[ Index ]

PHP Cross Reference of Unnamed Project




/se3-unattended/var/se3/unattended/install/linuxaux/opt/perl/lib/5.10.0/pod/ -> perlguts.pod (source)

   1  =head1 NAME
   3  perlguts - Introduction to the Perl API
   5  =head1 DESCRIPTION
   7  This document attempts to describe how to use the Perl API, as well as
   8  to provide some info on the basic workings of the Perl core. It is far
   9  from complete and probably contains many errors. Please refer any
  10  questions or comments to the author below.
  12  =head1 Variables
  14  =head2 Datatypes
  16  Perl has three typedefs that handle Perl's three main data types:
  18      SV  Scalar Value
  19      AV  Array Value
  20      HV  Hash Value
  22  Each typedef has specific routines that manipulate the various data types.
  24  =head2 What is an "IV"?
  26  Perl uses a special typedef IV which is a simple signed integer type that is
  27  guaranteed to be large enough to hold a pointer (as well as an integer).
  28  Additionally, there is the UV, which is simply an unsigned IV.
  30  Perl also uses two special typedefs, I32 and I16, which will always be at
  31  least 32-bits and 16-bits long, respectively. (Again, there are U32 and U16,
  32  as well.)  They will usually be exactly 32 and 16 bits long, but on Crays
  33  they will both be 64 bits.
  35  =head2 Working with SVs
  37  An SV can be created and loaded with one command.  There are five types of
  38  values that can be loaded: an integer value (IV), an unsigned integer
  39  value (UV), a double (NV), a string (PV), and another scalar (SV).
  41  The seven routines are:
  43      SV*  newSViv(IV);
  44      SV*  newSVuv(UV);
  45      SV*  newSVnv(double);
  46      SV*  newSVpv(const char*, STRLEN);
  47      SV*  newSVpvn(const char*, STRLEN);
  48      SV*  newSVpvf(const char*, ...);
  49      SV*  newSVsv(SV*);
  51  C<STRLEN> is an integer type (Size_t, usually defined as size_t in
  52  F<config.h>) guaranteed to be large enough to represent the size of
  53  any string that perl can handle.
  55  In the unlikely case of a SV requiring more complex initialisation, you
  56  can create an empty SV with newSV(len).  If C<len> is 0 an empty SV of
  57  type NULL is returned, else an SV of type PV is returned with len + 1 (for
  58  the NUL) bytes of storage allocated, accessible via SvPVX.  In both cases
  59  the SV has value undef.
  61      SV *sv = newSV(0);   /* no storage allocated  */
  62      SV *sv = newSV(10);  /* 10 (+1) bytes of uninitialised storage allocated  */
  64  To change the value of an I<already-existing> SV, there are eight routines:
  66      void  sv_setiv(SV*, IV);
  67      void  sv_setuv(SV*, UV);
  68      void  sv_setnv(SV*, double);
  69      void  sv_setpv(SV*, const char*);
  70      void  sv_setpvn(SV*, const char*, STRLEN)
  71      void  sv_setpvf(SV*, const char*, ...);
  72      void  sv_vsetpvfn(SV*, const char*, STRLEN, va_list *, SV **, I32, bool *);
  73      void  sv_setsv(SV*, SV*);
  75  Notice that you can choose to specify the length of the string to be
  76  assigned by using C<sv_setpvn>, C<newSVpvn>, or C<newSVpv>, or you may
  77  allow Perl to calculate the length by using C<sv_setpv> or by specifying
  78  0 as the second argument to C<newSVpv>.  Be warned, though, that Perl will
  79  determine the string's length by using C<strlen>, which depends on the
  80  string terminating with a NUL character.
  82  The arguments of C<sv_setpvf> are processed like C<sprintf>, and the
  83  formatted output becomes the value.
  85  C<sv_vsetpvfn> is an analogue of C<vsprintf>, but it allows you to specify
  86  either a pointer to a variable argument list or the address and length of
  87  an array of SVs.  The last argument points to a boolean; on return, if that
  88  boolean is true, then locale-specific information has been used to format
  89  the string, and the string's contents are therefore untrustworthy (see
  90  L<perlsec>).  This pointer may be NULL if that information is not
  91  important.  Note that this function requires you to specify the length of
  92  the format.
