++ed by:
MISHIN ILUX
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Author image Николай Мишин
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NAME/НАИМЕНОВАНИЕ

perlguts - Введение в Perl API

ОПИСАНИЕ

Этот документ пытается описать использование Perl API и дать некоторую информацию об основах работы ядра Perl. Он далеко не полон и, возможно, содержит много ошибок. Пожулайста, обращайтесь с любыми вопросами или коментариями к автору статьи.

Переменные

Типы данных

В Perl есть три определения типа для обработки трех основных типов данных Perl:

    SV  Скаляр (Scalar Value)
    AV  Массив (Array Value)
    HV  Хэш (Hash Value)

Для каждого типа имеются спецефические процедуры, которые манипулируют своими типами данных.

Что такое "IV"?

Perl использует определитель типа (typedef) IV для определения обычного знакового integer,что гарантирует достаточный размер для хранения указателя (и самого integer). Также есть определение UV, то есть просто беззнаковый IV.

Perl также использует два специальных определения типа I32 и I16, длина которых всегда будет как минимум 32 и 16 бит, соответственно. (Аналогично есть U32 и U16.) Обычно эти типы равны точно 32 и 16 битам, но на ОС Crays они оба будут занимать 64 бита.

Работа с SV

SV можно создать и инициализировать одной командой. ПОддерживается загрузка пяти типов значений: integer value (IV), беззнаковый unsigned integer value (UV), double (NМ), строка (PV) и другой скаляр (SV). ("PV" означает "Pointer Value". Вам может показаться, что для указателя только на строки это неверное название. Однако в PV возможно хранить что то другое, например, указатель на массив UV. Но при этом надо проявлять осторожность, так как большая часть внутренних механизмов ожидает в PV только строку. Например, часто происходит автоматическое добавление NUL. Использование PV с не-строками описано здесь только в этом параграфе.)

Семь процедур это:

    SV*  newSViv(IV);
    SV*  newSVuv(UV);
    SV*  newSVnv(double);
    SV*  newSVpv(const char*, STRLEN);
    SV*  newSVpvn(const char*, STRLEN);
    SV*  newSVpvf(const char*, ...);
    SV*  newSVsv(SV*);

STRLEN имеет тип integer (Size_t, обычно определяемый в config.h как size_t), достаточно большой для хранения любой строки, которую может обработать Perl.

В редких случаях, когда требуется более сложная инициализация, можно использовать newSV(len) для создания пустого SV. При len, равном нулю, возвращается пустой SV типа NULL, иначе возвращается SV типа PV с выделенными len + 1 (для С<NUL>) байтами памяти, доступными через SvPVX. В обоих случаях SV имеет undef значение.

    SV *sv = newSV(0);   /* память не выделяется */
    SV *sv = newSV(10);  /* выделяется 10 (+1) байт неинициализированной
                          * памяти allocated */

Для изменения значения уже существующего SV существует восемь функций:

    void  sv_setiv(SV*, IV);
    void  sv_setuv(SV*, UV);
    void  sv_setnv(SV*, double);
    void  sv_setpv(SV*, const char*);
    void  sv_setpvn(SV*, const char*, STRLEN)
    void  sv_setpvf(SV*, const char*, ...);
    void  sv_vsetpvfn(SV*, const char*, STRLEN, va_list *,
                                                    SV **, I32, bool *);
    void  sv_setsv(SV*, SV*);

обратите внимание, что можно либо передавать длину строки в sv_setpvn, newSVpvn и newSVpv, либо дать Perl вычислить её в sv_setpv или передавая 0 вторым агрументом newSVpv. Однако будьте внимательны, так как для вычисления длины используется strlen, которая зависит от наличия завершающего NUL и отсутствия других NUL внутри строки.

Аргументы sv_setpvf обрабатываются подобно аргументам sprintf, значением будет отформатированный результат.

sv_vsetpvfn является аналогом vsprintf, но позволяет определять либо указатель на список различных аргументов, либо адрес и длину массива SV. Последний аргумент, указатель на boolean, при возврате содержит true, если при форматировании использовалась локале-специфичная информация и поэтому содержимое строки ненадежно (см. perlsec). Если эта информация не важна, передавайте NULL. Также обратите внимание, что требуется определять длину строки формата.

Функции sv_set*() не являются достаточно обобщенными для оперирования значниями с "магией". Смотрите "Magic Virtual Tables" далее.

Во всех SV, хранящих строки, строки должны завершаться символом NUL, иначе есть риск падения в дамп или порчи программы из кода, который передает их в функции C или системные вызовы, ожидающие завершающиеся NUL строки. По этой причине собственные функции Perl обычно добавляют завершающий c<NUL>. Тем не менее, нвдо быть очень осторожным, передавая хранящиеся в SV строки в C-функии ил системные вызовы.

Для доступа к действиельному содержимому SV могут использоваться макросы:

    SvIV(SV*)
    SvUV(SV*)
    SvNV(SV*)
    SvPV(SV*, STRLEN len)
    SvPV_nolen(SV*)

которые автоматически преобразуют действительный типа скаляра в IV, UV, double или строку.

SvPV помещает длину возвращаемой строки в переменную len (это макрос, поэтому вам не нужно писать &len). Если длина вам не нужна, используйте SvPV_nolen. По историческим причинам в этом случае используется макрос SvPV с глобальной переменной PL_na. Но это может быть довольно неэффективно, поскольку PL_na должнв быть доступна в локальной памяти потока в потоковом Perl. В любом случае помните, что в Perl позволяет хранить в строках произвольные данные, в которых могут быть NUL внутри и не быть завершающего NUL.

Также помните, что в C нельзя безопасно написать foo(SvPV(s, len), len);. Это может проглотить ваш компилятор, но не проглотить другие. Разбивайте такую запись на отдельные иструкции:

    SV *s;
    STRLEN len;
    char *ptr;
    ptr = SvPV(s, len);
    foo(ptr, len);

Если вы хотите узнать, является ли скалярне значение TRUE, используйте:

    SvTRUE(SV*)
    

Хотя Perl автоматически наращивает строки, если вам понадобилось принудительно выделить большее количество памяти для SV, можно воспользоваться макросом

    SvGROW(SV*, STRLEN newlen)

который определит, нужно ли дополнительное выделение. Если да, будет вызвана функция sv_grow. Обратите внимание, что можно только увеличивать, не уменьшать, размер памяти для SV в SvGROW, и что при выделении памяти место для завершающего NUL не добавляется автоматически (строковые функции самого Perl обычно используют SvGROW(sv, len + 1)).

Если вы хотитите записать в буфер существующего SV и установить это значение, как строку, используйте SvPV_force() или однин из ее вариантом, чтобы принудить SV быть PV. При этом из SV будут удалены любые не приdовдимые к строке типы но сохранится содержимое. Этот функционал можно использовать ,например, при добавлении данных в буфер из функций API без дополнительного копирования:

    (void)SvPVbyte_force(sv, len);
    s = SvGROW(sv, len + needlen + 1);
    /* 
       Нечто, что изменяет до needlen байтов с s+len, но байты newlen
       something that modifies up to needlen bytes at s+len, but
       modifies newlen bytes
         eg. newlen = read(fd, s + len, needlen);
       в этом примере игнорируются ошибки
       ignoring errors for these examples
     */
    s[len + newlen] = '\0';
    SvCUR_set(sv, len + newlen);
    SvUTF8_off(sv);
    SvSETMAGIC(sv);

Если уже есть данные в памяти или вы хотите, что бы код был проще, используйте один из вариантов sv_catpvn(). Чтобы вставить данные внутрь строки, используйте sv_insert() или sv_insert_flags().

Если не нужно сохранять текущее содержимое SV, избежать копирования можно:

    sv_setpvn(sv, "", 0);
    s = SvGROW(sv, needlen + 1);
    /* something that modifies up to needlen bytes at s, but modifies
       newlen bytes
         eg. newlen = read(fd, s. needlen);
     */
    s[newlen] = '\0';
    SvCUR_set(sv, newlen);
    SvPOK_only(sv); /* also clears SVf_UTF8 */
    SvSETMAGIC(sv);

Повторим, что если у вас уже есть данные в памяти или вам не нравится сложность кода в пример выше, используйте sv_setpvn().

Если есть буфер, выделенный с помощью Newx() и вы хотите установить его, как значение SV, используйте sv_usepvn_flags(). Чтобы избежать реаллокации буфера при добавлении завершающего NUL, длжны быть соблюдены некоторые требования:

   Newx(buf, somesize+1, char);
   /* ... заполняем буфер ... */
   buf[somesize] = '\0';
   sv_usepvn_flags(sv, buf, somesize, SV_SMAGIC | SV_HAS_TRAILING_NUL);
   /* теперь буфер принадлежит perl, не освобождайте его */

Макросы для определения того, что Perl думает о типе данных в SV:

    SvIOK(SV*)
    SvNOK(SV*)
    SvPOK(SV*)

Макросы для получение и установка текущей длины сроки в SV:

    SvCUR(SV*)
    SvCUR_set(SV*, I32 val)
Также можно получить указатель на конец строки:

    SvEND(SV*)

Но имейте ввиду, что последнии три макроса правомерны только если SvPOK возвращает истину.

Макросы для добавления чего-либо в конец сроки в SV:

    void  sv_catpv(SV*, const char*);
    void  sv_catpvn(SV*, const char*, STRLEN);
    void  sv_catpvf(SV*, const char*, ...);
    void  sv_vcatpvfn(SV*, const char*, STRLEN, va_list *, SV **,
                                                             I32, bool);
    void  sv_catsv(SV*, SV*);

Первая функция вычисляет длину добавляемой строки с помощью strlen. Во вторую вы передаете длину сами. Третья обрабатывает аргументы подобно sprintf и добавляет отформатированный ресультат. Четвертая работает подобно vsprintf. Вместо аргумента va_list она может получать массив SV и его длину. Пятая функция расширяет строку в первом SV строкой из второго SV. Также она заставляет второй SV интерпретироваться как строка.

Функции семейства sv_cat*() недостаточно общие, чтобы работать с значениями с "магией". Смотрите секцию "Magic Virtual Table" далее.

Если известно имя переменной, можно получить указатель на SV:

    SV*  get_sv("package::varname", 0);

Если переменная не существует, возвращается NULL.

Следующий вызов позволяет узнать, действительно ли определена переменная:

    SvOK(SV*)

Скалярное значение undef хранится в экземпляре SV с именем PL_sv_undef.

Её адрес может использоваться там, где необходим SV. Убедитесь, что вы не пытаетесь сравнить случайное sv c PL_sv_undef. Например, при взаимодействии с кодом Perl это работает корректно:

    foo(undef)

а это нет:

    $x = undef;
    foo($x);

Так что повторим, что для проверки определенности всегда используйте SvOK().

Также будьте внимательны, используя &PL_sv_undef в качестве значений AV или HV (см. "AVs, HVs и неопределенные значения").

Есть еще два значения PL_sv_yes и PL_sv_no, которые содержат логические TRUE и FALSE, соответственно. Также как и в случае PL_sv_undef должны использоваться их адреса, когда требуется.

Вы ошибетесь, если решите что (SV *) 0 то же самое, что и &PL_sv_undef. Возьмем код:

    SV* sv = (SV*) 0;
    if (I-am-to-return-a-real-value) {
            sv = sv_2mortal(newSViv(42));
    }
    sv_setsv(ST(0), sv);

Этот код пытается вернуть новый SV (содержащий 42), если нужно реальное значение, или undef в ином случае. Вместо этого он вернет NULL, что где-нибудь ниже приведет к нарушению сегментации, ошибке шины или просто странным Замените ноль в первой строке на &PL_sv_undef и все станет нормально.

Для освобождения ранее созданного SV служит SvREFCNT_dec(SV*). Обычно этот вызов не требуется (смотрите "ПОдсчет Ссылок и Смертность").

Смещения

Для эффективного удаления символов из начала строки используется sv_chop; она принимает SV и указатель на какое-либо место внутри PV, и отбрасывает все до этого указателя. Эффективность достигется небольшим хаком: вместо реального удаления символов sv_chop устанавливает флаг OOK (offset OK). чтобы другие функции могли понять, что используется смещение. Указатель PV (SvPVX)перемещается вперед на число отброшенных символов, и соответствующим образом устанавливаются SvCUR и SvLEN. (Часть места между старым и новым PV используется для хранения числа отрезанных байтов)

С этого момента начало лежащего в памяти буфера находится по адресу SvPVX(sv) - SvIV(sv) и PV указывает куда то в его середину.

