%d/%Y", localtime);
+
+ # OK, let's go:
+
+ print pack("A10xA27xA7xA*", $date, "Totals", $tot_income, $tot_expend);
+
+=begin original
+
+Oh, hmm. That didn't quite work. Let's see what happened:
+
+=end original
+
+あら、ふうむ。
+これはうまく動きません。
+何が起こったのか見てみましょう:
+
+ 01/24/2001 Ahmed's Camel Emporium 1147.99
+ 01/28/2001 Flea spray 24.99
+ 01/29/2001 Camel rides to tourists 1235.00
+ 03/23/2001Totals 1235.001172.98
+
+=begin original
+
+OK, it's a start, but what happened to the spaces? We put C<x>, didn't
+we? Shouldn't it skip forward? Let's look at what L<perlfunc/pack> says:
+
+=end original
+
+まあ、これはスタートです。しかしスペースに何が起きたのでしょう?
+C<x> を指定しましたよね?これでは飛ばせない?
+L<perlfunc/pack> に書いていることを見てみましょう:
+
+ x A null byte.
+
+=begin original
+
+Urgh. No wonder. There's a big difference between "a null byte",
+character zero, and "a space", character 32. Perl's put something
+between the date and the description - but unfortunately, we can't see
+it!
+
+=end original
+
+うはあ。当たり前です。
+文字コード 0 の「ヌル文字」と、文字コード 32 の「空白」は全然違います。
+Perl は日付と説明の間に何かを書いたのです - しかし残念ながら、
+それは見えません!
+
+=begin original
+
+What we actually need to do is expand the width of the fields. The C<A>
+format pads any non-existent characters with spaces, so we can use the
+additional spaces to line up our fields, like this:
+
+=end original
+
+実際に必要なことはフィールドの幅を増やすことです。
+C<A> フォーマットは存在しない文字を空白でパッディングするので、
+以下のようにフィールドに空白の分だけ桁数を増やします:
+
+ print pack("A11 A28 A8 A*", $date, "Totals", $tot_income, $tot_expend);
+
+=begin original
+
+(Note that you can put spaces in the template to make it more readable,
+but they don't translate to spaces in the output.) Here's what we got
+this time:
+
+=end original
+
+(テンプレートには読みやすくするために空白を入れることができますが、
+出力には反映されないことに注意してください。)
+これで得られたのは以下のものです:
+
+ 01/24/2001 Ahmed's Camel Emporium 1147.99
+ 01/28/2001 Flea spray 24.99
+ 01/29/2001 Camel rides to tourists 1235.00
+ 03/23/2001 Totals 1235.00 1172.98
+
+=begin original
+
+That's a bit better, but we still have that last column which needs to
+be moved further over. There's an easy way to fix this up:
+unfortunately, we can't get C<pack> to right-justify our fields, but we
+can get C<sprintf> to do it:
+
+=end original
+
+これで少し良くなりましたが、まだ、最後の桁をもっと向こうに移動させる
+必要があります。
+これを修正する簡単な方法があります:
+残念ながら C<pack> でフィールドを右寄せにすることは出来ませんが、
+C<sprintf> を使えば出来ます:
+
+ $tot_income = sprintf("%.2f", $tot_income);
+ $tot_expend = sprintf("%12.2f", $tot_expend);
+ $date = POSIX::strftime("%m/%d/%Y", localtime);
+ print pack("A11 A28 A8 A*", $date, "Totals", $tot_income, $tot_expend);
+
+=begin original
+
+This time we get the right answer:
+
+=end original
+
+今度は正しい答えを得られました:
+
+ 01/28/2001 Flea spray 24.99
+ 01/29/2001 Camel rides to tourists 1235.00
+ 03/23/2001 Totals 1235.00 1172.98
+
+=begin original
+
+So that's how we consume and produce fixed-width data. Let's recap what
+we've seen of C<pack> and C<unpack> so far:
+
+=end original
+
+ということで、これが固定長データを読み書きする方法です。
+ここまでで C<pack> と C<unpack> について見たことを復習しましょう:
+
+=over 3
+
+=item *
+
+=begin original
+
+Use C<pack> to go from several pieces of data to one fixed-width
+version; use C<unpack> to turn a fixed-width-format string into several
+pieces of data.
+
+=end original
+
+
+=item *
+
+=begin original
+
+The pack format C<A> means "any character"; if you're C<pack>ing and
+you've run out of things to pack, C<pack> will fill the rest up with
+spaces.
+
+=end original
+
+
+=item *
+
+=begin original
+
+C<x> means "skip a byte" when C<unpack>ing; when C<pack>ing, it means
+"introduce a null byte" - that's probably not what you mean if you're
+dealing with plain text.
+
+=end original
+
+
+=item *
+
+=begin original
+
+You can follow the formats with numbers to say how many characters
+should be affected by that format: C<A12> means "take 12 characters";
+C<x6> means "skip 6 bytes" or "character 0, 6 times".
+
+=end original
+
+
+=item *
+
+=begin original
+
+Instead of a number, you can use C<*> to mean "consume everything else
+left".
+
+=end original
+
+
+=begin original
+
+B<Warning>: when packing multiple pieces of data, C<*> only means
+"consume all of the current piece of data". That's to say
+
+=end original
+
+
+ pack("A*A*", $one, $two)
+
+=begin original
+
+packs all of C<$one> into the first C<A*> and then all of C<$two> into
+the second. This is a general principle: each format character
+corresponds to one piece of data to be C<pack>ed.
+
+=end original
+
+
+=back
+
+
+
+=head1 Packing Numbers
+
+=begin original
+
+So much for textual data. Let's get onto the meaty stuff that C<pack>
+and C<unpack> are best at: handling binary formats for numbers. There is,
+of course, not just one binary format - life would be too simple - but
+Perl will do all the finicky labor for you.