  94  The C<sv_set*()> functions are not generic enough to operate on values
  95  that have "magic".  See L<Magic Virtual Tables> later in this document.
  97  All SVs that contain strings should be terminated with a NUL character.
  98  If it is not NUL-terminated there is a risk of
  99  core dumps and corruptions from code which passes the string to C
 100  functions or system calls which expect a NUL-terminated string.
 101  Perl's own functions typically add a trailing NUL for this reason.
 102  Nevertheless, you should be very careful when you pass a string stored
 103  in an SV to a C function or system call.
 105  To access the actual value that an SV points to, you can use the macros:
 107      SvIV(SV*)
 108      SvUV(SV*)
 109      SvNV(SV*)
 110      SvPV(SV*, STRLEN len)
 111      SvPV_nolen(SV*)
 113  which will automatically coerce the actual scalar type into an IV, UV, double,
 114  or string.
 116  In the C<SvPV> macro, the length of the string returned is placed into the
 117  variable C<len> (this is a macro, so you do I<not> use C<&len>).  If you do
 118  not care what the length of the data is, use the C<SvPV_nolen> macro.
 119  Historically the C<SvPV> macro with the global variable C<PL_na> has been
 120  used in this case.  But that can be quite inefficient because C<PL_na> must
 121  be accessed in thread-local storage in threaded Perl.  In any case, remember
 122  that Perl allows arbitrary strings of data that may both contain NULs and
 123  might not be terminated by a NUL.
 125  Also remember that C doesn't allow you to safely say C<foo(SvPV(s, len),
 126  len);>. It might work with your compiler, but it won't work for everyone.
 127  Break this sort of statement up into separate assignments:
 129      SV *s;
 130      STRLEN len;
 131      char * ptr;
 132      ptr = SvPV(s, len);
 133      foo(ptr, len);
 135  If you want to know if the scalar value is TRUE, you can use:
 137      SvTRUE(SV*)
 139  Although Perl will automatically grow strings for you, if you need to force
 140  Perl to allocate more memory for your SV, you can use the macro
 142      SvGROW(SV*, STRLEN newlen)
 144  which will determine if more memory needs to be allocated.  If so, it will
 145  call the function C<sv_grow>.  Note that C<SvGROW> can only increase, not
 146  decrease, the allocated memory of an SV and that it does not automatically
 147  add a byte for the a trailing NUL (perl's own string functions typically do
 148  C<SvGROW(sv, len + 1)>).
 150  If you have an SV and want to know what kind of data Perl thinks is stored
 151  in it, you can use the following macros to check the type of SV you have.
 153      SvIOK(SV*)
 154      SvNOK(SV*)
 155      SvPOK(SV*)
 157  You can get and set the current length of the string stored in an SV with
 158  the following macros:
 160      SvCUR(SV*)
 161      SvCUR_set(SV*, I32 val)
 163  You can also get a pointer to the end of the string stored in the SV
 164  with the macro:
 166      SvEND(SV*)
 168  But note that these last three macros are valid only if C<SvPOK()> is true.
 170  If you want to append something to the end of string stored in an C<SV*>,
 171  you can use the following functions:
 173      void  sv_catpv(SV*, const char*);
 174      void  sv_catpvn(SV*, const char*, STRLEN);
 175      void  sv_catpvf(SV*, const char*, ...);
 176      void  sv_vcatpvfn(SV*, const char*, STRLEN, va_list *, SV **, I32, bool);
 177      void  sv_catsv(SV*, SV*);
 179  The first function calculates the length of the string to be appended by
 180  using C<strlen>.  In the second, you specify the length of the string
 181  yourself.  The third function processes its arguments like C<sprintf> and
 182  appends the formatted output.  The fourth function works like C<vsprintf>.
 183  You can specify the address and length of an array of SVs instead of the
 184  va_list argument. The fifth function extends the string stored in the first
 185  SV with the string stored in the second SV.  It also forces the second SV
 186  to be interpreted as a string.
 188  The C<sv_cat*()> functions are not generic enough to operate on values that
 189  have "magic".  See L<Magic Virtual Tables> later in this document.