Лучше продемонстрировать сказанное примером. Обычно механизм copy-on-write препятствует использованию этого хака из оператора замены, но если вы сможете смастерить троку, на которой copy-on-write невозможен, то получится увидеть действии хака. В текущей реализации финальный байт строкового буфера используется в качестве счетчика ссылок copy-on-write. Если буфер недостаточно большой, copy-on-write пропускается. Возьмем для начала пустую строку:

  % ./perl -Ilib -MDevel::Peek -le '$a=""; $a .= ""; Dump $a'
  SV = PV(0x7ffb7c008a70) at 0x7ffb7c030390
    REFCNT = 1
    FLAGS = (POK,pPOK)
    PV = 0x7ffb7bc05b50 ""\0
    CUR = 0
    LEN = 10
Обратите внимание, что LEN равно 10. (На вышей платформе может быть другое значение.) Увеличим
строку до длины, на единицу меньше 10, и сделаем замену:

  % ./perl -Ilib -MDevel::Peek -le '$a=""; $a.="123456789"; $a=~s/.//; Dump($a)'
  SV = PV(0x7ffa04008a70) at 0x7ffa04030390
    REFCNT = 1
    FLAGS = (POK,OOK,pPOK)
    OFFSET = 1
    PV = 0x7ffa03c05b61 ( "\1" . ) "23456789"\0
    CUR = 8
    LEN = 9

  % ./perl -Ilib -MDevel::Peek -le '$a=""; $a.="123456789"; $a=~s/.//; Dump($a)'
  SV = PV(0x7ffa04008a70) at 0x7ffa04030390
    REFCNT = 1
    FLAGS = (POK,OOK,pPOK)
    OFFSET = 1
    PV = 0x7ffa03c05b61 ( "\1" . ) "23456789"\0
    CUR = 8
    LEN = 9

В OFFSET показано число отрезанных байтов. Часть строки между "реальным" и "фальшимым" началом показана в скобках, и значения SvCUR и SvLEN отражают фальшивое начало строки, а не реальное. (Первый символ строкового буфера изменяется на "\1", поскольку текущая реализация сохраняет там счетчик смещения. Замена такого поведения является темой дискурсий.)

Нечто подобное хаку со смещением применяется и c типом AV для реализации эффективного сдвига и замены в начале массива; в то время, как AvARRAY указывает на первый элемент массива, видимого в Perl, AvALLOC указывает на реальное начало C-массива. Обычно они равны, но операция shift может увеличить на единицу AvARRAY и уменьшить на единицу AvFILL и AvMAX. Опять таки, местонахождение реального начала массива C вступает в игру только при освобождении массива. Смотрите av_shift в ac.c.

Что на Самом Деле Хранится в SV?

Напомним, что для определения типа скаляра обычно используются макросы Sv*OK. Поскольку скаляр может быть и числом и строкой, эти маркросы почти вегда возвращают TRUE, а при вызове SV*V произойдет соответствующее преобразование строки в integer/double и наоборот.

Если вам на самом деле необходимо знать, находится ли в вашем SV interer, double или указатель на сроку, вместо Sv*OK используйте следующие три макроса:

    SvIOKp(SV*)
    SvNOKp(SV*)
    SvPOKp(SV*)

Они сообщат вам, что в действительности хранится в SV. "p" здесь означает private.

Есть несколько мест, где приватные и публичные флаги могут отличаться. Например, в perl 5.16 и более ранних версиях в слоте IV связанного SV могло находиться допустимое значение (SvIOKp - истина), но для доступу к данным необходимо использовать процедуру FETCH, а не получать их напрямую, поэтому SvIOK - ложь. (Начиная с perl 5.18, связанные скаляры используют флаги так же, как ине связанные.) Другой пример, это числовое преобразование с потерей точности: для 'lossy' значений устанавливается только приватный флаг. Поэтому, при преобразовании NV в IV с потерей точности, будут установлены флаги SvIOKp, SvNOKp и SvNOK , но не SvIOK.

В общем случае, однако, лучше использовать макросы Sv*V.

Работа с AV

Создать и загрузить AV можно двумя способами. Первый метода создает пустое AV:

    AV* newAV();

Второй метод создает AV и инициализирует его значениями SV:

    AV* av_make(SSize_t num, SV **prt)

Второй аргумент указывает на массив из num SV*. Как только массив создан, эти SV можно уничтожить, если хочется.

Следующие опреации доступны после создания AV:

    void  av_push(AV*, SV*);
    SV*   av_pop(AV*);
    SV*   av_shift(AV*);
    void  av_unshift(AV*, SSize_t num);

Все эти опреации, вероятно, очевидны, за исключением av_unshift. Эта процедура добавляет num элементов со значением undef в начало массива. Затем вы должны присвоить значения этим элементам с помощью описанной ниже av_store.

Еще несколько функций:

    SSize_t av_top_index(AV*);
    SV**    av_fetch(AV*, SSize_t key, I32 lval);
    SV**    av_store(AV*, SSize_t key, SV* val);

av_top_index возвращает наибольшее значение индекса массива (то же самое делает $#array в Perl), или -1, если массив пуст.

av_fetch вернет значение по индексу key, но также, если параметр lval не равен нулю, сохранит неопределенное значение по этому же индексу. av_store сохраняет val по индексу key, но не увеличивает счетчик ссылок val. Об этом должнв позаботиться вызывающая сторона, и если av_store вернул NULL, вызывающий дожне уменьшить счетчик ссылок для избежания утечки памяти. Заметьте, что av_fetch и av_store возвращают SV**, а не SV*.

Еще функции:

    void  av_clear(AV*);
    void  av_undef(AV*);
    void  av_extend(AV*, SSize_t key);

av_clear удаляет все елементы из массива AV*, но не удаляет сам массив. av_undef удалит все жлементы плюс сам массив. av_extend расшириряет массив, так что он будет содержать как минимум key+1 элементов. Если key+1 меньше текущей длины массива, av_extend ничего не делает.

Если вам известно имя переменной, можно получить указатель на AV:

    AV*  get_av("package::varname", 0);

Если переменная не существует, возвращается NULL.

В секция "Понимание Магии Связанных Хэшей и Массивов" содержится больше информации об использовании функций доступа со связанными массивами.

Работа с HV

Для создания HV используется следующаяч процедура:

    HV* newHV();

После создания на HV возможны следующие операции:

    SV**  hv_store(HV*, const char* key, U32 klen, SV* val, U32 hash);
    SV**  hv_fetch(HV*, const char* key, U32 klen, I32 lval);

Параметр klen содержит длину передаваемого ключа (заметьте, что в klen нельзя передавать 0, предлагая Perl самому вычислить длину ключа). Аргумент val содержит указатель типа SV на сохраняемый скаляр, и в hash - предварительно вычесленный хэш ключа, или 0, чтобы hv_store вычислила его сама. Параметр lval используется для указания, что операция получения в действительности является частью операции сохранения, в этом случае в HV будет добавлено новое неопределенное значение по заданному ключу, и hv_fetch вернет его, как если оно уже существовало.

Помните, что hv_store и hv_fetch возвращают SV**, а не просто SV*. Для доступа к скалярному значению необходимо разименововать вернувшееся значение. Однако, сначала нужно убедиться, что оно не NULL.

Первая из следующих двух функций проверяет существование записи в хэш-таблице, вторая удаляет запись:

    bool  hv_exists(HV*, const char* key, U32 klen);
    SV*   hv_delete(HV*, const char* key, U32 klen, I32 flags);

Если flags не включает флаг G_DISCARD, hv_delete создаст и вернет смертную копию удаляемого значения.

И еще функции:

    void   hv_clear(HV*);
    void   hv_undef(HV*);

Как и парные AV функции, hv_clear удаляет все записи хэш-таблицы, но не удаляет ее саму. hv_undef удаляет и записи и саму таблицу.

Perl хранит действительные данные в связанном списке структур типа typedef HE. Структура содержит указатели на ключ и значение (плюс дополнительный административный довесок). Ключ - указатель на строку; значением является SV*. Однако, как только у вас появляется HE*, для получения ключа и значения используйте определенные ниже процедуры:

    I32    hv_iterinit(HV*);
            /* Подготавливает точку старта для прохода по хэш-таблице */
    HE*    hv_iternext(HV*);
            /* Получает следующую запись и возвращает указатель на 
               структуру, хранящую ключ и значение */
    char*  hv_iterkey(HE* entry, I32* retlen);
            /* Получает ключ из структуры HE, а также возвращает длину строки с ключом */
    SV*    hv_iterval(HV*, HE* entry);
            /* Возвращает указатель на SV значения из структуры HE */
    SV*    hv_iternextsv(HV*, char** key, I32* retlen);
            /* Удобная процедура, комбинирующая hv_iternext, hv_iterkey и hv_iterval.
               Ключ и его длина возвращаются в аргументах key и retlen, значение 
               возвращается через SV* 
                */

Если известно имя переменной, можно получить указатель на HV:

    SV*  get_hv("package::varname", 0);

Если переменная не существует, возвращается NULL.

Алгоритм хеширования определен через макрос PERL_HASH:

    PERL_HASH(hash, key, klen)

Конкретная реализация этого макроса зависит от архитектуры системы и версии Perl, а возвращаемое значение может меняться при запуске интерпретатора, так что оно действительно только на время жизни одного процесса Perl.

В секция "Понимание Магии Связанных Хэшей и Массивов" содержится больше информации об использовании функций доступа со связанными массивами.

Расширенное API Хэшей

Начиная с версии 5.004 доступны следующие функции:

    HE*     hv_fetch_ent  (HV* tb, SV* key, I32 lval, U32 hash);
    HE*     hv_store_ent  (HV* tb, SV* key, SV* val, U32 hash);

    bool    hv_exists_ent (HV* tb, SV* key, U32 hash);
    SV*     hv_delete_ent (HV* tb, SV* key, I32 flags, U32 hash);

    SV*     hv_iterkeysv  (HE* entry);

Обратите внимание, что эти функции принимают ключ в виде SV*, что упрощает написание расширений, работающих с структурой хэша. Также они позволяют передавать SV* ключей в функции tie, не заставляя вас стрингифицировать ключи, в отличии от предыдущего набора функций.

Они также возвращают и принимают запись хэша целиком (HE*), что делает их более эффективными (поскольку не нужно каждый раз вычислять хэш для отдельных строк). Смотрите perlapi для детального описания.

Для доступа к содержимому записей хэша всегда должны использоваться описанные ниже макросы. Обратите внимание на то, что в аргументы этих макросов простые переменные, так как они могут определяться более одного раза. Смотрите perlapi для детального описания этих макросов.

    HePV(HE* he, STRLEN len)
    HeVAL(HE* he)
    HeHASH(HE* he)
    HeSVKEY(HE* he)
    HeSVKEY_force(HE* he)
    HeSVKEY_set(HE* he, SV* sv)

Далее определяются два низкоуровневых макроса, но они должны использовать только при работе с ключами, не являющимися SV*:

    HeKEY(HE* he)
    HeKLEN(HE* he)

Заметьте, что за увеличение счетчика ссылок val отвечает вызывающая строна,Ю так как hv_store и hv_store_ent не инкрементируют его. Если эти фукции возвращают NULL, вызывающему обычно нужно декрементировать счетчик ссылок val, дабы избежать утечки памяти.

AV, HV и Неопределенные Значения

Иногда нужно сохранить неопределенное значение в AV или HV. Хотя возможно это нужно в редких случаях, тут есть хитрости, поскольку вам нужно используете &PL_sv_undef для задания неопределенного SV.

Например, интуицитивно кажется, что следующий XS код:

    AV *av = newAV();
    av_store( av, 0, &PL_sv_undef );

эквивалентен Perl коду:

    my @av;
    $av[0] = undef;

Это не так, к сожалению. В perl 5.18 и ниже для AV &PL_sv_undef использовался как маркер, указывающий, что элемент массива еще не инициализирован. Поэтому exists $av[0] будет истинно для Perl кода, но ложно для массива в XS коде. В perl 5.20 сохранение &PL_sv_undef создаст read-only элемент, поскольку сохраняется сам скаляр &PL_sv_undef, а не его копия.

Те же самые проблемы могут возникнуть при сохранениии &PL_sv_undef в HV:

    hv_store( hv, "key", 3, &PL_sv_undef, 0 );

Этого достаточно, что бы получить undef значение, но при попытке модифицировать значение key вы получите ошибку:

    Modification of non-creatable hash value attempted

Мы сделаем покороче эту длинную историю, и сообщаем вам, что вы можете использовать специальные переменные &PL_sv_undef, &PL_sv_yes и &PL_sv_no с AV и HV, но вы должны быть уверены, что знаете, что делаете.

Итак, если вы хотите хранить неопределенное значение в AV или HV, вы не должны использовать &PL_sv_undef, вместо этого созайте новое неопределенное значение функцией newSV, например:

    av_store( av, 42, newSV(0) );
    hv_store( hv, "foo", 3, newSV(0), 0 );

Ссылки

Ссылки - это специальный тип скаляра, указывающего на другие типы данных (включая другие ссылки).

Для создание ссылки используются следующие функции:

    SV* newRV_inc((SV*) thing);
    SV* newRV_noinc((SV*) thing);

Аргумент thing может быть любым SV*, AV* или HV*. Функции идентичны, за исключением того, что newRV_inc увеличивает счетчик ссылок аргумента thing, а newRV_noinc нет. По историческим причинам newRV является синонимом newRV_inc.

Когда у вас есть ссылка, вы можете разименовать её, используя следующий макрос:

    SvRV(SV*)

и затем, если требуется, вызвать соответствующую процедуру для преобразования возвращаемого SV* в AV* или HV*.