+
+=end original
+
+
+
+=head2 Integers
+
+(整数)
+
+=begin original
+
+Packing and unpacking numbers implies conversion to and from some
+I<specific> binary representation. Leaving floating point numbers
+aside for the moment, the salient properties of any such representation
+are:
+
+=end original
+
+数値を pack や unpack するということは、I<特定の> バイナリ表現との間で
+変換するということを意味します。
+今のところ浮動小数点数は脇にやっておくとすると、このような表現の
+主要な性質としては:
+
+=over 4
+
+=item *
+
+=begin original
+
+the number of bytes used for storing the integer,
+
+=end original
+
+整数の保存に複数バイトを使う。
+
+=item *
+
+=begin original
+
+whether the contents are interpreted as a signed or unsigned number,
+
+=end original
+
+内容を符号なし数として解釈するか符号付き数として解釈するか。
+
+=item *
+
+=begin original
+
+the byte ordering: whether the first byte is the least or most
+significant byte (or: little-endian or big-endian, respectively).
+
+=end original
+
+バイト順序:最初のバイトは最下位バイトか最上位バイトか
+(言い換えると: それぞれリトルエンディアンかビッグエンディアンか)。
+
+=back
+
+=begin original
+
+So, for instance, to pack 20302 to a signed 16 bit integer in your
+computer's representation you write
+
+=end original
+
+それで、例えば、20302 をコンピューターの符号付き 16 ビット整数に
+pack するとすると、以下のように書きます:
+
+ my $ps = pack( 's', 20302 );
+
+=begin original
+
+Again, the result is a string, now containing 2 bytes. If you print
+this string (which is, generally, not recommended) you might see
+C<ON> or C<NO> (depending on your system's byte ordering) - or something
+entirely different if your computer doesn't use ASCII character encoding.
+Unpacking C<$ps> with the same template returns the original integer value:
+
+=end original
+
+再び、結果は 2 バイトからなる文字列です。
+もしこの文字列を表示する(これは一般的にはお勧めできません)と、
+C<ON> か C<NO> (システムのバイト順に依存します) - または、もし
+コンピューターが ASCII 文字エンコーディングを使っていないなら全く違う
+文字列が表示されます。
+C<$ps> を同じテンプレートで unpack すると、元の整数値が返ります:
+
+ my( $s ) = unpack( 's', $ps );
+
+=begin original
+
+This is true for all numeric template codes. But don't expect miracles:
+if the packed value exceeds the allotted byte capacity, high order bits
+are silently discarded, and unpack certainly won't be able to pull them
+back out of some magic hat. And, when you pack using a signed template
+code such as C<s>, an excess value may result in the sign bit
+getting set, and unpacking this will smartly return a negative value.
+
+=end original
+
+これは全ての数値テンプレートコードに対して真です。
+しかし奇跡を期待してはいけません:
+(TBT)
+
+=begin original
+
+16 bits won't get you too far with integers, but there is C<l> and C<L>
+for signed and unsigned 32-bit integers. And if this is not enough and
+your system supports 64 bit integers you can push the limits much closer
+to infinity with pack codes C<q> and C<Q>. A notable exception is provided
+by pack codes C<i> and C<I> for signed and unsigned integers of the
+"local custom" variety: Such an integer will take up as many bytes as
+a local C compiler returns for C<sizeof(int)>, but it'll use I<at least>
+32 bits.
+
+=end original
+
+
+=begin original
+
+Each of the integer pack codes C<sSlLqQ> results in a fixed number of bytes,
+no matter where you execute your program. This may be useful for some
+applications, but it does not provide for a portable way to pass data
+structures between Perl and C programs (bound to happen when you call
+XS extensions or the Perl function C<syscall>), or when you read or
+write binary files. What you'll need in this case are template codes that
+depend on what your local C compiler compiles when you code C<short> or
+C<unsigned long>, for instance. These codes and their corresponding
+byte lengths are shown in the table below. Since the C standard leaves
+much leeway with respect to the relative sizes of these data types, actual
+values may vary, and that's why the values are given as expressions in
+C and Perl. (If you'd like to use values from C<%Config> in your program
+you have to import it with C<use Config>.)
+
+=end original
+
+
+ signed unsigned byte length in C byte length in Perl
+ s! S! sizeof(short) $Config{shortsize}
+ i! I! sizeof(int) $Config{intsize}
+ l! L! sizeof(long) $Config{longsize}
+ q! Q! sizeof(long long) $Config{longlongsize}
+
+=begin original
+
+The C<i!> and C<I!> codes aren't different from C<i> and C<I>; they are
+tolerated for completeness' sake.
+
+=end original
+
+
+
+=head2 Unpacking a Stack Frame
+
+(スタックフレームを unpack する)
+
+=begin original
+
+Requesting a particular byte ordering may be necessary when you work with
+binary data coming from some specific architecture whereas your program could
+run on a totally different system. As an example, assume you have 24 bytes
+containing a stack frame as it happens on an Intel 8086:
+
+=end original
+
+
+ +---------+ +----+----+ +---------+
+ TOS: | IP | TOS+4:| FL | FH | FLAGS TOS+14:| SI |
+ +---------+ +----+----+ +---------+
+ | CS | | AL | AH | AX | DI |
+ +---------+ +----+----+ +---------+
+ | BL | BH | BX | BP |
+ +----+----+ +---------+
+ | CL | CH | CX | DS |
+ +----+----+ +---------+
+ | DL | DH | DX | ES |
+ +----+----+ +---------+
+
+=begin original
+
+First, we note that this time-honored 16-bit CPU uses little-endian order,
+and that's why the low order byte is stored at the lower address. To
+unpack such a (signed) short we'll have to use code C<v>. A repeat
+count unpacks all 12 shorts:
+
+=end original
+
+
+ my( $ip, $cs, $flags, $ax, $bx, $cd, $dx, $si, $di, $bp, $ds, $es ) =
+ unpack( 'v12', $frame );
+
+=begin original
+
+Alternatively, we could have used C<C> to unpack the individually
+accessible byte registers FL, FH, AL, AH, etc.:
+
+=end original
+
+
+ my( $fl, $fh, $al, $ah, $bl, $bh, $cl, $ch, $dl, $dh ) =
+ unpack( 'C10', substr( $frame, 4, 10 ) );
+
+=begin original
+
+It would be nice if we could do this in one fell swoop: unpack a short,
+back up a little, and then unpack 2 bytes. Since Perl I<is> nice, it
+proffers the template code C<X> to back up one byte. Putting this all
+together, we may now write:
+
+=end original
+
+
+ my( $ip, $cs,
+ $flags,$fl,$fh,
+ $ax,$al,$ah, $bx,$bl,$bh, $cx,$cl,$ch, $dx,$dl,$dh,
+ $si, $di, $bp, $ds, $es ) =
+ unpack( 'v2' . ('vXXCC' x 5) . 'v5', $frame );
+
+=begin original
+
+(The clumsy construction of the template can be avoided - just read on!)