 191  If you know the name of a scalar variable, you can get a pointer to its SV
 192  by using the following:
 194      SV*  get_sv("package::varname", FALSE);
 196  This returns NULL if the variable does not exist.
 198  If you want to know if this variable (or any other SV) is actually C<defined>,
 199  you can call:
 201      SvOK(SV*)
 203  The scalar C<undef> value is stored in an SV instance called C<PL_sv_undef>.
 205  Its address can be used whenever an C<SV*> is needed. Make sure that
 206  you don't try to compare a random sv with C<&PL_sv_undef>. For example
 207  when interfacing Perl code, it'll work correctly for:
 209    foo(undef);
 211  But won't work when called as:
 213    $x = undef;
 214    foo($x);
 216  So to repeat always use SvOK() to check whether an sv is defined.
 218  Also you have to be careful when using C<&PL_sv_undef> as a value in
 219  AVs or HVs (see L<AVs, HVs and undefined values>).
 221  There are also the two values C<PL_sv_yes> and C<PL_sv_no>, which contain
 222  boolean TRUE and FALSE values, respectively.  Like C<PL_sv_undef>, their
 223  addresses can be used whenever an C<SV*> is needed.
 225  Do not be fooled into thinking that C<(SV *) 0> is the same as C<&PL_sv_undef>.
 226  Take this code:
 228      SV* sv = (SV*) 0;
 229      if (I-am-to-return-a-real-value) {
 230              sv = sv_2mortal(newSViv(42));
 231      }
 232      sv_setsv(ST(0), sv);
 234  This code tries to return a new SV (which contains the value 42) if it should
 235  return a real value, or undef otherwise.  Instead it has returned a NULL
 236  pointer which, somewhere down the line, will cause a segmentation violation,
 237  bus error, or just weird results.  Change the zero to C<&PL_sv_undef> in the
 238  first line and all will be well.
 240  To free an SV that you've created, call C<SvREFCNT_dec(SV*)>.  Normally this
 241  call is not necessary (see L<Reference Counts and Mortality>).
 243  =head2 Offsets
 245  Perl provides the function C<sv_chop> to efficiently remove characters
 246  from the beginning of a string; you give it an SV and a pointer to
 247  somewhere inside the PV, and it discards everything before the
 248  pointer. The efficiency comes by means of a little hack: instead of
 249  actually removing the characters, C<sv_chop> sets the flag C<OOK>
 250  (offset OK) to signal to other functions that the offset hack is in
 251  effect, and it puts the number of bytes chopped off into the IV field
 252  of the SV. It then moves the PV pointer (called C<SvPVX>) forward that
 253  many bytes, and adjusts C<SvCUR> and C<SvLEN>.
 255  Hence, at this point, the start of the buffer that we allocated lives
 256  at C<SvPVX(sv) - SvIV(sv)> in memory and the PV pointer is pointing
 257  into the middle of this allocated storage.
 259  This is best demonstrated by example:
 261    % ./perl -Ilib -MDevel::Peek -le '$a="12345"; $a=~s/.//; Dump($a)'
 262    SV = PVIV(0x8128450) at 0x81340f0
 263      REFCNT = 1
 264      FLAGS = (POK,OOK,pPOK)
 265      IV = 1  (OFFSET)
 266      PV = 0x8135781 ( "1" . ) "2345"\0
 267      CUR = 4
 268      LEN = 5
 270  Here the number of bytes chopped off (1) is put into IV, and
 271  C<Devel::Peek::Dump> helpfully reminds us that this is an offset. The
 272  portion of the string between the "real" and the "fake" beginnings is
 273  shown in parentheses, and the values of C<SvCUR> and C<SvLEN> reflect
 274  the fake beginning, not the real one.
 276  Something similar to the offset hack is performed on AVs to enable
 277  efficient shifting and splicing off the beginning of the array; while
 278  C<AvARRAY> points to the first element in the array that is visible from
 279  Perl, C<AvALLOC> points to the real start of the C array. These are
 280  usually the same, but a C<shift> operation can be carried out by
 281  increasing C<AvARRAY> by one and decreasing C<AvFILL> and C<AvLEN>.
 282  Again, the location of the real start of the C array only comes into
 283  play when freeing the array. See C<av_shift> in F<av.c>.
 285  =head2 What's Really Stored in an SV?
 287  Recall that the usual method of determining the type of scalar you have is
 288  to use C<Sv*OK> macros.  Because a scalar can be both a number and a string,
 289  usually these macros will always return TRUE and calling the C<Sv*V>
 290  macros will do the appropriate conversion of string to integer/double or
 291  integer/double to string.