Макрос для определения, является ли SV ссылкой:

    SvRV(SV*)

Для определения типа, на который ссылается ссылка, исследуйте возвращаемое значение следующего макроса:

    SvTYPE(SvRV(SV*))

Большинство полезных типов вернется как:

    < SVt_PVAV  Scalar
    SVt_PVAV    Array
    SVt_PVHV    Hash
    SVt_PVCV    Code
    SVt_PVGV    Glob (возможно файловый указатель)

Смотрите подробности в "svtype" in perlapi.

Освященные Ссылки и Объекты Класса

Ссылки используются также для поддержки объектно-ориентированного программирования. На лексиконе perl OO объект является просто ссылкой, которая была освящена (blessed) в пакет (или класс). После освящения программист может использовать ссылку для доступа к различным методам в классе.

Освятить ссылку в пакет можно следующим образом:

    SV* sv_bless(SV* sv, HV* stash);

Аргумент sv должен быть ссылкой. Аргумент stash определяет, какому классу будет принадлежать ссылка. Информация о преобразовании имен классов преобразуются в стэши находится в секции "Стэши и Глобы".

/* Все еще в процессе создания */

Обновление rv до ссылки, если нужно. Создается новый SV, ссылающийся на rv. Если classname не-null, SV освещается в заданный класс. Возвращается SV.

    SV* newRVrv(SV* rv, const char* classname);

Следующие три функции копируют integer, unsigned integer или double в SV, ссылка на который находится в rv. SV освящается, если classname не-null.

        SV* sv_setref_iv(SV* rv, const char* classname, IV iv);
        SV* sv_setref_uv(SV* rv, const char* classname, UV uv);
        SV* sv_setref_nv(SV* rv, const char* classname, NV iv);

Копирование указателя (адреса, не строки!) в SV, на который ссылается rv. SV освящается, если classname не-null.

        SV* sv_setref_pv(SV* rv, const char* classname, void* pv);

Копирование строки в SV, на который ссылается rv. Если length установлен в 0, Perl Perl сам вычисли длину строки. SV освящается, если classname не-null.

    SV* sv_setref_pvn(SV* rv, const char* classname, char* pv,
                                                         STRLEN length);

Проверяет, является ли SV освященной в данный класс. Иерархия наследования не проверяется.

        int  sv_isa(SV* sv, const char* name);

Проверить, является ли SV ссылкой на освященный объект.

        int  sv_isobject(SV* sv);

Узнать, наследуется ли SV от определенного класса. SV может быть либо ссылкой на освященный объект, либо строкой, содержащей имя класса. Эта функция реализуется функциональность UNIVERSAL::isa.

    bool sv_derived_from(SV* sv, const char* name);

Проверку того, что объект является наследником определенного класса, можно написать так:

    if (sv_isobject(sv) && sv_derived_from(sv, class)) { ... }

Создание Новых Переменных

С помощью сдедующих трех функций можно создать новую переменную с неопределенным значением, которая будет доступна в Perl коде:

    SV*  get_sv("package::varname", GV_ADD);
    AV*  get_av("package::varname", GV_ADD);
    HV*  get_hv("package::varname", GV_ADD);

Обратите внимание, что во втором параметре GV_ADD. Установить значение новой переменной можно с помощью соответствующие типу данных процедур.

Существуют дополнительные макросы для включения специальных возможностей, объединяемые побитовым OR с GV_ADD.

GV_ADDMULTI

Маркировать переменную, как определямую неоднократно, что подавит вывод такого предупредения:

  Name <varname> used only once: possible typo
GV_ADDWARN

Вывод предупреждения:

  Had to create <varname> unexpectedly

если переменная не существует до вызова функции.

Подсчет Ссылок и Смертность

Механизм сборки мусора в Perl управляется через подсчет ссылок. SV, AV или HV (далее xV для краткости) начинают жизнь со счетчиком ссылок, равным единиеце. Если счетчик ссылок какого-либо xV становится равен нулю, xV уничтожается и занимаемая им память становится доступна для повторного использования.

Обычно в perl-коде этого не бывает, пока переменная не является неопределенной, или последняя переменная, удерживающая ссылку на что-либо, не изменяется или не перезаписывается. На внутреннем уровне подсчетом ссылок можно управлять через макросы:

    int SvREFCNT(SV* sv);
    SV* SvREFCNT_inc(SV* sv);
    void SvREFCNT_dec(SV* sv);

Есть еще одна функция, манипулирующая счетчиком ссылок своего аргумента. newRV_inc, если помните, создает ссылку на свой аргумент и, в качестве побочного эффекта, увеличивает его счетчик ссылок. Если вы не хотите увеличивать счетчик ссылок, вместо нее используйте newRV_noinc.

Например, представьте себе, что вы хотите вернуть ссылку из XSUB функции. Внутри XSUB вы создаете SV, ее счетчик ссылок изначально равен единице. Затем вы вызываете newRV_inc и передаете ей только что созданный SV. newRV_inc возвращает ссылку в новом SV, но счетчик ссылок вашего SV увеличился и стал равен двум. Далее вы возвращаете ссылку из XSUB и забываете об вашем SV. То Perl ничего не забывает! Всякий раз, когда возвращаемая ссылка уничтожается, счетчик ссылок оригинального SV уменьшается на единицу, и больше ничего не происходит. SV будет торчать в памяти, не имея никакого способа доступа, пока сам Perl не завершится. Это утечка памяти.

В данном случае правильным выбором будет использовать newRV_noinc вместо newRV_inc. Тогда после уничтожения последней ссылки счетчик ссылок SV достигет нуля и SV уничтожится, и утечки памяти не будет.

Есть несколько удобных функций, помогающих уничтожать SV. Эти функции вводят концепцию "смертости" (mortal SV). Счетчик ссылок смертного SV помечен, как подлежащий уменьшению, но реальное его уменьшение произойдет через "короткий отрезок времени". Термин "короткий отрезок времени" обычно означает выполнение одной инструкции Perl, такой, как вызов XSUB. Фактичесий момент уменьшения счетчика ссылок SV зависит от двух макросов, SAVETMPS и FREETMPS. Смотрите perlcall и perlxs.

В самом простом случае "мортализация", это отложенный вызов SvREFCNT_dec. Если, однако, вы "мортализируете" переменную дважды, в последствии счетчик ссылок также уменьшится дважды.

"Мортализация" в основном используется для SV, хранящихся в стеке Perl. Например, SV, созданный только лишь для передачи числа в вызываемую функцию, делается смертным для автоматической зачистки его после выталкивания из стека. Также, часто делается смертным результат (хранящийся на стеке) вызова XSUB.

Создание смертной переменной:

    SV*  sv_newmortal()
    SV*  sv_2mortal(SV*)
    SV*  sv_mortalcopy(SV*)

Первый вызов создает пустое смертное SV, второй конвертирует обычное SV в смертное (и таким образом, создает отложенный вызов SvREFCNT_dec), третий создает смертную копию SV. Так как sv_newmortal дает нам новое SV без значения, значение нужно установить обычным образом через sv_setpv, sv_setiv и т.д.:

    SV *tmp = sv_newmortal();
    sv_setiv(tmp, an_integer);

Зачастую вместо двух C инстукций встречается следующая идиома:

    SV *tmp = sv_2mortal(newSViv(an_integer));

Нужно быть внимательным при создании смертных переменных. Странные вещи могут происходить, если одно и тоже значение сделать смертым в нескольких контекстах, или или несколько раз. Думайте о "мортализации", как об откладывании вызова SvREFCNT_dec, это уменьшит количество проблем. Например, если вы знаете, что счетчик ссылок переменной достаточно велик и она переживет передачу через стек, не нужно использовать мортализацию. Если уверенности нет, вызовите sv_REFCNT_dec и sv_2mortal, или sv_mortalcopy для надежности.

Смертные функции используются не только с SV: в sv_2mortal и sv_mortalcopy можно передавать адреса AV и HV (приведя их к типу SV*).

Стеши и Глобы

Стеш - это хеш, содержащий все определенные в пакете переменные. Каждый ключ в хеше является именем символа (используется для всех объектов различных типов с тем же именем), значения в хеш-таблице представляют собой GV (Glob Value). GV, в свою очередь, содержит сылки на объекты различных типов, включая следующие (есть и другие типы):

    Scalar Value
    Array Value
    Hash Value
    I/O Handle
    Format
    Subroutine

Сущности пакета main находятся в глобальном хеше PL_defstash. Для доступа к элементам других пакетов используется имя пакета с добавлением символов '::'. Для элементов пакета Foo это будет Foo:: в PL_defstash. Элементы Bar::Baz находятся в стеше Baz стеша Bar.

Получить указатель на стеш конкретного пакета можно при помощи следующих функций:

    HV*  gv_stashpv(const char* name, I32 flags)
    HV*  gv_stashsv(SV*, I32 flags)

Первая принимает литеральную строку, вторая использует строку из SV. Помните, стеш является обычным хешом, поэтому на выходе возвращается HV*.

The first function takes a literal string, the second uses the string stored in the SV. Remember that a stash is just a hash table, so you get back an HV*. The flags flag will create a new package if it is set to GV_ADD.

The name that gv_stash*v wants is the name of the package whose symbol table you want. The default package is called main. If you have multiply nested packages, pass their names to gv_stash*v, separated by :: as in the Perl language itself.

Alternately, if you have an SV that is a blessed reference, you can find out the stash pointer by using:

    HV*  SvSTASH(SvRV(SV*));

then use the following to get the package name itself:

    char*  HvNAME(HV* stash);

If you need to bless or re-bless an object you can use the following function:

    SV*  sv_bless(SV*, HV* stash)

where the first argument, an SV*, must be a reference, and the second argument is a stash. The returned SV* can now be used in the same way as any other SV.

For more information on references and blessings, consult perlref.

Double-Typed SVs

Scalar variables normally contain only one type of value, an integer, double, pointer, or reference. Perl will automatically convert the actual scalar data from the stored type into the requested type.

Some scalar variables contain more than one type of scalar data. For example, the variable $! contains either the numeric value of errno or its string equivalent from either strerror or sys_errlist[].

To force multiple data values into an SV, you must do two things: use the sv_set*v routines to add the additional scalar type, then set a flag so that Perl will believe it contains more than one type of data. The four macros to set the flags are:

        SvIOK_on
        SvNOK_on
        SvPOK_on
        SvROK_on

The particular macro you must use depends on which sv_set*v routine you called first. This is because every sv_set*v routine turns on only the bit for the particular type of data being set, and turns off all the rest.

For example, to create a new Perl variable called "dberror" that contains both the numeric and descriptive string error values, you could use the following code:

    extern int  dberror;
    extern char *dberror_list;

    SV* sv = get_sv("dberror", GV_ADD);
    sv_setiv(sv, (IV) dberror);
    sv_setpv(sv, dberror_list[dberror]);
    SvIOK_on(sv);

If the order of sv_setiv and sv_setpv had been reversed, then the macro SvPOK_on would need to be called instead of SvIOK_on.

Magic Variables

[This section still under construction. Ignore everything here. Post no bills. Everything not permitted is forbidden.]

Any SV may be magical, that is, it has special features that a normal SV does not have. These features are stored in the SV structure in a linked list of struct magic's, typedef'ed to MAGIC.

    struct magic {
        MAGIC*      mg_moremagic;
        MGVTBL*     mg_virtual;
        U16         mg_private;
        char        mg_type;
        U8          mg_flags;
        I32         mg_len;
        SV*         mg_obj;
        char*       mg_ptr;
    };

Note this is current as of patchlevel 0, and could change at any time.

Assigning Magic

Perl adds magic to an SV using the sv_magic function:

  void sv_magic(SV* sv, SV* obj, int how, const char* name, I32 namlen);

The sv argument is a pointer to the SV that is to acquire a new magical feature.

If sv is not already magical, Perl uses the SvUPGRADE macro to convert sv to type SVt_PVMG. Perl then continues by adding new magic to the beginning of the linked list of magical features. Any prior entry of the same type of magic is deleted. Note that this can be overridden, and multiple instances of the same type of magic can be associated with an SV.

The name and namlen arguments are used to associate a string with the magic, typically the name of a variable. namlen is stored in the mg_len field and if name is non-null then either a savepvn copy of name or name itself is stored in the mg_ptr field, depending on whether namlen is greater than zero or equal to zero respectively. As a special case, if (name && namlen == HEf_SVKEY) then name is assumed to contain an SV* and is stored as-is with its REFCNT incremented.

The sv_magic function uses how to determine which, if any, predefined "Magic Virtual Table" should be assigned to the mg_virtual field. See the "Magic Virtual Tables" section below. The how argument is also stored in the mg_type field. The value of how should be chosen from the set of macros PERL_MAGIC_foo found in perl.h. Note that before these macros were added, Perl internals used to directly use character literals, so you may occasionally come across old code or documentation referring to 'U' magic rather than PERL_MAGIC_uvar for example.

The obj argument is stored in the mg_obj field of the MAGIC structure. If it is not the same as the sv argument, the reference count of the obj object is incremented. If it is the same, or if the how argument is PERL_MAGIC_arylen, or if it is a NULL pointer, then obj is merely stored, without the reference count being incremented.