+
+=end original
+
+
+=begin original
+
+We've taken some pains to construct the template so that it matches
+the contents of our frame buffer. Otherwise we'd either get undefined values,
+or C<unpack> could not unpack all. If C<pack> runs out of items, it will
+supply null strings (which are coerced into zeroes whenever the pack code
+says so).
+
+=end original
+
+
+
+=head2 How to Eat an Egg on a Net
+
+(インターネットの卵の食べ方)
+
+=begin original
+
+The pack code for big-endian (high order byte at the lowest address) is
+C<n> for 16 bit and C<N> for 32 bit integers. You use these codes
+if you know that your data comes from a compliant architecture, but,
+surprisingly enough, you should also use these pack codes if you
+exchange binary data, across the network, with some system that you
+know next to nothing about. The simple reason is that this
+order has been chosen as the I<network order>, and all standard-fearing
+programs ought to follow this convention. (This is, of course, a stern
+backing for one of the Lilliputian parties and may well influence the
+political development there.) So, if the protocol expects you to send
+a message by sending the length first, followed by just so many bytes,
+you could write:
+
+=end original
+
+
+ my $buf = pack( 'N', length( $msg ) ) . $msg;
+
+=begin original
+
+or even:
+
+=end original
+
+
+ my $buf = pack( 'NA*', length( $msg ), $msg );
+
+=begin original
+
+and pass C<$buf> to your send routine. Some protocols demand that the
+count should include the length of the count itself: then just add 4
+to the data length. (But make sure to read L<"Lengths and Widths"> before
+you really code this!)
+
+=end original
+
+
+
+=head2 Floating point Numbers
+
+(浮動小数点数)
+
+=begin original
+
+For packing floating point numbers you have the choice between the
+pack codes C<f> and C<d> which pack into (or unpack from) single-precision or
+double-precision representation as it is provided by your system. (There
+is no such thing as a network representation for reals, so if you want
+to send your real numbers across computer boundaries, you'd better stick
+to ASCII representation, unless you're absolutely sure what's on the other
+end of the line.)
+
+=end original
+
+
+
+
+=head1 Exotic Templates
+
+(風変わりなテンプレート)
+
+=head2 Bit Strings
+
+(ビット文字列)
+
+=begin original
+
+Bits are the atoms in the memory world. Access to individual bits may
+have to be used either as a last resort or because it is the most
+convenient way to handle your data. Bit string (un)packing converts
+between strings containing a series of C<0> and C<1> characters and
+a sequence of bytes each containing a group of 8 bits. This is almost
+as simple as it sounds, except that there are two ways the contents of
+a byte may be written as a bit string. Let's have a look at an annotated
+byte:
+
+=end original
+
+
+ 7 6 5 4 3 2 1 0
+ +-----------------+
+ | 1 0 0 0 1 1 0 0 |
+ +-----------------+
+ MSB LSB
+
+=begin original
+
+It's egg-eating all over again: Some think that as a bit string this should
+be written "10001100" i.e. beginning with the most significant bit, others
+insist on "00110001". Well, Perl isn't biased, so that's why we have two bit
+string codes:
+
+=end original
+
+
+ $byte = pack( 'B8', '10001100' ); # start with MSB
+ $byte = pack( 'b8', '00110001' ); # start with LSB
+
+=begin original
+
+It is not possible to pack or unpack bit fields - just integral bytes.
+C<pack> always starts at the next byte boundary and "rounds up" to the
+next multiple of 8 by adding zero bits as required. (If you do want bit
+fields, there is L<perlfunc/vec>. Or you could implement bit field
+handling at the character string level, using split, substr, and
+concatenation on unpacked bit strings.)
+
+=end original
+
+
+=begin original
+
+To illustrate unpacking for bit strings, we'll decompose a simple
+status register (a "-" stands for a "reserved" bit):
+
+=end original
+
+
+ +-----------------+-----------------+
+ | S Z - A - P - C | - - - - O D I T |
+ +-----------------+-----------------+
+ MSB LSB MSB LSB
+
+=begin original
+
+Converting these two bytes to a string can be done with the unpack
+template C<'b16'>. To obtain the individual bit values from the bit
+string we use C<split> with the "empty" separator pattern which dissects
+into individual characters. Bit values from the "reserved" positions are
+simply assigned to C<undef>, a convenient notation for "I don't care where
+this goes".
+
+=end original
+
+
+ ($carry, undef, $parity, undef, $auxcarry, undef, $zero, $sign,
+ $trace, $interrupt, $direction, $overflow) =
+ split( //, unpack( 'b16', $status ) );
+
+=begin original
+
+We could have used an unpack template C<'b12'> just as well, since the
+last 4 bits can be ignored anyway.