 293  If you I<really> need to know if you have an integer, double, or string
 294  pointer in an SV, you can use the following three macros instead:
 296      SvIOKp(SV*)
 297      SvNOKp(SV*)
 298      SvPOKp(SV*)
 300  These will tell you if you truly have an integer, double, or string pointer
 301  stored in your SV.  The "p" stands for private.
 303  The are various ways in which the private and public flags may differ.
 304  For example, a tied SV may have a valid underlying value in the IV slot
 305  (so SvIOKp is true), but the data should be accessed via the FETCH
 306  routine rather than directly, so SvIOK is false. Another is when
 307  numeric conversion has occurred and precision has been lost: only the
 308  private flag is set on 'lossy' values. So when an NV is converted to an
 309  IV with loss, SvIOKp, SvNOKp and SvNOK will be set, while SvIOK wont be.
 311  In general, though, it's best to use the C<Sv*V> macros.
 313  =head2 Working with AVs
 315  There are two ways to create and load an AV.  The first method creates an
 316  empty AV:
 318      AV*  newAV();
 320  The second method both creates the AV and initially populates it with SVs:
 322      AV*  av_make(I32 num, SV **ptr);
 324  The second argument points to an array containing C<num> C<SV*>'s.  Once the
 325  AV has been created, the SVs can be destroyed, if so desired.
 327  Once the AV has been created, the following operations are possible on AVs:
 329      void  av_push(AV*, SV*);
 330      SV*   av_pop(AV*);
 331      SV*   av_shift(AV*);
 332      void  av_unshift(AV*, I32 num);
 334  These should be familiar operations, with the exception of C<av_unshift>.
 335  This routine adds C<num> elements at the front of the array with the C<undef>
 336  value.  You must then use C<av_store> (described below) to assign values
 337  to these new elements.
 339  Here are some other functions:
 341      I32   av_len(AV*);
 342      SV**  av_fetch(AV*, I32 key, I32 lval);
 343      SV**  av_store(AV*, I32 key, SV* val);
 345  The C<av_len> function returns the highest index value in array (just
 346  like $#array in Perl).  If the array is empty, -1 is returned.  The
 347  C<av_fetch> function returns the value at index C<key>, but if C<lval>
 348  is non-zero, then C<av_fetch> will store an undef value at that index.
 349  The C<av_store> function stores the value C<val> at index C<key>, and does
 350  not increment the reference count of C<val>.  Thus the caller is responsible
 351  for taking care of that, and if C<av_store> returns NULL, the caller will
 352  have to decrement the reference count to avoid a memory leak.  Note that
 353  C<av_fetch> and C<av_store> both return C<SV**>'s, not C<SV*>'s as their
 354  return value.
 356      void  av_clear(AV*);
 357      void  av_undef(AV*);
 358      void  av_extend(AV*, I32 key);
 360  The C<av_clear> function deletes all the elements in the AV* array, but
 361  does not actually delete the array itself.  The C<av_undef> function will
 362  delete all the elements in the array plus the array itself.  The
 363  C<av_extend> function extends the array so that it contains at least C<key+1>
 364  elements.  If C<key+1> is less than the currently allocated length of the array,
 365  then nothing is done.
 367  If you know the name of an array variable, you can get a pointer to its AV
 368  by using the following:
 370      AV*  get_av("package::varname", FALSE);
 372  This returns NULL if the variable does not exist.
 374  See L<Understanding the Magic of Tied Hashes and Arrays> for more
 375  information on how to use the array access functions on tied arrays.
 377  =head2 Working with HVs
 379  To create an HV, you use the following routine:
 381      HV*  newHV();
 383  Once the HV has been created, the following operations are possible on HVs:
 385      SV**  hv_store(HV*, const char* key, U32 klen, SV* val, U32 hash);
 386      SV**  hv_fetch(HV*, const char* key, U32 klen, I32 lval);
 388  The C<klen> parameter is the length of the key being passed in (Note that
 389  you cannot pass 0 in as a value of C<klen> to tell Perl to measure the
 390  length of the key).  The C<val> argument contains the SV pointer to the
 391  scalar being stored, and C<hash> is the precomputed hash value (zero if
 392  you want C<hv_store> to calculate it for you).  The C<lval> parameter
 393  indicates whether this fetch is actually a part of a store operation, in
 394  which case a new undefined value will be added to the HV with the supplied
 395  key and C<hv_fetch> will return as if the value had already existed.