See also sv_magicext in perlapi for a more flexible way to add magic to an SV.

There is also a function to add magic to an HV:

    void hv_magic(HV *hv, GV *gv, int how);

This simply calls sv_magic and coerces the gv argument into an SV.

To remove the magic from an SV, call the function sv_unmagic:

    int sv_unmagic(SV *sv, int type);

The type argument should be equal to the how value when the SV was initially made magical.

However, note that sv_unmagic removes all magic of a certain type from the SV. If you want to remove only certain magic of a type based on the magic virtual table, use sv_unmagicext instead:

    int sv_unmagicext(SV *sv, int type, MGVTBL *vtbl);

Magic Virtual Tables

The mg_virtual field in the MAGIC structure is a pointer to an MGVTBL, which is a structure of function pointers and stands for "Magic Virtual Table" to handle the various operations that might be applied to that variable.

The MGVTBL has five (or sometimes eight) pointers to the following routine types:

    int  (*svt_get)(SV* sv, MAGIC* mg);
    int  (*svt_set)(SV* sv, MAGIC* mg);
    U32  (*svt_len)(SV* sv, MAGIC* mg);
    int  (*svt_clear)(SV* sv, MAGIC* mg);
    int  (*svt_free)(SV* sv, MAGIC* mg);

    int  (*svt_copy)(SV *sv, MAGIC* mg, SV *nsv,
                                          const char *name, I32 namlen);
    int  (*svt_dup)(MAGIC *mg, CLONE_PARAMS *param);
    int  (*svt_local)(SV *nsv, MAGIC *mg);

This MGVTBL structure is set at compile-time in perl.h and there are currently 32 types. These different structures contain pointers to various routines that perform additional actions depending on which function is being called.

   Function pointer    Action taken
   ----------------    ------------
   svt_get             Do something before the value of the SV is
                       retrieved.
   svt_set             Do something after the SV is assigned a value.
   svt_len             Report on the SV's length.
   svt_clear           Clear something the SV represents.
   svt_free            Free any extra storage associated with the SV.

   svt_copy            copy tied variable magic to a tied element
   svt_dup             duplicate a magic structure during thread cloning
   svt_local           copy magic to local value during 'local'

For instance, the MGVTBL structure called vtbl_sv (which corresponds to an mg_type of PERL_MAGIC_sv) contains:

    { magic_get, magic_set, magic_len, 0, 0 }

Thus, when an SV is determined to be magical and of type PERL_MAGIC_sv, if a get operation is being performed, the routine magic_get is called. All the various routines for the various magical types begin with magic_. NOTE: the magic routines are not considered part of the Perl API, and may not be exported by the Perl library.

The last three slots are a recent addition, and for source code compatibility they are only checked for if one of the three flags MGf_COPY, MGf_DUP or MGf_LOCAL is set in mg_flags. This means that most code can continue declaring a vtable as a 5-element value. These three are currently used exclusively by the threading code, and are highly subject to change.

The current kinds of Magic Virtual Tables are:

 mg_type
 (old-style char and macro)   MGVTBL         Type of magic
 --------------------------   ------         -------------
 \0 PERL_MAGIC_sv             vtbl_sv        Special scalar variable
 #  PERL_MAGIC_arylen         vtbl_arylen    Array length ($#ary)
 %  PERL_MAGIC_rhash          (none)         extra data for restricted
                                             hashes
 &  PERL_MAGIC_proto          (none)         my sub prototype CV
 .  PERL_MAGIC_pos            vtbl_pos       pos() lvalue
 :  PERL_MAGIC_symtab         (none)         extra data for symbol
                                             tables
 <  PERL_MAGIC_backref        vtbl_backref   for weak ref data
 @  PERL_MAGIC_arylen_p       (none)         to move arylen out of XPVAV
 B  PERL_MAGIC_bm             vtbl_regexp    Boyer-Moore 
                                             (fast string search)
 c  PERL_MAGIC_overload_table vtbl_ovrld     Holds overload table 
                                             (AMT) on stash
 D  PERL_MAGIC_regdata        vtbl_regdata   Regex match position data 
                                             (@+ and @- vars)
 d  PERL_MAGIC_regdatum       vtbl_regdatum  Regex match position data
                                             element
 E  PERL_MAGIC_env            vtbl_env       %ENV hash
 e  PERL_MAGIC_envelem        vtbl_envelem   %ENV hash element
 f  PERL_MAGIC_fm             vtbl_regexp    Formline 
                                             ('compiled' format)
 g  PERL_MAGIC_regex_global   vtbl_mglob     m//g target
 H  PERL_MAGIC_hints          vtbl_hints     %^H hash
 h  PERL_MAGIC_hintselem      vtbl_hintselem %^H hash element
 I  PERL_MAGIC_isa            vtbl_isa       @ISA array
 i  PERL_MAGIC_isaelem        vtbl_isaelem   @ISA array element
 k  PERL_MAGIC_nkeys          vtbl_nkeys     scalar(keys()) lvalue
 L  PERL_MAGIC_dbfile         (none)         Debugger %_<filename
 l  PERL_MAGIC_dbline         vtbl_dbline    Debugger %_<filename
                                             element
 N  PERL_MAGIC_shared         (none)         Shared between threads
 n  PERL_MAGIC_shared_scalar  (none)         Shared between threads
 o  PERL_MAGIC_collxfrm       vtbl_collxfrm  Locale transformation
 P  PERL_MAGIC_tied           vtbl_pack      Tied array or hash
 p  PERL_MAGIC_tiedelem       vtbl_packelem  Tied array or hash element
 q  PERL_MAGIC_tiedscalar     vtbl_packelem  Tied scalar or handle
 r  PERL_MAGIC_qr             vtbl_regexp    precompiled qr// regex
 S  PERL_MAGIC_sig            (none)         %SIG hash
 s  PERL_MAGIC_sigelem        vtbl_sigelem   %SIG hash element
 t  PERL_MAGIC_taint          vtbl_taint     Taintedness
 U  PERL_MAGIC_uvar           vtbl_uvar      Available for use by
                                             extensions
 u  PERL_MAGIC_uvar_elem      (none)         Reserved for use by
                                             extensions
 V  PERL_MAGIC_vstring        (none)         SV was vstring literal
 v  PERL_MAGIC_vec            vtbl_vec       vec() lvalue
 w  PERL_MAGIC_utf8           vtbl_utf8      Cached UTF-8 information
 x  PERL_MAGIC_substr         vtbl_substr    substr() lvalue
 y  PERL_MAGIC_defelem        vtbl_defelem   Shadow "foreach" iterator
                                             variable / smart parameter
                                             vivification
 ]  PERL_MAGIC_checkcall      vtbl_checkcall inlining/mutation of call
                                             to this CV
 ~  PERL_MAGIC_ext            (none)         Available for use by
                                             extensions

When an uppercase and lowercase letter both exist in the table, then the uppercase letter is typically used to represent some kind of composite type (a list or a hash), and the lowercase letter is used to represent an element of that composite type. Some internals code makes use of this case relationship. However, 'v' and 'V' (vec and v-string) are in no way related.

The PERL_MAGIC_ext and PERL_MAGIC_uvar magic types are defined specifically for use by extensions and will not be used by perl itself. Extensions can use PERL_MAGIC_ext magic to 'attach' private information to variables (typically objects). This is especially useful because there is no way for normal perl code to corrupt this private information (unlike using extra elements of a hash object).

Similarly, PERL_MAGIC_uvar magic can be used much like tie() to call a C function any time a scalar's value is used or changed. The MAGIC's mg_ptr field points to a ufuncs structure:

    struct ufuncs {
        I32 (*uf_val)(pTHX_ IV, SV*);
        I32 (*uf_set)(pTHX_ IV, SV*);
        IV uf_index;
    };

When the SV is read from or written to, the uf_val or uf_set function will be called with uf_index as the first arg and a pointer to the SV as the second. A simple example of how to add PERL_MAGIC_uvar magic is shown below. Note that the ufuncs structure is copied by sv_magic, so you can safely allocate it on the stack.

    void
    Umagic(sv)
        SV *sv;
    PREINIT:
        struct ufuncs uf;
    CODE:
        uf.uf_val   = &my_get_fn;
        uf.uf_set   = &my_set_fn;
        uf.uf_index = 0;
        sv_magic(sv, 0, PERL_MAGIC_uvar, (char*)&uf, sizeof(uf));

Attaching PERL_MAGIC_uvar to arrays is permissible but has no effect.

For hashes there is a specialized hook that gives control over hash keys (but not values). This hook calls PERL_MAGIC_uvar 'get' magic if the "set" function in the ufuncs structure is NULL. The hook is activated whenever the hash is accessed with a key specified as an SV through the functions hv_store_ent, hv_fetch_ent, hv_delete_ent, and hv_exists_ent. Accessing the key as a string through the functions without the ..._ent suffix circumvents the hook. See "GUTS" in Hash::Util::FieldHash for a detailed description.

Note that because multiple extensions may be using PERL_MAGIC_ext or PERL_MAGIC_uvar magic, it is important for extensions to take extra care to avoid conflict. Typically only using the magic on objects blessed into the same class as the extension is sufficient. For PERL_MAGIC_ext magic, it is usually a good idea to define an MGVTBL, even if all its fields will be 0, so that individual MAGIC pointers can be identified as a particular kind of magic using their magic virtual table. mg_findext provides an easy way to do that:

    STATIC MGVTBL my_vtbl = { 0, 0, 0, 0, 0, 0, 0, 0 };

    MAGIC *mg;
    if ((mg = mg_findext(sv, PERL_MAGIC_ext, &my_vtbl))) {
        /* this is really ours, not another module's PERL_MAGIC_ext */
        my_priv_data_t *priv = (my_priv_data_t *)mg->mg_ptr;
        ...
    }

Also note that the sv_set*() and sv_cat*() functions described earlier do not invoke 'set' magic on their targets. This must be done by the user either by calling the SvSETMAGIC() macro after calling these functions, or by using one of the sv_set*_mg() or sv_cat*_mg() functions. Similarly, generic C code must call the SvGETMAGIC() macro to invoke any 'get' magic if they use an SV obtained from external sources in functions that don't handle magic. See perlapi for a description of these functions. For example, calls to the sv_cat*() functions typically need to be followed by SvSETMAGIC(), but they don't need a prior SvGETMAGIC() since their implementation handles 'get' magic.

Finding Magic

    MAGIC *mg_find(SV *sv, int type); /* Finds the magic pointer of that
                                       * type */

This routine returns a pointer to a MAGIC structure stored in the SV. If the SV does not have that magical feature, NULL is returned. If the SV has multiple instances of that magical feature, the first one will be returned. mg_findext can be used to find a MAGIC structure of an SV based on both its magic type and its magic virtual table:

    MAGIC *mg_findext(SV *sv, int type, MGVTBL *vtbl);

Also, if the SV passed to mg_find or mg_findext is not of type SVt_PVMG, Perl may core dump.

    int mg_copy(SV* sv, SV* nsv, const char* key, STRLEN klen);

This routine checks to see what types of magic sv has. If the mg_type field is an uppercase letter, then the mg_obj is copied to nsv, but the mg_type field is changed to be the lowercase letter.

Understanding the Magic of Tied Hashes and Arrays

Tied hashes and arrays are magical beasts of the PERL_MAGIC_tied magic type.

WARNING: As of the 5.004 release, proper usage of the array and hash access functions requires understanding a few caveats. Some of these caveats are actually considered bugs in the API, to be fixed in later releases, and are bracketed with [MAYCHANGE] below. If you find yourself actually applying such information in this section, be aware that the behavior may change in the future, umm, without warning.

The perl tie function associates a variable with an object that implements the various GET, SET, etc methods. To perform the equivalent of the perl tie function from an XSUB, you must mimic this behaviour. The code below carries out the necessary steps - firstly it creates a new hash, and then creates a second hash which it blesses into the class which will implement the tie methods. Lastly it ties the two hashes together, and returns a reference to the new tied hash. Note that the code below does NOT call the TIEHASH method in the MyTie class - see "Calling Perl Routines from within C Programs" for details on how to do this.

    SV*
    mytie()
    PREINIT:
        HV *hash;
        HV *stash;
        SV *tie;
    CODE:
        hash = newHV();
        tie = newRV_noinc((SV*)newHV());
        stash = gv_stashpv("MyTie", GV_ADD);
        sv_bless(tie, stash);
        hv_magic(hash, (GV*)tie, PERL_MAGIC_tied);
        RETVAL = newRV_noinc(hash);
    OUTPUT:
        RETVAL

The av_store function, when given a tied array argument, merely copies the magic of the array onto the value to be "stored", using mg_copy. It may also return NULL, indicating that the value did not actually need to be stored in the array. [MAYCHANGE] After a call to av_store on a tied array, the caller will usually need to call mg_set(val) to actually invoke the perl level "STORE" method on the TIEARRAY object. If av_store did return NULL, a call to SvREFCNT_dec(val) will also be usually necessary to avoid a memory leak. [/MAYCHANGE]

The previous paragraph is applicable verbatim to tied hash access using the hv_store and hv_store_ent functions as well.

av_fetch and the corresponding hash functions hv_fetch and hv_fetch_ent actually return an undefined mortal value whose magic has been initialized using mg_copy. Note the value so returned does not need to be deallocated, as it is already mortal. [MAYCHANGE] But you will need to call mg_get() on the returned value in order to actually invoke the perl level "FETCH" method on the underlying TIE object. Similarly, you may also call mg_set() on the return value after possibly assigning a suitable value to it using sv_setsv, which will invoke the "STORE" method on the TIE object. [/MAYCHANGE]

[MAYCHANGE] In other words, the array or hash fetch/store functions don't really fetch and store actual values in the case of tied arrays and hashes. They merely call mg_copy to attach magic to the values that were meant to be "stored" or "fetched". Later calls to mg_get and mg_set actually do the job of invoking the TIE methods on the underlying objects. Thus the magic mechanism currently implements a kind of lazy access to arrays and hashes.