+
+=end original
+
+
+
+=head2 Uuencoding
+
+(uuencode)
+
+=begin original
+
+Another odd-man-out in the template alphabet is C<u>, which packs an
+"uuencoded string". ("uu" is short for Unix-to-Unix.) Chances are that
+you won't ever need this encoding technique which was invented to overcome
+the shortcomings of old-fashioned transmission mediums that do not support
+other than simple ASCII data. The essential recipe is simple: Take three
+bytes, or 24 bits. Split them into 4 six-packs, adding a space (0x20) to
+each. Repeat until all of the data is blended. Fold groups of 4 bytes into
+lines no longer than 60 and garnish them in front with the original byte count
+(incremented by 0x20) and a C<"\n"> at the end. - The C<pack> chef will
+prepare this for you, a la minute, when you select pack code C<u> on the menu:
+
+=end original
+
+
+ my $uubuf = pack( 'u', $bindat );
+
+=begin original
+
+A repeat count after C<u> sets the number of bytes to put into an
+uuencoded line, which is the maximum of 45 by default, but could be
+set to some (smaller) integer multiple of three. C<unpack> simply ignores
+the repeat count.
+
+=end original
+
+
+
+=head2 Doing Sums
+
+(合計を計算する)
+
+=begin original
+
+An even stranger template code is C<%>E<lt>I<number>E<gt>. First, because
+it's used as a prefix to some other template code. Second, because it
+cannot be used in C<pack> at all, and third, in C<unpack>, doesn't return the
+data as defined by the template code it precedes. Instead it'll give you an
+integer of I<number> bits that is computed from the data value by
+doing sums. For numeric unpack codes, no big feat is achieved:
+
+=end original
+
+
+ my $buf = pack( 'iii', 100, 20, 3 );
+ print unpack( '%32i3', $buf ), "\n"; # prints 123
+
+=begin original
+
+For string values, C<%> returns the sum of the byte values saving
+you the trouble of a sum loop with C<substr> and C<ord>:
+
+=end original
+
+
+ print unpack( '%32A*', "\x01\x10" ), "\n"; # prints 17
+
+=begin original
+
+Although the C<%> code is documented as returning a "checksum":
+don't put your trust in such values! Even when applied to a small number
+of bytes, they won't guarantee a noticeable Hamming distance.
+
+=end original
+
+
+=begin original
+
+In connection with C<b> or C<B>, C<%> simply adds bits, and this can be put
+to good use to count set bits efficiently:
+
+=end original
+
+
+ my $bitcount = unpack( '%32b*', $mask );
+
+=begin original
+
+And an even parity bit can be determined like this:
+
+=end original
+
+
+ my $evenparity = unpack( '%1b*', $mask );
+
+
+=head2 Unicode
+
+=begin original
+
+Unicode is a character set that can represent most characters in most of
+the world's languages, providing room for over one million different
+characters. Unicode 3.1 specifies 94,140 characters: The Basic Latin
+characters are assigned to the numbers 0 - 127. The Latin-1 Supplement with
+characters that are used in several European languages is in the next
+range, up to 255. After some more Latin extensions we find the character
+sets from languages using non-Roman alphabets, interspersed with a
+variety of symbol sets such as currency symbols, Zapf Dingbats or Braille.
+(You might want to visit L<www.unicode.org> for a look at some of
+them - my personal favourites are Telugu and Kannada.)
+
+=end original
+
+
+=begin original
+
+The Unicode character sets associates characters with integers. Encoding
+these numbers in an equal number of bytes would more than double the
+requirements for storing texts written in Latin alphabets.
+The UTF-8 encoding avoids this by storing the most common (from a western
+point of view) characters in a single byte while encoding the rarer
+ones in three or more bytes.
+
+=end original
+
+
+=begin original
+
+So what has this got to do with C<pack>? Well, if you want to convert
+between a Unicode number and its UTF-8 representation you can do so by
+using template code C<U>. As an example, let's produce the UTF-8
+representation of the Euro currency symbol (code number 0x20AC):
+
+=end original
+
+
+ $UTF8{Euro} = pack( 'U', 0x20AC );
+
+=begin original
+
+Inspecting C<$UTF8{Euro}> shows that it contains 3 bytes: "\xe2\x82\xac". The
+round trip can be completed with C<unpack>:
+
+=end original
+
+
+ $Unicode{Euro} = unpack( 'U', $UTF8{Euro} );
+
+=begin original
+
+Usually you'll want to pack or unpack UTF-8 strings:
+
+=end original
+
+
+ # pack and unpack the Hebrew alphabet
+ my $alefbet = pack( 'U*', 0x05d0..0x05ea );
+ my @hebrew = unpack( 'U*', $utf );
+
+
+=head2 Another Portable Binary Encoding
+
+(その他の移植性のあるバイナリエンコーディング)
+
+=begin original
+
+The pack code C<w> has been added to support a portable binary data
+encoding scheme that goes way beyond simple integers. (Details can
+be found at L<Casbah.org>, the Scarab project.) A BER (Binary Encoded
+Representation) compressed unsigned integer stores base 128
+digits, most significant digit first, with as few digits as possible.
+Bit eight (the high bit) is set on each byte except the last. There
+is no size limit to BER encoding, but Perl won't go to extremes.
+
+=end original
+
+
+ my $berbuf = pack( 'w*', 1, 128, 128+1, 128*128+127 );
+
+=begin original
+
+A hex dump of C<$berbuf>, with spaces inserted at the right places,
+shows 01 8100 8101 81807F. Since the last byte is always less than
+128, C<unpack> knows where to stop.
+
+=end original
+
+C<$berbuf> を、適切な位置に空白を入れつつ 16 進ダンプを取ると、
+01 8100 8101 81807F となります。
+最後のバイトは常に 128 より小さくなるので、C<unpack> は停止する位置が
+わかります。
+
+=head1 Template Grouping
+
+(テンプレートのグループ化)
+
+=begin original
+
+Prior to Perl 5.8, repetitions of templates had to be made by
+C<x>-multiplication of template strings. Now there is a better way as
+we may use the pack codes C<(> and C<)> combined with a repeat count.