 397  Remember that C<hv_store> and C<hv_fetch> return C<SV**>'s and not just
 398  C<SV*>.  To access the scalar value, you must first dereference the return
 399  value.  However, you should check to make sure that the return value is
 400  not NULL before dereferencing it.
 402  These two functions check if a hash table entry exists, and deletes it.
 404      bool  hv_exists(HV*, const char* key, U32 klen);
 405      SV*   hv_delete(HV*, const char* key, U32 klen, I32 flags);
 407  If C<flags> does not include the C<G_DISCARD> flag then C<hv_delete> will
 408  create and return a mortal copy of the deleted value.
 410  And more miscellaneous functions:
 412      void   hv_clear(HV*);
 413      void   hv_undef(HV*);
 415  Like their AV counterparts, C<hv_clear> deletes all the entries in the hash
 416  table but does not actually delete the hash table.  The C<hv_undef> deletes
 417  both the entries and the hash table itself.
 419  Perl keeps the actual data in linked list of structures with a typedef of HE.
 420  These contain the actual key and value pointers (plus extra administrative
 421  overhead).  The key is a string pointer; the value is an C<SV*>.  However,
 422  once you have an C<HE*>, to get the actual key and value, use the routines
 423  specified below.
 425      I32    hv_iterinit(HV*);
 426              /* Prepares starting point to traverse hash table */
 427      HE*    hv_iternext(HV*);
 428              /* Get the next entry, and return a pointer to a
 429                 structure that has both the key and value */
 430      char*  hv_iterkey(HE* entry, I32* retlen);
 431              /* Get the key from an HE structure and also return
 432                 the length of the key string */
 433      SV*    hv_iterval(HV*, HE* entry);
 434              /* Return an SV pointer to the value of the HE
 435                 structure */
 436      SV*    hv_iternextsv(HV*, char** key, I32* retlen);
 437              /* This convenience routine combines hv_iternext,
 438             hv_iterkey, and hv_iterval.  The key and retlen
 439             arguments are return values for the key and its
 440             length.  The value is returned in the SV* argument */
 442  If you know the name of a hash variable, you can get a pointer to its HV
 443  by using the following:
 445      HV*  get_hv("package::varname", FALSE);
 447  This returns NULL if the variable does not exist.
 449  The hash algorithm is defined in the C<PERL_HASH(hash, key, klen)> macro:
 451      hash = 0;
 452      while (klen--)
 453      hash = (hash * 33) + *key++;
 454      hash = hash + (hash >> 5);            /* after 5.6 */
 456  The last step was added in version 5.6 to improve distribution of
 457  lower bits in the resulting hash value.
 459  See L<Understanding the Magic of Tied Hashes and Arrays> for more
 460  information on how to use the hash access functions on tied hashes.
 462  =head2 Hash API Extensions
 464  Beginning with version 5.004, the following functions are also supported:
 466      HE*     hv_fetch_ent  (HV* tb, SV* key, I32 lval, U32 hash);
 467      HE*     hv_store_ent  (HV* tb, SV* key, SV* val, U32 hash);
 469      bool    hv_exists_ent (HV* tb, SV* key, U32 hash);
 470      SV*     hv_delete_ent (HV* tb, SV* key, I32 flags, U32 hash);
 472      SV*     hv_iterkeysv  (HE* entry);
 474  Note that these functions take C<SV*> keys, which simplifies writing
 475  of extension code that deals with hash structures.  These functions
 476  also allow passing of C<SV*> keys to C<tie> functions without forcing
 477  you to stringify the keys (unlike the previous set of functions).
 479  They also return and accept whole hash entries (C<HE*>), making their
 480  use more efficient (since the hash number for a particular string
 481  doesn't have to be recomputed every time).  See L<perlapi> for detailed
 482  descriptions.
 484  The following macros must always be used to access the contents of hash
 485  entries.  Note that the arguments to these macros must be simple
 486  variables, since they may get evaluated more than once.  See
 487  L<perlapi> for detailed descriptions of these macros.
 489      HePV(HE* he, STRLEN len)
 490      HeVAL(HE* he)
 491      HeHASH(HE* he)
 492      HeSVKEY(HE* he)
 493      HeSVKEY_force(HE* he)
 494      HeSVKEY_set(HE* he, SV* sv)
 496  These two lower level macros are defined, but must only be used when
 497  dealing with keys that are not C<SV*>s:
 499      HeKEY(HE* he)
 500      HeKLEN(HE* he)
 502  Note that both C<hv_store> and C<hv_store_ent> do not increment the
 503  reference count of the stored C<val>, which is the caller's responsibility.