Currently (as of perl version 5.004), use of the hash and array access functions requires the user to be aware of whether they are operating on "normal" hashes and arrays, or on their tied variants. The API may be changed to provide more transparent access to both tied and normal data types in future versions. [/MAYCHANGE]

You would do well to understand that the TIEARRAY and TIEHASH interfaces are mere sugar to invoke some perl method calls while using the uniform hash and array syntax. The use of this sugar imposes some overhead (typically about two to four extra opcodes per FETCH/STORE operation, in addition to the creation of all the mortal variables required to invoke the methods). This overhead will be comparatively small if the TIE methods are themselves substantial, but if they are only a few statements long, the overhead will not be insignificant.

Localizing changes

Perl has a very handy construction

  {
    local $var = 2;
    ...
  }

This construction is approximately equivalent to

  {
    my $oldvar = $var;
    $var = 2;
    ...
    $var = $oldvar;
  }

The biggest difference is that the first construction would reinstate the initial value of $var, irrespective of how control exits the block: goto, return, die/eval, etc. It is a little bit more efficient as well.

There is a way to achieve a similar task from C via Perl API: create a pseudo-block, and arrange for some changes to be automatically undone at the end of it, either explicit, or via a non-local exit (via die()). A block-like construct is created by a pair of ENTER/LEAVE macros (see "Returning a Scalar" in perlcall). Such a construct may be created specially for some important localized task, or an existing one (like boundaries of enclosing Perl subroutine/block, or an existing pair for freeing TMPs) may be used. (In the second case the overhead of additional localization must be almost negligible.) Note that any XSUB is automatically enclosed in an ENTER/LEAVE pair.

Inside such a pseudo-block the following service is available:

SAVEINT(int i)
SAVEIV(IV i)
SAVEI32(I32 i)
SAVELONG(long i)

These macros arrange things to restore the value of integer variable i at the end of enclosing pseudo-block.

SAVESPTR(s)
SAVEPPTR(p)

These macros arrange things to restore the value of pointers s and p. s must be a pointer of a type which survives conversion to SV* and back, p should be able to survive conversion to char* and back.

SAVEFREESV(SV *sv)

The refcount of sv would be decremented at the end of pseudo-block. This is similar to sv_2mortal in that it is also a mechanism for doing a delayed SvREFCNT_dec. However, while sv_2mortal extends the lifetime of sv until the beginning of the next statement, SAVEFREESV extends it until the end of the enclosing scope. These lifetimes can be wildly different.

Also compare SAVEMORTALIZESV.

SAVEMORTALIZESV(SV *sv)

Just like SAVEFREESV, but mortalizes sv at the end of the current scope instead of decrementing its reference count. This usually has the effect of keeping sv alive until the statement that called the currently live scope has finished executing.

SAVEFREEOP(OP *op)

The OP * is op_free()ed at the end of pseudo-block.

SAVEFREEPV(p)

The chunk of memory which is pointed to by p is Safefree()ed at the end of pseudo-block.

SAVECLEARSV(SV *sv)

Clears a slot in the current scratchpad which corresponds to sv at the end of pseudo-block.

SAVEDELETE(HV *hv, char *key, I32 length)

The key key of hv is deleted at the end of pseudo-block. The string pointed to by key is Safefree()ed. If one has a key in short-lived storage, the corresponding string may be reallocated like this:

  SAVEDELETE(PL_defstash, savepv(tmpbuf), strlen(tmpbuf));
SAVEDESTRUCTOR(DESTRUCTORFUNC_NOCONTEXT_t f, void *p)

At the end of pseudo-block the function f is called with the only argument p.

SAVEDESTRUCTOR_X(DESTRUCTORFUNC_t f, void *p)

At the end of pseudo-block the function f is called with the implicit context argument (if any), and p.

SAVESTACK_POS()

The current offset on the Perl internal stack (cf. SP) is restored at the end of pseudo-block.

The following API list contains functions, thus one needs to provide pointers to the modifiable data explicitly (either C pointers, or Perlish GV *s). Where the above macros take int, a similar function takes int *.

SV* save_scalar(GV *gv)

Equivalent to Perl code local $gv.

AV* save_ary(GV *gv)
HV* save_hash(GV *gv)

Similar to save_scalar, but localize @gv and %gv.

void save_item(SV *item)

Duplicates the current value of SV, on the exit from the current ENTER/LEAVE pseudo-block will restore the value of SV using the stored value. It doesn't handle magic. Use save_scalar if magic is affected.

void save_list(SV **sarg, I32 maxsarg)

A variant of save_item which takes multiple arguments via an array sarg of SV* of length maxsarg.

SV* save_svref(SV **sptr)

Similar to save_scalar, but will reinstate an SV *.

void save_aptr(AV **aptr)
void save_hptr(HV **hptr)

Similar to save_svref, but localize AV * and HV *.

The Alias module implements localization of the basic types within the caller's scope. People who are interested in how to localize things in the containing scope should take a look there too.

Subroutines

XSUBs and the Argument Stack

The XSUB mechanism is a simple way for Perl programs to access C subroutines. An XSUB routine will have a stack that contains the arguments from the Perl program, and a way to map from the Perl data structures to a C equivalent.

The stack arguments are accessible through the ST(n) macro, which returns the n'th stack argument. Argument 0 is the first argument passed in the Perl subroutine call. These arguments are SV*, and can be used anywhere an SV* is used.

Most of the time, output from the C routine can be handled through use of the RETVAL and OUTPUT directives. However, there are some cases where the argument stack is not already long enough to handle all the return values. An example is the POSIX tzname() call, which takes no arguments, but returns two, the local time zone's standard and summer time abbreviations.

To handle this situation, the PPCODE directive is used and the stack is extended using the macro:

    EXTEND(SP, num);

where SP is the macro that represents the local copy of the stack pointer, and num is the number of elements the stack should be extended by.

Now that there is room on the stack, values can be pushed on it using PUSHs macro. The pushed values will often need to be "mortal" (See "Reference Counts and Mortality"):

    PUSHs(sv_2mortal(newSViv(an_integer)))
    PUSHs(sv_2mortal(newSVuv(an_unsigned_integer)))
    PUSHs(sv_2mortal(newSVnv(a_double)))
    PUSHs(sv_2mortal(newSVpv("Some String",0)))
    /* Although the last example is better written as the more
     * efficient: */
    PUSHs(newSVpvs_flags("Some String", SVs_TEMP))

And now the Perl program calling tzname, the two values will be assigned as in:

    ($standard_abbrev, $summer_abbrev) = POSIX::tzname;

An alternate (and possibly simpler) method to pushing values on the stack is to use the macro:

    XPUSHs(SV*)

This macro automatically adjusts the stack for you, if needed. Thus, you do not need to call EXTEND to extend the stack.

Despite their suggestions in earlier versions of this document the macros (X)PUSH[iunp] are not suited to XSUBs which return multiple results. For that, either stick to the (X)PUSHs macros shown above, or use the new m(X)PUSH[iunp] macros instead; see "Putting a C value on Perl stack".

For more information, consult perlxs and perlxstut.

Autoloading with XSUBs

If an AUTOLOAD routine is an XSUB, as with Perl subroutines, Perl puts the fully-qualified name of the autoloaded subroutine in the $AUTOLOAD variable of the XSUB's package.

But it also puts the same information in certain fields of the XSUB itself:

    HV *stash           = CvSTASH(cv);
    const char *subname = SvPVX(cv);
    STRLEN name_length  = SvCUR(cv); /* in bytes */
    U32 is_utf8         = SvUTF8(cv);

SvPVX(cv) contains just the sub name itself, not including the package. For an AUTOLOAD routine in UNIVERSAL or one of its superclasses, CvSTASH(cv) returns NULL during a method call on a nonexistent package.

Note: Setting $AUTOLOAD stopped working in 5.6.1, which did not support XS AUTOLOAD subs at all. Perl 5.8.0 introduced the use of fields in the XSUB itself. Perl 5.16.0 restored the setting of $AUTOLOAD. If you need to support 5.8-5.14, use the XSUB's fields.

Calling Perl Routines from within C Programs

There are four routines that can be used to call a Perl subroutine from within a C program. These four are:

    I32  call_sv(SV*, I32);
    I32  call_pv(const char*, I32);
    I32  call_method(const char*, I32);
    I32  call_argv(const char*, I32, char**);

The routine most often used is call_sv. The SV* argument contains either the name of the Perl subroutine to be called, or a reference to the subroutine. The second argument consists of flags that control the context in which the subroutine is called, whether or not the subroutine is being passed arguments, how errors should be trapped, and how to treat return values.

All four routines return the number of arguments that the subroutine returned on the Perl stack.

These routines used to be called perl_call_sv, etc., before Perl v5.6.0, but those names are now deprecated; macros of the same name are provided for compatibility.

When using any of these routines (except call_argv), the programmer must manipulate the Perl stack. These include the following macros and functions:

    dSP
    SP
    PUSHMARK()
    PUTBACK
    SPAGAIN
    ENTER
    SAVETMPS
    FREETMPS
    LEAVE
    XPUSH*()
    POP*()

For a detailed description of calling conventions from C to Perl, consult perlcall.

Memory Allocation

Allocation

All memory meant to be used with the Perl API functions should be manipulated using the macros described in this section. The macros provide the necessary transparency between differences in the actual malloc implementation that is used within perl.

It is suggested that you enable the version of malloc that is distributed with Perl. It keeps pools of various sizes of unallocated memory in order to satisfy allocation requests more quickly. However, on some platforms, it may cause spurious malloc or free errors.

The following three macros are used to initially allocate memory :

    Newx(pointer, number, type);
    Newxc(pointer, number, type, cast);
    Newxz(pointer, number, type);

The first argument pointer should be the name of a variable that will point to the newly allocated memory.

The second and third arguments number and type specify how many of the specified type of data structure should be allocated. The argument type is passed to sizeof. The final argument to Newxc, cast, should be used if the pointer argument is different from the type argument.

Unlike the Newx and Newxc macros, the Newxz macro calls memzero to zero out all the newly allocated memory.

Reallocation

    Renew(pointer, number, type);
    Renewc(pointer, number, type, cast);
    Safefree(pointer)

These three macros are used to change a memory buffer size or to free a piece of memory no longer needed. The arguments to Renew and Renewc match those of New and Newc with the exception of not needing the "magic cookie" argument.

Moving

    Move(source, dest, number, type);
    Copy(source, dest, number, type);
    Zero(dest, number, type);

These three macros are used to move, copy, or zero out previously allocated memory. The source and dest arguments point to the source and destination starting points. Perl will move, copy, or zero out number instances of the size of the type data structure (using the sizeof function).

PerlIO

The most recent development releases of Perl have been experimenting with removing Perl's dependency on the "normal" standard I/O suite and allowing other stdio implementations to be used. This involves creating a new abstraction layer that then calls whichever implementation of stdio Perl was compiled with. All XSUBs should now use the functions in the PerlIO abstraction layer and not make any assumptions about what kind of stdio is being used.

For a complete description of the PerlIO abstraction, consult perlapio.

Putting a C value on Perl stack

A lot of opcodes (this is an elementary operation in the internal perl stack machine) put an SV* on the stack. However, as an optimization the corresponding SV is (usually) not recreated each time. The opcodes reuse specially assigned SVs (targets) which are (as a corollary) not constantly freed/created.

Each of the targets is created only once (but see "Scratchpads and recursion" below), and when an opcode needs to put an integer, a double, or a string on stack, it just sets the corresponding parts of its target and puts the target on stack.

The macro to put this target on stack is PUSHTARG, and it is directly used in some opcodes, as well as indirectly in zillions of others, which use it via (X)PUSH[iunp].

Because the target is reused, you must be careful when pushing multiple values on the stack. The following code will not do what you think:

    XPUSHi(10);
    XPUSHi(20);

This translates as "set TARG to 10, push a pointer to TARG onto the stack; set TARG to 20, push a pointer to TARG onto the stack". At the end of the operation, the stack does not contain the values 10 and 20, but actually contains two pointers to TARG, which we have set to 20.