+The C<unpack> template from the Stack Frame example can simply
+be written like this:
+
+=end original
+
+
+ unpack( 'v2 (vXXCC)5 v5', $frame )
+
+=begin original
+
+Let's explore this feature a little more. We'll begin with the equivalent of
+
+=end original
+
+
+ join( '', map( substr( $_, 0, 1 ), @str ) )
+
+=begin original
+
+which returns a string consisting of the first character from each string.
+Using pack, we can write
+
+=end original
+
+
+ pack( '(A)'. ＠ str, @str )
+
+=begin original
+
+or, because a repeat count C<*> means "repeat as often as required",
+simply
+
+=end original
+
+
+ pack( '(A)*', @str )
+
+=begin original
+
+(Note that the template C<A*> would only have packed C<$str[0]> in full
+length.)
+
+=end original
+
+
+=begin original
+
+To pack dates stored as triplets ( day, month, year ) in an array C<@dates>
+into a sequence of byte, byte, short integer we can write
+
+=end original
+
+
+ $pd = pack( '(CCS)*', map( @$_, @dates ) );
+
+=begin original
+
+To swap pairs of characters in a string (with even length) one could use
+several techniques. First, let's use C<x> and C<X> to skip forward and back:
+
+=end original
+
+
+ $s = pack( '(A)*', unpack( '(xAXXAx)*', $s ) );
+
+=begin original
+
+We can also use C<@> to jump to an offset, with 0 being the position where
+we were when the last C<(> was encountered:
+
+=end original
+
+
+ $s = pack( '(A)*', unpack( '(@1A @0A @2)*', $s ) );
+
+=begin original
+
+Finally, there is also an entirely different approach by unpacking big
+endian shorts and packing them in the reverse byte order:
+
+=end original
+
+
+ $s = pack( '(v)*', unpack( '(n)*', $s );
+
+
+=head1 Lengths and Widths
+
+(長さと幅)
+
+=head2 String Lengths
+
+(文字列の長さ)
+
+=begin original
+
+In the previous section we've seen a network message that was constructed
+by prefixing the binary message length to the actual message. You'll find
+that packing a length followed by so many bytes of data is a
+frequently used recipe since appending a null byte won't work
+if a null byte may be part of the data. Here is an example where both
+techniques are used: after two null terminated strings with source and
+destination address, a Short Message (to a mobile phone) is sent after
+a length byte:
+
+=end original
+
+
+ my $msg = pack( 'Z*Z*CA*', $src, $dst, length( $sm ), $sm );
+
+=begin original
+
+Unpacking this message can be done with the same template:
+
+=end original
+
+このメッセージを unpack するには同じテンプレートで可能です:
+
+ ( $src, $dst, $len, $sm ) = unpack( 'Z*Z*CA*', $msg );
+
+=begin original
+
+There's a subtle trap lurking in the offing: Adding another field after
+the Short Message (in variable C<$sm>) is all right when packing, but this
+cannot be unpacked naively:
+
+=end original
+
+
+ # pack a message
+ my $msg = pack( 'Z*Z*CA*C', $src, $dst, length( $sm ), $sm, $prio );
+
+ # unpack fails - $prio remains undefined!
+ ( $src, $dst, $len, $sm, $prio ) = unpack( 'Z*Z*CA*C', $msg );
+
+=begin original
+
+The pack code C<A*> gobbles up all remaining bytes, and C<$prio> remains
+undefined! Before we let disappointment dampen the morale: Perl's got
+the trump card to make this trick too, just a little further up the sleeve.
+Watch this:
+
+=end original
+
+
+ # pack a message: ASCIIZ, ASCIIZ, length/string, byte
+ my $msg = pack( 'Z* Z* C/A* C', $src, $dst, $sm, $prio );
+
+ # unpack
+ ( $src, $dst, $sm, $prio ) = unpack( 'Z* Z* C/A* C', $msg );
+
+=begin original
+
+Combining two pack codes with a slash (C</>) associates them with a single
+value from the argument list. In C<pack>, the length of the argument is
+taken and packed according to the first code while the argument itself
+is added after being converted with the template code after the slash.
+This saves us the trouble of inserting the C<length> call, but it is
+in C<unpack> where we really score: The value of the length byte marks the
+end of the string to be taken from the buffer. Since this combination
+doesn't make sense except when the second pack code isn't C<a*>, C<A*>
+or C<Z*>, Perl won't let you.
+
+=end original
+
+
+=begin original
+
+The pack code preceding C</> may be anything that's fit to represent a
+number: All the numeric binary pack codes, and even text codes such as
+C<A4> or C<Z*>:
+
+=end original
+
+
+ # pack/unpack a string preceded by its length in ASCII
+ my $buf = pack( 'A4/A*', "Humpty-Dumpty" );
+ # unpack $buf: '13 Humpty-Dumpty'
+ my $txt = unpack( 'A4/A*', $buf );
+
+=begin original
+
+C</> is not implemented in Perls before 5.6, so if your code is required to
+work on older Perls you'll need to C<unpack( 'Z* Z* C')> to get the length,
+then use it to make a new unpack string. For example
+
+=end original
+
+
+ # pack a message: ASCIIZ, ASCIIZ, length, string, byte (5.005 compatible)
+ my $msg = pack( 'Z* Z* C A* C', $src, $dst, length $sm, $sm, $prio );
+
+ # unpack
+ ( undef, undef, $len) = unpack( 'Z* Z* C', $msg );
+ ($src, $dst, $sm, $prio) = unpack ( "Z* Z* x A$len C", $msg );
+
+=begin original
+
+But that second C<unpack> is rushing ahead. It isn't using a simple literal
+string for the template. So maybe we should introduce...