 504  If these functions return a NULL value, the caller will usually have to
 505  decrement the reference count of C<val> to avoid a memory leak.
 507  =head2 AVs, HVs and undefined values
 509  Sometimes you have to store undefined values in AVs or HVs. Although
 510  this may be a rare case, it can be tricky. That's because you're
 511  used to using C<&PL_sv_undef> if you need an undefined SV.
 513  For example, intuition tells you that this XS code:
 515      AV *av = newAV();
 516      av_store( av, 0, &PL_sv_undef );
 518  is equivalent to this Perl code:
 520      my @av;
 521      $av[0] = undef;
 523  Unfortunately, this isn't true. AVs use C<&PL_sv_undef> as a marker
 524  for indicating that an array element has not yet been initialized.
 525  Thus, C<exists $av[0]> would be true for the above Perl code, but
 526  false for the array generated by the XS code.
 528  Other problems can occur when storing C<&PL_sv_undef> in HVs:
 530      hv_store( hv, "key", 3, &PL_sv_undef, 0 );
 532  This will indeed make the value C<undef>, but if you try to modify
 533  the value of C<key>, you'll get the following error:
 535      Modification of non-creatable hash value attempted
 537  In perl 5.8.0, C<&PL_sv_undef> was also used to mark placeholders
 538  in restricted hashes. This caused such hash entries not to appear
 539  when iterating over the hash or when checking for the keys
 540  with the C<hv_exists> function.
 542  You can run into similar problems when you store C<&PL_sv_true> or
 543  C<&PL_sv_false> into AVs or HVs. Trying to modify such elements
 544  will give you the following error:
 546      Modification of a read-only value attempted
 548  To make a long story short, you can use the special variables
 549  C<&PL_sv_undef>, C<&PL_sv_true> and C<&PL_sv_false> with AVs and
 550  HVs, but you have to make sure you know what you're doing.
 552  Generally, if you want to store an undefined value in an AV
 553  or HV, you should not use C<&PL_sv_undef>, but rather create a
 554  new undefined value using the C<newSV> function, for example:
 556      av_store( av, 42, newSV(0) );
 557      hv_store( hv, "foo", 3, newSV(0), 0 );
 559  =head2 References
 561  References are a special type of scalar that point to other data types
 562  (including references).
 564  To create a reference, use either of the following functions:
 566      SV* newRV_inc((SV*) thing);
 567      SV* newRV_noinc((SV*) thing);
 569  The C<thing> argument can be any of an C<SV*>, C<AV*>, or C<HV*>.  The
 570  functions are identical except that C<newRV_inc> increments the reference
 571  count of the C<thing>, while C<newRV_noinc> does not.  For historical
 572  reasons, C<newRV> is a synonym for C<newRV_inc>.
 574  Once you have a reference, you can use the following macro to dereference
 575  the reference:
 577      SvRV(SV*)
 579  then call the appropriate routines, casting the returned C<SV*> to either an
 580  C<AV*> or C<HV*>, if required.
 582  To determine if an SV is a reference, you can use the following macro:
 584      SvROK(SV*)
 586  To discover what type of value the reference refers to, use the following
 587  macro and then check the return value.
 589      SvTYPE(SvRV(SV*))
 591  The most useful types that will be returned are:
 593      SVt_IV    Scalar
 594      SVt_NV    Scalar
 595      SVt_PV    Scalar
 596      SVt_RV    Scalar
 597      SVt_PVAV  Array
 598      SVt_PVHV  Hash
 599      SVt_PVCV  Code
 600      SVt_PVGV  Glob (possible a file handle)
 601      SVt_PVMG  Blessed or Magical Scalar
 603      See the sv.h header file for more details.
 605  =head2 Blessed References and Class Objects
 607  References are also used to support object-oriented programming.  In perl's
 608  OO lexicon, an object is simply a reference that has been blessed into a
 609  package (or class).  Once blessed, the programmer may now use the reference
 610  to access the various methods in the class.