If you need to push multiple different values then you should either use the (X)PUSHs macros, or else use the new m(X)PUSH[iunp] macros, none of which make use of TARG. The (X)PUSHs macros simply push an SV* on the stack, which, as noted under "XSUBs and the Argument Stack", will often need to be "mortal". The new m(X)PUSH[iunp] macros make this a little easier to achieve by creating a new mortal for you (via (X)PUSHmortal), pushing that onto the stack (extending it if necessary in the case of the mXPUSH[iunp] macros), and then setting its value. Thus, instead of writing this to "fix" the example above:

    XPUSHs(sv_2mortal(newSViv(10)))
    XPUSHs(sv_2mortal(newSViv(20)))

you can simply write:

    mXPUSHi(10)
    mXPUSHi(20)

On a related note, if you do use (X)PUSH[iunp], then you're going to need a dTARG in your variable declarations so that the *PUSH* macros can make use of the local variable TARG. See also dTARGET and dXSTARG.

Scratchpads

The question remains on when the SVs which are targets for opcodes are created. The answer is that they are created when the current unit--a subroutine or a file (for opcodes for statements outside of subroutines)--is compiled. During this time a special anonymous Perl array is created, which is called a scratchpad for the current unit.

A scratchpad keeps SVs which are lexicals for the current unit and are targets for opcodes. One can deduce that an SV lives on a scratchpad by looking on its flags: lexicals have SVs_PADMY set, and targets have SVs_PADTMP set.

The correspondence between OPs and targets is not 1-to-1. Different OPs in the compile tree of the unit can use the same target, if this would not conflict with the expected life of the temporary.

Scratchpads and recursion

In fact it is not 100% true that a compiled unit contains a pointer to the scratchpad AV. In fact it contains a pointer to an AV of (initially) one element, and this element is the scratchpad AV. Why do we need an extra level of indirection?

The answer is recursion, and maybe threads. Both these can create several execution pointers going into the same subroutine. For the subroutine-child not write over the temporaries for the subroutine-parent (lifespan of which covers the call to the child), the parent and the child should have different scratchpads. (And the lexicals should be separate anyway!)

So each subroutine is born with an array of scratchpads (of length 1). On each entry to the subroutine it is checked that the current depth of the recursion is not more than the length of this array, and if it is, new scratchpad is created and pushed into the array.

The targets on this scratchpad are undefs, but they are already marked with correct flags.

Compiled code

Code tree

Here we describe the internal form your code is converted to by Perl. Start with a simple example:

  $a = $b + $c;

This is converted to a tree similar to this one:

             assign-to
           /           \
          +             $a
        /   \
      $b     $c

(but slightly more complicated). This tree reflects the way Perl parsed your code, but has nothing to do with the execution order. There is an additional "thread" going through the nodes of the tree which shows the order of execution of the nodes. In our simplified example above it looks like:

     $b ---> $c ---> + ---> $a ---> assign-to

But with the actual compile tree for $a = $b + $c it is different: some nodes optimized away. As a corollary, though the actual tree contains more nodes than our simplified example, the execution order is the same as in our example.

Examining the tree

If you have your perl compiled for debugging (usually done with -DDEBUGGING on the Configure command line), you may examine the compiled tree by specifying -Dx on the Perl command line. The output takes several lines per node, and for $b+$c it looks like this:

    5           TYPE = add  ===> 6
                TARG = 1
                FLAGS = (SCALAR,KIDS)
                {
                    TYPE = null  ===> (4)
                      (was rv2sv)
                    FLAGS = (SCALAR,KIDS)
                    {
    3                   TYPE = gvsv  ===> 4
                        FLAGS = (SCALAR)
                        GV = main::b
                    }
                }
                {
                    TYPE = null  ===> (5)
                      (was rv2sv)
                    FLAGS = (SCALAR,KIDS)
                    {
    4                   TYPE = gvsv  ===> 5
                        FLAGS = (SCALAR)
                        GV = main::c
                    }
                }

This tree has 5 nodes (one per TYPE specifier), only 3 of them are not optimized away (one per number in the left column). The immediate children of the given node correspond to {} pairs on the same level of indentation, thus this listing corresponds to the tree:

                   add
                 /     \
               null    null
                |       |
               gvsv    gvsv

The execution order is indicated by ===> marks, thus it is 3 4 5 6 (node 6 is not included into above listing), i.e., gvsv gvsv add whatever.

Each of these nodes represents an op, a fundamental operation inside the Perl core. The code which implements each operation can be found in the pp*.c files; the function which implements the op with type gvsv is pp_gvsv, and so on. As the tree above shows, different ops have different numbers of children: add is a binary operator, as one would expect, and so has two children. To accommodate the various different numbers of children, there are various types of op data structure, and they link together in different ways.

The simplest type of op structure is OP: this has no children. Unary operators, UNOPs, have one child, and this is pointed to by the op_first field. Binary operators (BINOPs) have not only an op_first field but also an op_last field. The most complex type of op is a LISTOP, which has any number of children. In this case, the first child is pointed to by op_first and the last child by op_last. The children in between can be found by iteratively following the op_sibling pointer from the first child to the last.

There are also two other op types: a PMOP holds a regular expression, and has no children, and a LOOP may or may not have children. If the op_children field is non-zero, it behaves like a LISTOP. To complicate matters, if a UNOP is actually a null op after optimization (see "Compile pass 2: context propagation") it will still have children in accordance with its former type.

Another way to examine the tree is to use a compiler back-end module, such as B::Concise.

Compile pass 1: check routines

The tree is created by the compiler while yacc code feeds it the constructions it recognizes. Since yacc works bottom-up, so does the first pass of perl compilation.

What makes this pass interesting for perl developers is that some optimization may be performed on this pass. This is optimization by so-called "check routines". The correspondence between node names and corresponding check routines is described in opcode.pl (do not forget to run make regen_headers if you modify this file).

A check routine is called when the node is fully constructed except for the execution-order thread. Since at this time there are no back-links to the currently constructed node, one can do most any operation to the top-level node, including freeing it and/or creating new nodes above/below it.

The check routine returns the node which should be inserted into the tree (if the top-level node was not modified, check routine returns its argument).

By convention, check routines have names ck_*. They are usually called from new*OP subroutines (or convert) (which in turn are called from perly.y).

Compile pass 1a: constant folding

Immediately after the check routine is called the returned node is checked for being compile-time executable. If it is (the value is judged to be constant) it is immediately executed, and a constant node with the "return value" of the corresponding subtree is substituted instead. The subtree is deleted.

If constant folding was not performed, the execution-order thread is created.

Compile pass 2: context propagation

When a context for a part of compile tree is known, it is propagated down through the tree. At this time the context can have 5 values (instead of 2 for runtime context): void, boolean, scalar, list, and lvalue. In contrast with the pass 1 this pass is processed from top to bottom: a node's context determines the context for its children.

Additional context-dependent optimizations are performed at this time. Since at this moment the compile tree contains back-references (via "thread" pointers), nodes cannot be free()d now. To allow optimized-away nodes at this stage, such nodes are null()ified instead of free()ing (i.e. their type is changed to OP_NULL).

Compile pass 3: peephole optimization

After the compile tree for a subroutine (or for an eval or a file) is created, an additional pass over the code is performed. This pass is neither top-down or bottom-up, but in the execution order (with additional complications for conditionals). Optimizations performed at this stage are subject to the same restrictions as in the pass 2.

Peephole optimizations are done by calling the function pointed to by the global variable PL_peepp. By default, PL_peepp just calls the function pointed to by the global variable PL_rpeepp. By default, that performs some basic op fixups and optimisations along the execution-order op chain, and recursively calls PL_rpeepp for each side chain of ops (resulting from conditionals). Extensions may provide additional optimisations or fixups, hooking into either the per-subroutine or recursive stage, like this:

    static peep_t prev_peepp;
    static void my_peep(pTHX_ OP *o)
    {
        /* custom per-subroutine optimisation goes here */
        prev_peepp(aTHX_ o);
        /* custom per-subroutine optimisation may also go here */
    }
    BOOT:
        prev_peepp = PL_peepp;
        PL_peepp = my_peep;

    static peep_t prev_rpeepp;
    static void my_rpeep(pTHX_ OP *o)
    {
        OP *orig_o = o;
        for(; o; o = o->op_next) {
            /* custom per-op optimisation goes here */
        }
        prev_rpeepp(aTHX_ orig_o);
    }
    BOOT:
        prev_rpeepp = PL_rpeepp;
        PL_rpeepp = my_rpeep;

Pluggable runops

The compile tree is executed in a runops function. There are two runops functions, in run.c and in dump.c. Perl_runops_debug is used with DEBUGGING and Perl_runops_standard is used otherwise. For fine control over the execution of the compile tree it is possible to provide your own runops function.

It's probably best to copy one of the existing runops functions and change it to suit your needs. Then, in the BOOT section of your XS file, add the line:

  PL_runops = my_runops;

This function should be as efficient as possible to keep your programs running as fast as possible.

Compile-time scope hooks

As of perl 5.14 it is possible to hook into the compile-time lexical scope mechanism using Perl_blockhook_register. This is used like this:

    STATIC void my_start_hook(pTHX_ int full);
    STATIC BHK my_hooks;

    BOOT:
        BhkENTRY_set(&my_hooks, bhk_start, my_start_hook);
        Perl_blockhook_register(aTHX_ &my_hooks);

This will arrange to have my_start_hook called at the start of compiling every lexical scope. The available hooks are:

void bhk_start(pTHX_ int full)

This is called just after starting a new lexical scope. Note that Perl code like

    if ($x) { ... }

creates two scopes: the first starts at the ( and has full == 1, the second starts at the { and has full == 0. Both end at the }, so calls to start and pre/post_end will match. Anything pushed onto the save stack by this hook will be popped just before the scope ends (between the pre_ and post_end hooks, in fact).

void bhk_pre_end(pTHX_ OP **o)

This is called at the end of a lexical scope, just before unwinding the stack. o is the root of the optree representing the scope; it is a double pointer so you can replace the OP if you need to.

void bhk_post_end(pTHX_ OP **o)

This is called at the end of a lexical scope, just after unwinding the stack. o is as above. Note that it is possible for calls to pre_ and post_end to nest, if there is something on the save stack that calls string eval.

void bhk_eval(pTHX_ OP *const o)

This is called just before starting to compile an eval STRING, do FILE, require or use, after the eval has been set up. o is the OP that requested the eval, and will normally be an OP_ENTEREVAL, OP_DOFILE or OP_REQUIRE.

Once you have your hook functions, you need a BHK structure to put them in. It's best to allocate it statically, since there is no way to free it once it's registered. The function pointers should be inserted into this structure using the BhkENTRY_set macro, which will also set flags indicating which entries are valid. If you do need to allocate your BHK dynamically for some reason, be sure to zero it before you start.

Once registered, there is no mechanism to switch these hooks off, so if that is necessary you will need to do this yourself. An entry in %^H is probably the best way, so the effect is lexically scoped; however it is also possible to use the BhkDISABLE and BhkENABLE macros to temporarily switch entries on and off. You should also be aware that generally speaking at least one scope will have opened before your extension is loaded, so you will see some pre/post_end pairs that didn't have a matching start.

Examining internal data structures with the dump functions

To aid debugging, the source file dump.c contains a number of functions which produce formatted output of internal data structures.

The most commonly used of these functions is Perl_sv_dump; it's used for dumping SVs, AVs, HVs, and CVs. The Devel::Peek module calls sv_dump to produce debugging output from Perl-space, so users of that module should already be familiar with its format.

Perl_op_dump can be used to dump an OP structure or any of its derivatives, and produces output similar to perl -Dx; in fact, Perl_dump_eval will dump the main root of the code being evaluated, exactly like -Dx.

Other useful functions are Perl_dump_sub, which turns a GV into an op tree, Perl_dump_packsubs which calls Perl_dump_sub on all the subroutines in a package like so: (Thankfully, these are all xsubs, so there is no op tree)

    (gdb) print Perl_dump_packsubs(PL_defstash)

    SUB attributes::bootstrap = (xsub 0x811fedc 0)

    SUB UNIVERSAL::can = (xsub 0x811f50c 0)

    SUB UNIVERSAL::isa = (xsub 0x811f304 0)

    SUB UNIVERSAL::VERSION = (xsub 0x811f7ac 0)

    SUB DynaLoader::boot_DynaLoader = (xsub 0x805b188 0)

and Perl_dump_all, which dumps all the subroutines in the stash and the op tree of the main root.

How multiple interpreters and concurrency are supported

Background and PERL_IMPLICIT_CONTEXT

The Perl interpreter can be regarded as a closed box: it has an API for feeding it code or otherwise making it do things, but it also has functions for its own use. This smells a lot like an object, and there are ways for you to build Perl so that you can have multiple interpreters, with one interpreter represented either as a C structure, or inside a thread-specific structure. These structures contain all the context, the state of that interpreter.

One macro controls the major Perl build flavor: MULTIPLICITY. The MULTIPLICITY build has a C structure that packages all the interpreter state. With multiplicity-enabled perls, PERL_IMPLICIT_CONTEXT is also normally defined, and enables the support for passing in a "hidden" first argument that represents all three data structures. MULTIPLICITY makes multi-threaded perls possible (with the ithreads threading model, related to the macro USE_ITHREADS.)