+
+=end original
+
+
+=head2 Dynamic Templates
+
+(動的テンプレート)
+
+=begin original
+
+So far, we've seen literals used as templates. If the list of pack
+items doesn't have fixed length, an expression constructing the
+template is required (whenever, for some reason, C<()*> cannot be used).
+Here's an example: To store named string values in a way that can be
+conveniently parsed by a C program, we create a sequence of names and
+null terminated ASCII strings, with C<=> between the name and the value,
+followed by an additional delimiting null byte. Here's how:
+
+=end original
+
+
+ my $env = pack( '(A*A*Z*)' . keys( %Env ) . 'C',
+ map( { ( $_, '=', $Env{$_} ) } keys( %Env ) ), 0 );
+
+=begin original
+
+Let's examine the cogs of this byte mill, one by one. There's the C<map>
+call, creating the items we intend to stuff into the C<$env> buffer:
+to each key (in C<$_>) it adds the C<=> separator and the hash entry value.
+Each triplet is packed with the template code sequence C<A*A*Z*> that
+is repeated according to the number of keys. (Yes, that's what the C<keys>
+function returns in scalar context.) To get the very last null byte,
+we add a C<0> at the end of the C<pack> list, to be packed with C<C>.
+(Attentive readers may have noticed that we could have omitted the 0.)
+
+=end original
+
+
+=begin original
+
+For the reverse operation, we'll have to determine the number of items
+in the buffer before we can let C<unpack> rip it apart:
+
+=end original
+
+
+ my $n = $env =~ tr/\0// - 1;
+ my %env = map( split( /=/, $_ ), unpack( "(Z*)$n", $env ) );
+
+=begin original
+
+The C<tr> counts the null bytes. The C<unpack> call returns a list of
+name-value pairs each of which is taken apart in the C<map> block.
+
+=end original
+
+
+
+=head2 Counting Repetitions
+
+(繰り返しを数える)
+
+=begin original
+
+Rather than storing a sentinel at the end of a data item (or a list of items),
+we could precede the data with a count. Again, we pack keys and values of
+a hash, preceding each with an unsigned short length count, and up front
+we store the number of pairs:
+
+=end original
+
+
+ my $env = pack( 'S(S/A* S/A*)*', scalar keys( %Env ), %Env );
+
+=begin original
+
+This simplifies the reverse operation as the number of repetitions can be
+unpacked with the C</> code:
+
+=end original
+
+
+ my %env = unpack( 'S/(S/A* S/A*)', $env );
+
+=begin original
+
+Note that this is one of the rare cases where you cannot use the same
+template for C<pack> and C<unpack> because C<pack> can't determine
+a repeat count for a C<()>-group.
+
+=end original
+
+
+
+=head1 Packing and Unpacking C Structures
+
+(C の構造体を pack/unpack する)
+
+=begin original
+
+In previous sections we have seen how to pack numbers and character
+strings. If it were not for a couple of snags we could conclude this
+section right away with the terse remark that C structures don't
+contain anything else, and therefore you already know all there is to it.
+Sorry, no: read on, please.
+
+=end original
+
+
+=head2 The Alignment Pit
+
+(アライメントの落とし穴)
+
+=begin original
+
+In the consideration of speed against memory requirements the balance
+has been tilted in favor of faster execution. This has influenced the
+way C compilers allocate memory for structures: On architectures
+where a 16-bit or 32-bit operand can be moved faster between places in
+memory, or to or from a CPU register, if it is aligned at an even or
+multiple-of-four or even at a multiple-of eight address, a C compiler
+will give you this speed benefit by stuffing extra bytes into structures.
+If you don't cross the C shoreline this is not likely to cause you any
+grief (although you should care when you design large data structures,
+or you want your code to be portable between architectures (you do want
+that, don't you?)).
+
+=end original
+
+
+=begin original
+
+To see how this affects C<pack> and C<unpack>, we'll compare these two
+C structures:
+
+=end original
+
+
+ typedef struct {
+ char c1;
+ short s;
+ char c2;
+ long l;
+ } gappy_t;
+
+ typedef struct {
+ long l;
+ short s;
+ char c1;
+ char c2;
+ } dense_t;
+
+=begin original
+
+Typically, a C compiler allocates 12 bytes to a C<gappy_t> variable, but
+requires only 8 bytes for a C<dense_t>. After investigating this further,
+we can draw memory maps, showing where the extra 4 bytes are hidden:
+
+=end original
+
+
+ 0 +4 +8 +12
+ +--+--+--+--+--+--+--+--+--+--+--+--+
+ |c1|xx| s |c2|xx|xx|xx| l | xx = fill byte
+ +--+--+--+--+--+--+--+--+--+--+--+--+
+ gappy_t
+
+ 0 +4 +8
+ +--+--+--+--+--+--+--+--+
+ | l | h |c1|c2|
+ +--+--+--+--+--+--+--+--+
+ dense_t
+
+=begin original
+
+And that's where the first quirk strikes: C<pack> and C<unpack>
+templates have to be stuffed with C<x> codes to get those extra fill bytes.
+
+=end original
+
+
+=begin original
+
+The natural question: "Why can't Perl compensate for the gaps?" warrants
+an answer. One good reason is that C compilers might provide (non-ANSI)
+extensions permitting all sorts of fancy control over the way structures
+are aligned, even at the level of an individual structure field. And, if
+this were not enough, there is an insidious thing called C<union> where
+the amount of fill bytes cannot be derived from the alignment of the next
+item alone.
+
+=end original
+
+
+=begin original
+
+OK, so let's bite the bullet. Here's one way to get the alignment right
+by inserting template codes C<x>, which don't take a corresponding item
+from the list:
+
+=end original
+
+
+ my $gappy = pack( 'cxs cxxx l!', $c1, $s, $c2, $l );
+
+=begin original
+
+Note the C<!> after C<l>: We want to make sure that we pack a long
+integer as it is compiled by our C compiler. And even now, it will only
+work for the platforms where the compiler aligns things as above.