 612  A reference can be blessed into a package with the following function:
 614      SV* sv_bless(SV* sv, HV* stash);
 616  The C<sv> argument must be a reference value.  The C<stash> argument
 617  specifies which class the reference will belong to.  See
 618  L<Stashes and Globs> for information on converting class names into stashes.
 620  /* Still under construction */
 622  Upgrades rv to reference if not already one.  Creates new SV for rv to
 623  point to.  If C<classname> is non-null, the SV is blessed into the specified
 624  class.  SV is returned.
 626      SV* newSVrv(SV* rv, const char* classname);
 628  Copies integer, unsigned integer or double into an SV whose reference is C<rv>.  SV is blessed
 629  if C<classname> is non-null.
 631      SV* sv_setref_iv(SV* rv, const char* classname, IV iv);
 632      SV* sv_setref_uv(SV* rv, const char* classname, UV uv);
 633      SV* sv_setref_nv(SV* rv, const char* classname, NV iv);
 635  Copies the pointer value (I<the address, not the string!>) into an SV whose
 636  reference is rv.  SV is blessed if C<classname> is non-null.
 638      SV* sv_setref_pv(SV* rv, const char* classname, PV iv);
 640  Copies string into an SV whose reference is C<rv>.  Set length to 0 to let
 641  Perl calculate the string length.  SV is blessed if C<classname> is non-null.
 643      SV* sv_setref_pvn(SV* rv, const char* classname, PV iv, STRLEN length);
 645  Tests whether the SV is blessed into the specified class.  It does not
 646  check inheritance relationships.
 648      int  sv_isa(SV* sv, const char* name);
 650  Tests whether the SV is a reference to a blessed object.
 652      int  sv_isobject(SV* sv);
 654  Tests whether the SV is derived from the specified class. SV can be either
 655  a reference to a blessed object or a string containing a class name. This
 656  is the function implementing the C<UNIVERSAL::isa> functionality.
 658      bool sv_derived_from(SV* sv, const char* name);
 660  To check if you've got an object derived from a specific class you have
 661  to write:
 663      if (sv_isobject(sv) && sv_derived_from(sv, class)) { ... }
 665  =head2 Creating New Variables
 667  To create a new Perl variable with an undef value which can be accessed from
 668  your Perl script, use the following routines, depending on the variable type.
 670      SV*  get_sv("package::varname", TRUE);
 671      AV*  get_av("package::varname", TRUE);
 672      HV*  get_hv("package::varname", TRUE);
 674  Notice the use of TRUE as the second parameter.  The new variable can now
 675  be set, using the routines appropriate to the data type.
 677  There are additional macros whose values may be bitwise OR'ed with the
 678  C<TRUE> argument to enable certain extra features.  Those bits are:
 680  =over
 682  =item GV_ADDMULTI
 684  Marks the variable as multiply defined, thus preventing the:
 686    Name <varname> used only once: possible typo
 688  warning.
 690  =item GV_ADDWARN
 692  Issues the warning:
 694    Had to create <varname> unexpectedly
 696  if the variable did not exist before the function was called.
 698  =back
 700  If you do not specify a package name, the variable is created in the current
 701  package.
 703  =head2 Reference Counts and Mortality
 705  Perl uses a reference count-driven garbage collection mechanism. SVs,
 706  AVs, or HVs (xV for short in the following) start their life with a
 707  reference count of 1.  If the reference count of an xV ever drops to 0,
 708  then it will be destroyed and its memory made available for reuse.
 710  This normally doesn't happen at the Perl level unless a variable is
 711  undef'ed or the last variable holding a reference to it is changed or
 712  overwritten.  At the internal level, however, reference counts can be
 713  manipulated with the following macros:
 715      int SvREFCNT(SV* sv);
 716      SV* SvREFCNT_inc(SV* sv);
 717      void SvREFCNT_dec(SV* sv);
 719  However, there is one other function which manipulates the reference
 720  count of its argument.  The C<newRV_inc> function, you will recall,
 721  creates a reference to the specified argument.  As a side effect,
 722  it increments the argument's reference count.  If this is not what
 723  you want, use C<newRV_noinc> instead.
 725  For example, imagine you want to return a reference from an XSUB function.
 726  Inside the XSUB routine, you create an SV which initially has a reference
 727  count of one.  Then you call C<newRV_inc>, passing it the just-created SV.