Two other "encapsulation" macros are the PERL_GLOBAL_STRUCT and PERL_GLOBAL_STRUCT_PRIVATE (the latter turns on the former, and the former turns on MULTIPLICITY.) The PERL_GLOBAL_STRUCT causes all the internal variables of Perl to be wrapped inside a single global struct, struct perl_vars, accessible as (globals) &PL_Vars or PL_VarsPtr or the function Perl_GetVars(). The PERL_GLOBAL_STRUCT_PRIVATE goes one step further, there is still a single struct (allocated in main() either from heap or from stack) but there are no global data symbols pointing to it. In either case the global struct should be initialised as the very first thing in main() using Perl_init_global_struct() and correspondingly tear it down after perl_free() using Perl_free_global_struct(), please see miniperlmain.c for usage details. You may also need to use dVAR in your coding to "declare the global variables" when you are using them. dTHX does this for you automatically.

To see whether you have non-const data you can use a BSD-compatible nm:

  nm libperl.a | grep -v ' [TURtr] '

If this displays any D or d symbols, you have non-const data.

For backward compatibility reasons defining just PERL_GLOBAL_STRUCT doesn't actually hide all symbols inside a big global struct: some PerlIO_xxx vtables are left visible. The PERL_GLOBAL_STRUCT_PRIVATE then hides everything (see how the PERLIO_FUNCS_DECL is used).

All this obviously requires a way for the Perl internal functions to be either subroutines taking some kind of structure as the first argument, or subroutines taking nothing as the first argument. To enable these two very different ways of building the interpreter, the Perl source (as it does in so many other situations) makes heavy use of macros and subroutine naming conventions.

First problem: deciding which functions will be public API functions and which will be private. All functions whose names begin S_ are private (think "S" for "secret" or "static"). All other functions begin with "Perl_", but just because a function begins with "Perl_" does not mean it is part of the API. (See "Internal Functions".) The easiest way to be sure a function is part of the API is to find its entry in perlapi. If it exists in perlapi, it's part of the API. If it doesn't, and you think it should be (i.e., you need it for your extension), send mail via perlbug explaining why you think it should be.

Second problem: there must be a syntax so that the same subroutine declarations and calls can pass a structure as their first argument, or pass nothing. To solve this, the subroutines are named and declared in a particular way. Here's a typical start of a static function used within the Perl guts:

  STATIC void
  S_incline(pTHX_ char *s)

STATIC becomes "static" in C, and may be #define'd to nothing in some configurations in the future.

A public function (i.e. part of the internal API, but not necessarily sanctioned for use in extensions) begins like this:

  void
  Perl_sv_setiv(pTHX_ SV* dsv, IV num)

pTHX_ is one of a number of macros (in perl.h) that hide the details of the interpreter's context. THX stands for "thread", "this", or "thingy", as the case may be. (And no, George Lucas is not involved. :-) The first character could be 'p' for a prototype, 'a' for argument, or 'd' for declaration, so we have pTHX, aTHX and dTHX, and their variants.

When Perl is built without options that set PERL_IMPLICIT_CONTEXT, there is no first argument containing the interpreter's context. The trailing underscore in the pTHX_ macro indicates that the macro expansion needs a comma after the context argument because other arguments follow it. If PERL_IMPLICIT_CONTEXT is not defined, pTHX_ will be ignored, and the subroutine is not prototyped to take the extra argument. The form of the macro without the trailing underscore is used when there are no additional explicit arguments.

When a core function calls another, it must pass the context. This is normally hidden via macros. Consider sv_setiv. It expands into something like this:

    #ifdef PERL_IMPLICIT_CONTEXT
      #define sv_setiv(a,b)      Perl_sv_setiv(aTHX_ a, b)
      /* can't do this for vararg functions, see below */
    #else
      #define sv_setiv           Perl_sv_setiv
    #endif

This works well, and means that XS authors can gleefully write:

    sv_setiv(foo, bar);

and still have it work under all the modes Perl could have been compiled with.

This doesn't work so cleanly for varargs functions, though, as macros imply that the number of arguments is known in advance. Instead we either need to spell them out fully, passing aTHX_ as the first argument (the Perl core tends to do this with functions like Perl_warner), or use a context-free version.

The context-free version of Perl_warner is called Perl_warner_nocontext, and does not take the extra argument. Instead it does dTHX; to get the context from thread-local storage. We #define warner Perl_warner_nocontext so that extensions get source compatibility at the expense of performance. (Passing an arg is cheaper than grabbing it from thread-local storage.)

You can ignore [pad]THXx when browsing the Perl headers/sources. Those are strictly for use within the core. Extensions and embedders need only be aware of [pad]THX.

So what happened to dTHR?

dTHR was introduced in perl 5.005 to support the older thread model. The older thread model now uses the THX mechanism to pass context pointers around, so dTHR is not useful any more. Perl 5.6.0 and later still have it for backward source compatibility, but it is defined to be a no-op.

How do I use all this in extensions?

When Perl is built with PERL_IMPLICIT_CONTEXT, extensions that call any functions in the Perl API will need to pass the initial context argument somehow. The kicker is that you will need to write it in such a way that the extension still compiles when Perl hasn't been built with PERL_IMPLICIT_CONTEXT enabled.

There are three ways to do this. First, the easy but inefficient way, which is also the default, in order to maintain source compatibility with extensions: whenever XSUB.h is #included, it redefines the aTHX and aTHX_ macros to call a function that will return the context. Thus, something like:

        sv_setiv(sv, num);

in your extension will translate to this when PERL_IMPLICIT_CONTEXT is in effect:

        Perl_sv_setiv(Perl_get_context(), sv, num);

or to this otherwise:

        Perl_sv_setiv(sv, num);

You don't have to do anything new in your extension to get this; since the Perl library provides Perl_get_context(), it will all just work.

The second, more efficient way is to use the following template for your Foo.xs:

        #define PERL_NO_GET_CONTEXT     /* we want efficiency */
        #include "EXTERN.h"
        #include "perl.h"
        #include "XSUB.h"

        STATIC void my_private_function(int arg1, int arg2);

        STATIC void
        my_private_function(int arg1, int arg2)
        {
            dTHX;       /* fetch context */
            ... call many Perl API functions ...
        }

        [... etc ...]

        MODULE = Foo            PACKAGE = Foo

        /* typical XSUB */

        void
        my_xsub(arg)
                int arg
            CODE:
                my_private_function(arg, 10);

Note that the only two changes from the normal way of writing an extension is the addition of a #define PERL_NO_GET_CONTEXT before including the Perl headers, followed by a dTHX; declaration at the start of every function that will call the Perl API. (You'll know which functions need this, because the C compiler will complain that there's an undeclared identifier in those functions.) No changes are needed for the XSUBs themselves, because the XS() macro is correctly defined to pass in the implicit context if needed.

The third, even more efficient way is to ape how it is done within the Perl guts:

        #define PERL_NO_GET_CONTEXT     /* we want efficiency */
        #include "EXTERN.h"
        #include "perl.h"
        #include "XSUB.h"

        /* pTHX_ only needed for functions that call Perl API */
        STATIC void my_private_function(pTHX_ int arg1, int arg2);

        STATIC void
        my_private_function(pTHX_ int arg1, int arg2)
        {
            /* dTHX; not needed here, because THX is an argument */
            ... call Perl API functions ...
        }

        [... etc ...]

        MODULE = Foo            PACKAGE = Foo

        /* typical XSUB */

        void
        my_xsub(arg)
                int arg
            CODE:
                my_private_function(aTHX_ arg, 10);

This implementation never has to fetch the context using a function call, since it is always passed as an extra argument. Depending on your needs for simplicity or efficiency, you may mix the previous two approaches freely.

Never add a comma after pTHX yourself--always use the form of the macro with the underscore for functions that take explicit arguments, or the form without the argument for functions with no explicit arguments.

If one is compiling Perl with the -DPERL_GLOBAL_STRUCT the dVAR definition is needed if the Perl global variables (see perlvars.h or globvar.sym) are accessed in the function and dTHX is not used (the dTHX includes the dVAR if necessary). One notices the need for dVAR only with the said compile-time define, because otherwise the Perl global variables are visible as-is.

Should I do anything special if I call perl from multiple threads?

If you create interpreters in one thread and then proceed to call them in another, you need to make sure perl's own Thread Local Storage (TLS) slot is initialized correctly in each of those threads.

The perl_alloc and perl_clone API functions will automatically set the TLS slot to the interpreter they created, so that there is no need to do anything special if the interpreter is always accessed in the same thread that created it, and that thread did not create or call any other interpreters afterwards. If that is not the case, you have to set the TLS slot of the thread before calling any functions in the Perl API on that particular interpreter. This is done by calling the PERL_SET_CONTEXT macro in that thread as the first thing you do:

        /* do this before doing anything else with some_perl */
        PERL_SET_CONTEXT(some_perl);

        ... other Perl API calls on some_perl go here ...

Future Plans and PERL_IMPLICIT_SYS

Just as PERL_IMPLICIT_CONTEXT provides a way to bundle up everything that the interpreter knows about itself and pass it around, so too are there plans to allow the interpreter to bundle up everything it knows about the environment it's running on. This is enabled with the PERL_IMPLICIT_SYS macro. Currently it only works with USE_ITHREADS on Windows.

This allows the ability to provide an extra pointer (called the "host" environment) for all the system calls. This makes it possible for all the system stuff to maintain their own state, broken down into seven C structures. These are thin wrappers around the usual system calls (see win32/perllib.c) for the default perl executable, but for a more ambitious host (like the one that would do fork() emulation) all the extra work needed to pretend that different interpreters are actually different "processes", would be done here.

The Perl engine/interpreter and the host are orthogonal entities. There could be one or more interpreters in a process, and one or more "hosts", with free association between them.

Internal Functions

All of Perl's internal functions which will be exposed to the outside world are prefixed by Perl_ so that they will not conflict with XS functions or functions used in a program in which Perl is embedded. Similarly, all global variables begin with PL_. (By convention, static functions start with S_.)

Inside the Perl core (PERL_CORE defined), you can get at the functions either with or without the Perl_ prefix, thanks to a bunch of defines that live in embed.h. Note that extension code should not set PERL_CORE; this exposes the full perl internals, and is likely to cause breakage of the XS in each new perl release.

The file embed.h is generated automatically from embed.pl and embed.fnc. embed.pl also creates the prototyping header files for the internal functions, generates the documentation and a lot of other bits and pieces. It's important that when you add a new function to the core or change an existing one, you change the data in the table in embed.fnc as well. Here's a sample entry from that table:

    Apd |SV**   |av_fetch   |AV* ar|I32 key|I32 lval

The second column is the return type, the third column the name. Columns after that are the arguments. The first column is a set of flags:

A

This function is a part of the public API. All such functions should also have 'd', very few do not.

p

This function has a Perl_ prefix; i.e. it is defined as Perl_av_fetch.

d

This function has documentation using the apidoc feature which we'll look at in a second. Some functions have 'd' but not 'A'; docs are good.

Other available flags are:

s

This is a static function and is defined as STATIC S_whatever, and usually called within the sources as whatever(...).

n

This does not need an interpreter context, so the definition has no pTHX, and it follows that callers don't use aTHX. (See "Background and PERL_IMPLICIT_CONTEXT".)

r

This function never returns; croak, exit and friends.

f

This function takes a variable number of arguments, printf style. The argument list should end with ..., like this:

    Afprd   |void   |croak          |const char* pat|...
M

This function is part of the experimental development API, and may change or disappear without notice.

o

This function should not have a compatibility macro to define, say, Perl_parse to parse. It must be called as Perl_parse.

x

This function isn't exported out of the Perl core.

m

This is implemented as a macro.

X

This function is explicitly exported.

E

This function is visible to extensions included in the Perl core.

b

Binary backward compatibility; this function is a macro but also has a Perl_ implementation (which is exported).

others

See the comments at the top of embed.fnc for others.

If you edit embed.pl or embed.fnc, you will need to run make regen_headers to force a rebuild of embed.h and other auto-generated files.

Formatted Printing of IVs, UVs, and NVs

If you are printing IVs, UVs, or NVS instead of the stdio(3) style formatting codes like %d, %ld, %f, you should use the following macros for portability

        IVdf            IV in decimal
        UVuf            UV in decimal
        UVof            UV in octal
        UVxf            UV in hexadecimal
        NVef            NV %e-like
        NVff            NV %f-like
        NVgf            NV %g-like

These will take care of 64-bit integers and long doubles. For example:

        printf("IV is %"IVdf"\n", iv);

The IVdf will expand to whatever is the correct format for the IVs.

If you are printing addresses of pointers, use UVxf combined with PTR2UV(), do not use %lx or %p.

Pointer-To-Integer and Integer-To-Pointer

Because pointer size does not necessarily equal integer size, use the follow macros to do it right.