+And somebody somewhere has a platform where it doesn't.
+[Probably a Cray, where C<short>s, C<int>s and C<long>s are all 8 bytes. :-)]
+
+=end original
+
+
+=begin original
+
+Counting bytes and watching alignments in lengthy structures is bound to
+be a drag. Isn't there a way we can create the template with a simple
+program? Here's a C program that does the trick:
+
+=end original
+
+
+ #include <stdio.h>
+ #include <stddef.h>
+
+ typedef struct {
+ char fc1;
+ short fs;
+ char fc2;
+ long fl;
+ } gappy_t;
+
+ #define Pt(struct,field,tchar) \
+ printf( "@%d%s ", offsetof(struct,field), # tchar );
+
+ int main() {
+ Pt( gappy_t, fc1, c );
+ Pt( gappy_t, fs, s! );
+ Pt( gappy_t, fc2, c );
+ Pt( gappy_t, fl, l! );
+ printf( "\n" );
+ }
+
+=begin original
+
+The output line can be used as a template in a C<pack> or C<unpack> call:
+
+=end original
+
+出力行は C<pack> や C<unpack> 呼び出しのテンプレートとして使えます。
+
+ my $gappy = pack( '@0c @2s! @4c @8l!', $c1, $s, $c2, $l );
+
+=begin original
+
+Gee, yet another template code - as if we hadn't plenty. But
+C<@> saves our day by enabling us to specify the offset from the beginning
+of the pack buffer to the next item: This is just the value
+the C<offsetof> macro (defined in C<E<lt>stddef.hE<gt>>) returns when
+given a C<struct> type and one of its field names ("member-designator" in
+C standardese).
+
+=end original
+
+
+=begin original
+
+Neither using offsets nor adding C<x>'s to bridge the gaps is satisfactory.
+(Just imagine what happens if the structure changes.) What we really need
+is a way of saying "skip as many bytes as required to the next multiple of N".
+In fluent Templatese, you say this with C<x!N> where N is replaced by the
+appropriate value. Here's the next version of our struct packaging:
+
+=end original
+
+
+ my $gappy = pack( 'c x!2 s c x!4 l!', $c1, $s, $c2, $l );
+
+=begin original
+
+That's certainly better, but we still have to know how long all the
+integers are, and portability is far away. Rather than C<2>,
+for instance, we want to say "however long a short is". But this can be
+done by enclosing the appropriate pack code in brackets: C<[s]>. So, here's
+the very best we can do:
+
+=end original
+
+
+ my $gappy = pack( 'c x![s] s c x![l!] l!', $c1, $s, $c2, $l );
+
+
+=head2 Alignment, Take 2
+
+(アライメント、第二幕)
+
+=begin original
+
+I'm afraid that we're not quite through with the alignment catch yet. The
+hydra raises another ugly head when you pack arrays of structures:
+
+=end original
+
+アライメントの捕捉について、十分に説明していないのではないかと
+心配しています。
+構造体の配列を pack しようとすると、ヒドラはまた別の醜い頭をもたげてきます。
+
+ typedef struct {
+ short count;
+ char glyph;
+ } cell_t;
+
+ typedef cell_t buffer_t[BUFLEN];
+
+=begin original
+
+Where's the catch? Padding is neither required before the first field C<count>,
+nor between this and the next field C<glyph>, so why can't we simply pack
+like this:
+
+=end original
+
+
+ # something goes wrong here:
+ pack( 's!a' x @buffer,
+ map{ ( $_->{count}, $_->{glyph} ) } @buffer );
+
+=begin original
+
+This packs C<3*@buffer> bytes, but it turns out that the size of
+C<buffer_t> is four times C<BUFLEN>! The moral of the story is that
+the required alignment of a structure or array is propagated to the
+next higher level where we have to consider padding I<at the end>
+of each component as well. Thus the correct template is:
+
+=end original
+
+
+ pack( 's!ax' x @buffer,
+ map{ ( $_->{count}, $_->{glyph} ) } @buffer );
+
+=head2 Alignment, Take 3
+
+(アライメント、第三幕)
+
+=begin original
+
+And even if you take all the above into account, ANSI still lets this:
+
+=end original
+
+上記のことを全て頭に入れたとしても、ANSI は以下のような場合:
+
+ typedef struct {
+ char foo[2];
+ } foo_t;
+
+=begin original
+
+vary in size. The alignment constraint of the structure can be greater than
+any of its elements. [And if you think that this doesn't affect anything
+common, dismember the next cellphone that you see. Many have ARM cores, and
+the ARM structure rules make C<sizeof (foo_t)> == 4]
+
+=end original
+
+サイズは様々であるとしています。
+(TBT)
+
+=head2 Pointers for How to Use Them
+
+(ポインタをどう扱うかのポインタ)
+
+=begin original
+
+The title of this section indicates the second problem you may run into
+sooner or later when you pack C structures. If the function you intend
+to call expects a, say, C<void *> value, you I<cannot> simply take
+a reference to a Perl variable. (Although that value certainly is a
+memory address, it's not the address where the variable's contents are
+stored.)
+
+=end original
+
+
+=begin original
+
+Template code C<P> promises to pack a "pointer to a fixed length string".
+Isn't this what we want? Let's try:
+
+=end original
+
+
+ # allocate some storage and pack a pointer to it
+ my $memory = "\x00" x $size;
+ my $memptr = pack( 'P', $memory );
+
+=begin original
+
+But wait: doesn't C<pack> just return a sequence of bytes? How can we pass this
+string of bytes to some C code expecting a pointer which is, after all,
+nothing but a number? The answer is simple: We have to obtain the numeric
+address from the bytes returned by C<pack>.
+
+=end original
+
+
+ my $ptr = unpack( 'L!', $memptr );
+
+=begin original
+
+Obviously this assumes that it is possible to typecast a pointer
+to an unsigned long and vice versa, which frequently works but should not
+be taken as a universal law. - Now that we have this pointer the next question
+is: How can we put it to good use? We need a call to some C function
+where a pointer is expected. The read(2) system call comes to mind:
+
+=end original
+
+
+ ssize_t read(int fd, void *buf, size_t count);
+
+=begin original
+
+After reading L<perlfunc> explaining how to use C<syscall> we can write
+this Perl function copying a file to standard output:
+
+=end original
+
+L<perlfunc> にある C<syscall> の使い方の説明を読んだ後、ファイルを
+標準出力にコピーする Perl 関数を書けます:
+
+ require 'syscall.ph';
+ sub cat($){
+ my $path = shift();
+ my $size = -s $path;
+ my $memory = "\x00" x $size; # allocate some memory
+ my $ptr = unpack( 'L', pack( 'P', $memory ) );
+ open( F, $path ) || die( "$path: cannot open ($!)\n" );
+ my $fd = fileno(F);
+ my $res = syscall( &SYS_read, fileno(F), $ptr, $size );
+ print $memory;
+ close( F );
+ }
+
+=begin original
+
+This is neither a specimen of simplicity nor a paragon of portability but
+it illustrates the point: We are able to sneak behind the scenes and
+access Perl's otherwise well-guarded memory! (Important note: Perl's
+C<syscall> does I<not> require you to construct pointers in this roundabout
+way. You simply pass a string variable, and Perl forwards the address.)
+
+=end original
+
+
+=begin original
+
+How does C<unpack> with C<P> work? Imagine some pointer in the buffer
+about to be unpacked: If it isn't the null pointer (which will smartly
+produce the C<undef> value) we have a start address - but then what?
+Perl has no way of knowing how long this "fixed length string" is, so
+it's up to you to specify the actual size as an explicit length after C<P>.
+
+=end original
+
+
+ my $mem = "abcdefghijklmn";
+ print unpack( 'P5', pack( 'P', $mem ) ); # prints "abcde"
+
+=begin original
+
+As a consequence, C<pack> ignores any number or C<*> after C<P>.
+
+=end original
+
+
+
+=begin original
+
+Now that we have seen C<P> at work, we might as well give C<p> a whirl.
+Why do we need a second template code for packing pointers at all? The
+answer lies behind the simple fact that an C<unpack> with C<p> promises
+a null-terminated string starting at the address taken from the buffer,
+and that implies a length for the data item to be returned:
+
+=end original
+
+
+ my $buf = pack( 'p', "abc\x00efhijklmn" );
+ print unpack( 'p', $buf ); # prints "abc"
+
+
+
+=begin original
+
+Albeit this is apt to be confusing: As a consequence of the length being
+implied by the string's length, a number after pack code C<p> is a repeat
+count, not a length as after C<P>.
+
+=end original
+
+
+
+=begin original
+
+Using C<pack(..., $x)> with C<P> or C<p> to get the address where C<$x> is
+actually stored must be used with circumspection. Perl's internal machinery
+considers the relation between a variable and that address as its very own
+private matter and doesn't really care that we have obtained a copy. Therefore:
+
+=end original
+
+
+=over 4
+
+=item *
+
+=begin original
+
+Do not use C<pack> with C<p> or C<P> to obtain the address of variable
+that's bound to go out of scope (and thereby freeing its memory) before you
+are done with using the memory at that address.
+
+=end original
+
+
+=item *
+
+=begin original
+
+Be very careful with Perl operations that change the value of the
+variable. Appending something to the variable, for instance, might require
+reallocation of its storage, leaving you with a pointer into no-man's land.
+
+=end original
+
+
+=item *
+
+=begin original
+
+Don't think that you can get the address of a Perl variable
+when it is stored as an integer or double number! C<pack('P', $x)> will
+force the variable's internal representation to string, just as if you
+had written something like C<$x .= ''>.
+
+=end original
+
+
+=back
+
+=begin original
+
+It's safe, however, to P- or p-pack a string literal, because Perl simply
+allocates an anonymous variable.
+
+=end original
+
+
+
+
+=head1 Pack Recipes
+
+(pack レシピ)
+
+=begin original
+
+Here are a collection of (possibly) useful canned recipes for C<pack>
+and C<unpack>:
+
+=end original
+
+
+ # Convert IP address for socket functions
+ pack( "C4", split /\./, "123.4.5.6" );
+
+ # Count the bits in a chunk of memory (e.g. a select vector)
+ unpack( '%32b*', $mask );
+
+ # Determine the endianness of your system
+ $is_little_endian = unpack( 'c', pack( 's', 1 ) );
+ $is_big_endian = unpack( 'xc', pack( 's', 1 ) );
+
+ # Determine the number of bits in a native integer
+ $bits = unpack( '%32I!', ~0 );
+
+ # Prepare argument for the nanosleep system call
+ my $timespec = pack( 'L!L!', $secs, $nanosecs );
+
+=begin original
+
+For a simple memory dump we unpack some bytes into just as
+many pairs of hex digits, and use C<map> to handle the traditional
+spacing - 16 bytes to a line:
+
+=end original
+
+
+ my $i;
+ print map( ++$i % 16 ? "$_ " : "$_\n",
+ unpack( 'H2' x length( $mem ), $mem ) ),
+ length( $mem ) % 16 ? "\n" : '';
+
+
+=head1 Funnies Section
+
+(ネタ部門)
+
+ # Pulling digits out of nowhere...
+ print unpack( 'C', pack( 'x' ) ),
+ unpack( '%B*', pack( 'A' ) ),
+ unpack( 'H', pack( 'A' ) ),
+ unpack( 'A', unpack( 'C', pack( 'A' ) ) ), "\n";
+
+ # One for the road ;-)
+ my $advice = pack( 'all u can in a van' );
+
+=head1 Authors
+
+=begin original
+
+Simon Cozens and Wolfgang Laun.
+
+=end original
+
+Simon Cozens と Wolfgang Laun。