        PTR2UV(pointer)
        PTR2IV(pointer)
        PTR2NV(pointer)
        INT2PTR(pointertotype, integer)

For example:

        IV  iv = ...;
        SV *sv = INT2PTR(SV*, iv);

and

        AV *av = ...;
        UV  uv = PTR2UV(av);

Exception Handling

There are a couple of macros to do very basic exception handling in XS modules. You have to define NO_XSLOCKS before including XSUB.h to be able to use these macros:

        #define NO_XSLOCKS
        #include "XSUB.h"

You can use these macros if you call code that may croak, but you need to do some cleanup before giving control back to Perl. For example:

        dXCPT;    /* set up necessary variables */

        XCPT_TRY_START {
          code_that_may_croak();
        } XCPT_TRY_END

        XCPT_CATCH
        {
          /* do cleanup here */
          XCPT_RETHROW;
        }

Note that you always have to rethrow an exception that has been caught. Using these macros, it is not possible to just catch the exception and ignore it. If you have to ignore the exception, you have to use the call_* function.

The advantage of using the above macros is that you don't have to setup an extra function for call_*, and that using these macros is faster than using call_*.

Source Documentation

There's an effort going on to document the internal functions and automatically produce reference manuals from them - perlapi is one such manual which details all the functions which are available to XS writers. perlintern is the autogenerated manual for the functions which are not part of the API and are supposedly for internal use only.

Source documentation is created by putting POD comments into the C source, like this:

 /*
 =for apidoc sv_setiv

 Copies an integer into the given SV.  Does not handle 'set' magic.  See
 C<sv_setiv_mg>.

 =cut
 */

Please try and supply some documentation if you add functions to the Perl core.

Backwards compatibility

The Perl API changes over time. New functions are added or the interfaces of existing functions are changed. The Devel::PPPort module tries to provide compatibility code for some of these changes, so XS writers don't have to code it themselves when supporting multiple versions of Perl.

Devel::PPPort generates a C header file ppport.h that can also be run as a Perl script. To generate ppport.h, run:

    perl -MDevel::PPPort -eDevel::PPPort::WriteFile

Besides checking existing XS code, the script can also be used to retrieve compatibility information for various API calls using the --api-info command line switch. For example:

  % perl ppport.h --api-info=sv_magicext

For details, see perldoc ppport.h.

Unicode Support

Perl 5.6.0 introduced Unicode support. It's important for porters and XS writers to understand this support and make sure that the code they write does not corrupt Unicode data.

What is Unicode, anyway?

In the olden, less enlightened times, we all used to use ASCII. Most of us did, anyway. The big problem with ASCII is that it's American. Well, no, that's not actually the problem; the problem is that it's not particularly useful for people who don't use the Roman alphabet. What used to happen was that particular languages would stick their own alphabet in the upper range of the sequence, between 128 and 255. Of course, we then ended up with plenty of variants that weren't quite ASCII, and the whole point of it being a standard was lost.

Worse still, if you've got a language like Chinese or Japanese that has hundreds or thousands of characters, then you really can't fit them into a mere 256, so they had to forget about ASCII altogether, and build their own systems using pairs of numbers to refer to one character.

To fix this, some people formed Unicode, Inc. and produced a new character set containing all the characters you can possibly think of and more. There are several ways of representing these characters, and the one Perl uses is called UTF-8. UTF-8 uses a variable number of bytes to represent a character. You can learn more about Unicode and Perl's Unicode model in perlunicode.

How can I recognise a UTF-8 string?

You can't. This is because UTF-8 data is stored in bytes just like non-UTF-8 data. The Unicode character 200, (0xC8 for you hex types) capital E with a grave accent, is represented by the two bytes v196.172. Unfortunately, the non-Unicode string chr(196).chr(172) has that byte sequence as well. So you can't tell just by looking - this is what makes Unicode input an interesting problem.

In general, you either have to know what you're dealing with, or you have to guess. The API function is_utf8_string can help; it'll tell you if a string contains only valid UTF-8 characters. However, it can't do the work for you. On a character-by-character basis, is_utf8_char_buf will tell you whether the current character in a string is valid UTF-8.

How does UTF-8 represent Unicode characters?

As mentioned above, UTF-8 uses a variable number of bytes to store a character. Characters with values 0...127 are stored in one byte, just like good ol' ASCII. Character 128 is stored as v194.128; this continues up to character 191, which is v194.191. Now we've run out of bits (191 is binary 10111111) so we move on; 192 is v195.128. And so it goes on, moving to three bytes at character 2048.

Assuming you know you're dealing with a UTF-8 string, you can find out how long the first character in it is with the UTF8SKIP macro:

    char *utf = "\305\233\340\240\201";
    I32 len;

    len = UTF8SKIP(utf); /* len is 2 here */
    utf += len;
    len = UTF8SKIP(utf); /* len is 3 here */

Another way to skip over characters in a UTF-8 string is to use utf8_hop, which takes a string and a number of characters to skip over. You're on your own about bounds checking, though, so don't use it lightly.

All bytes in a multi-byte UTF-8 character will have the high bit set, so you can test if you need to do something special with this character like this (the UTF8_IS_INVARIANT() is a macro that tests whether the byte can be encoded as a single byte even in UTF-8):

    U8 *utf;
    U8 *utf_end; /* 1 beyond buffer pointed to by utf */
    UV uv;      /* Note: a UV, not a U8, not a char */
    STRLEN len; /* length of character in bytes */

    if (!UTF8_IS_INVARIANT(*utf))
        /* Must treat this as UTF-8 */
        uv = utf8_to_uvchr_buf(utf, utf_end, &len);
    else
        /* OK to treat this character as a byte */
        uv = *utf;

You can also see in that example that we use utf8_to_uvchr_buf to get the value of the character; the inverse function uvchr_to_utf8 is available for putting a UV into UTF-8:

    if (!UTF8_IS_INVARIANT(uv))
        /* Must treat this as UTF8 */
        utf8 = uvchr_to_utf8(utf8, uv);
    else
        /* OK to treat this character as a byte */
        *utf8++ = uv;

You must convert characters to UVs using the above functions if you're ever in a situation where you have to match UTF-8 and non-UTF-8 characters. You may not skip over UTF-8 characters in this case. If you do this, you'll lose the ability to match hi-bit non-UTF-8 characters; for instance, if your UTF-8 string contains v196.172, and you skip that character, you can never match a chr(200) in a non-UTF-8 string. So don't do that!

How does Perl store UTF-8 strings?

Currently, Perl deals with Unicode strings and non-Unicode strings slightly differently. A flag in the SV, SVf_UTF8, indicates that the string is internally encoded as UTF-8. Without it, the byte value is the codepoint number and vice versa (in other words, the string is encoded as iso-8859-1, but use feature 'unicode_strings' is needed to get iso-8859-1 semantics). You can check and manipulate this flag with the following macros:

    SvUTF8(sv)
    SvUTF8_on(sv)
    SvUTF8_off(sv)

This flag has an important effect on Perl's treatment of the string: if Unicode data is not properly distinguished, regular expressions, length, substr and other string handling operations will have undesirable results.

The problem comes when you have, for instance, a string that isn't flagged as UTF-8, and contains a byte sequence that could be UTF-8 - especially when combining non-UTF-8 and UTF-8 strings.

Never forget that the SVf_UTF8 flag is separate to the PV value; you need be sure you don't accidentally knock it off while you're manipulating SVs. More specifically, you cannot expect to do this:

    SV *sv;
    SV *nsv;
    STRLEN len;
    char *p;

    p = SvPV(sv, len);
    frobnicate(p);
    nsv = newSVpvn(p, len);

The char* string does not tell you the whole story, and you can't copy or reconstruct an SV just by copying the string value. Check if the old SV has the UTF8 flag set, and act accordingly:

    p = SvPV(sv, len);
    frobnicate(p);
    nsv = newSVpvn(p, len);
    if (SvUTF8(sv))
        SvUTF8_on(nsv);

In fact, your frobnicate function should be made aware of whether or not it's dealing with UTF-8 data, so that it can handle the string appropriately.

Since just passing an SV to an XS function and copying the data of the SV is not enough to copy the UTF8 flags, even less right is just passing a char * to an XS function.

How do I convert a string to UTF-8?

If you're mixing UTF-8 and non-UTF-8 strings, it is necessary to upgrade one of the strings to UTF-8. If you've got an SV, the easiest way to do this is:

    sv_utf8_upgrade(sv);

However, you must not do this, for example:

    if (!SvUTF8(left))
        sv_utf8_upgrade(left);

If you do this in a binary operator, you will actually change one of the strings that came into the operator, and, while it shouldn't be noticeable by the end user, it can cause problems in deficient code.

Instead, bytes_to_utf8 will give you a UTF-8-encoded copy of its string argument. This is useful for having the data available for comparisons and so on, without harming the original SV. There's also utf8_to_bytes to go the other way, but naturally, this will fail if the string contains any characters above 255 that can't be represented in a single byte.

Is there anything else I need to know?

Not really. Just remember these things:

  • There's no way to tell if a string is UTF-8 or not. You can tell if an SV is UTF-8 by looking at its SvUTF8 flag. Don't forget to set the flag if something should be UTF-8. Treat the flag as part of the PV, even though it's not - if you pass on the PV to somewhere, pass on the flag too.

  • If a string is UTF-8, always use utf8_to_uvchr_buf to get at the value, unless UTF8_IS_INVARIANT(*s) in which case you can use *s.

  • When writing a character uv to a UTF-8 string, always use uvchr_to_utf8, unless UTF8_IS_INVARIANT(uv)) in which case you can use *s = uv.

  • Mixing UTF-8 and non-UTF-8 strings is tricky. Use bytes_to_utf8 to get a new string which is UTF-8 encoded, and then combine them.

Custom Operators

Custom operator support is an experimental feature that allows you to define your own ops. This is primarily to allow the building of interpreters for other languages in the Perl core, but it also allows optimizations through the creation of "macro-ops" (ops which perform the functions of multiple ops which are usually executed together, such as gvsv, gvsv, add.)

This feature is implemented as a new op type, OP_CUSTOM. The Perl core does not "know" anything special about this op type, and so it will not be involved in any optimizations. This also means that you can define your custom ops to be any op structure - unary, binary, list and so on - you like.

It's important to know what custom operators won't do for you. They won't let you add new syntax to Perl, directly. They won't even let you add new keywords, directly. In fact, they won't change the way Perl compiles a program at all. You have to do those changes yourself, after Perl has compiled the program. You do this either by manipulating the op tree using a CHECK block and the B::Generate module, or by adding a custom peephole optimizer with the optimize module.

When you do this, you replace ordinary Perl ops with custom ops by creating ops with the type OP_CUSTOM and the op_ppaddr of your own PP function. This should be defined in XS code, and should look like the PP ops in pp_*.c. You are responsible for ensuring that your op takes the appropriate number of values from the stack, and you are responsible for adding stack marks if necessary.

You should also "register" your op with the Perl interpreter so that it can produce sensible error and warning messages. Since it is possible to have multiple custom ops within the one "logical" op type OP_CUSTOM, Perl uses the value of o->op_ppaddr to determine which custom op it is dealing with. You should create an XOP structure for each ppaddr you use, set the properties of the custom op with XopENTRY_set, and register the structure against the ppaddr using Perl_custom_op_register. A trivial example might look like:

    static XOP my_xop;
    static OP *my_pp(pTHX);

    BOOT:
        XopENTRY_set(&my_xop, xop_name, "myxop");
        XopENTRY_set(&my_xop, xop_desc, "Useless custom op");
        Perl_custom_op_register(aTHX_ my_pp, &my_xop);

The available fields in the structure are:

xop_name

A short name for your op. This will be included in some error messages, and will also be returned as $op->name by the B module, so it will appear in the output of module like B::Concise.

xop_desc

A short description of the function of the op.

xop_class

Which of the various *OP structures this op uses. This should be one of the OA_* constants from op.h, namely

OA_BASEOP
OA_UNOP
OA_BINOP
OA_LOGOP
OA_LISTOP
OA_PMOP
OA_SVOP
OA_PADOP
OA_PVOP_OR_SVOP

This should be interpreted as 'PVOP' only. The _OR_SVOP is because the only core PVOP, OP_TRANS, can sometimes be a SVOP instead.

OA_LOOP
OA_COP

The other OA_* constants should not be used.

xop_peep

This member is of type Perl_cpeep_t, which expands to void (*Perl_cpeep_t)(aTHX_ OP *o, OP *oldop). If it is set, this function will be called from Perl_rpeep when ops of this type are encountered by the peephole optimizer. o is the OP that needs optimizing; oldop is the previous OP optimized, whose op_next points to o.

B::Generate directly supports the creation of custom ops by name.

AUTHORS

Until May 1997, this document was maintained by Jeff Okamoto <okamoto@corp.hp.com>. It is now maintained as part of Perl itself by the Perl 5 Porters <perl5-porters@perl.org>.

With lots of help and suggestions from Dean Roehrich, Malcolm Beattie, Andreas Koenig, Paul Hudson, Ilya Zakharevich, Paul Marquess, Neil Bowers, Matthew Green, Tim Bunce, Spider Boardman, Ulrich Pfeifer, Stephen McCamant, and Gurusamy Sarathy.

SEE ALSO

perlapi, perlintern, perlxs, perlembed

ПЕРЕВОДЧИКИ

  • Андрей Асякин <asan999 at gmail.com>

  • Николай Мишин <mi at ya.ru>