 728  This returns the reference as a new SV, but the reference count of the
 729  SV you passed to C<newRV_inc> has been incremented to two.  Now you
 730  return the reference from the XSUB routine and forget about the SV.
 731  But Perl hasn't!  Whenever the returned reference is destroyed, the
 732  reference count of the original SV is decreased to one and nothing happens.
 733  The SV will hang around without any way to access it until Perl itself
 734  terminates.  This is a memory leak.
 736  The correct procedure, then, is to use C<newRV_noinc> instead of
 737  C<newRV_inc>.  Then, if and when the last reference is destroyed,
 738  the reference count of the SV will go to zero and it will be destroyed,
 739  stopping any memory leak.
 741  There are some convenience functions available that can help with the
 742  destruction of xVs.  These functions introduce the concept of "mortality".
 743  An xV that is mortal has had its reference count marked to be decremented,
 744  but not actually decremented, until "a short time later".  Generally the
 745  term "short time later" means a single Perl statement, such as a call to
 746  an XSUB function.  The actual determinant for when mortal xVs have their
 747  reference count decremented depends on two macros, SAVETMPS and FREETMPS.
 748  See L<perlcall> and L<perlxs> for more details on these macros.
 750  "Mortalization" then is at its simplest a deferred C<SvREFCNT_dec>.
 751  However, if you mortalize a variable twice, the reference count will
 752  later be decremented twice.
 754  "Mortal" SVs are mainly used for SVs that are placed on perl's stack.
 755  For example an SV which is created just to pass a number to a called sub
 756  is made mortal to have it cleaned up automatically when it's popped off
 757  the stack. Similarly, results returned by XSUBs (which are pushed on the
 758  stack) are often made mortal.
 760  To create a mortal variable, use the functions:
 762      SV*  sv_newmortal()
 763      SV*  sv_2mortal(SV*)
 764      SV*  sv_mortalcopy(SV*)
 766  The first call creates a mortal SV (with no value), the second converts an existing
 767  SV to a mortal SV (and thus defers a call to C<SvREFCNT_dec>), and the
 768  third creates a mortal copy of an existing SV.
 769  Because C<sv_newmortal> gives the new SV no value,it must normally be given one
 770  via C<sv_setpv>, C<sv_setiv>, etc. :
 772      SV *tmp = sv_newmortal();
 773      sv_setiv(tmp, an_integer);
 775  As that is multiple C statements it is quite common so see this idiom instead:
 777      SV *tmp = sv_2mortal(newSViv(an_integer));
 780  You should be careful about creating mortal variables.  Strange things
 781  can happen if you make the same value mortal within multiple contexts,
 782  or if you make a variable mortal multiple times. Thinking of "Mortalization"
 783  as deferred C<SvREFCNT_dec> should help to minimize such problems.
 784  For example if you are passing an SV which you I<know> has high enough REFCNT
 785  to survive its use on the stack you need not do any mortalization.
 786  If you are not sure then doing an C<SvREFCNT_inc> and C<sv_2mortal>, or
 787  making a C<sv_mortalcopy> is safer.
 789  The mortal routines are not just for SVs -- AVs and HVs can be
 790  made mortal by passing their address (type-casted to C<SV*>) to the
 791  C<sv_2mortal> or C<sv_mortalcopy> routines.
 793  =head2 Stashes and Globs
 795  A B<stash> is a hash that contains all variables that are defined
 796  within a package.  Each key of the stash is a symbol
 797  name (shared by all the different types of objects that have the same
 798  name), and each value in the hash table is a GV (Glob Value).  This GV
 799  in turn contains references to the various objects of that name,
 800  including (but not limited to) the following:
 802      Scalar Value
 803      Array Value
 804      Hash Value
 805      I/O Handle
 806      Format
 807      Subroutine
 809  There is a single stash called C<PL_defstash> that holds the items that exist
 810  in the C<main> package.  To get at the items in other packages, append the
 811  string "::" to the package name.  The items in the C<Foo> package are in
 812  the stash C<Foo::> in PL_defstash.  The items in the C<Bar::Baz> package are
 813  in the stash C<Baz::> in C<Bar::>'s stash.
 815  To get the stash pointer for a particular package, use the function:
 817      HV*  gv_stashpv(const char* name, I32 flags)
 818      HV*  gv_stashsv(SV*, I32 flags)
 820  The first function takes a literal string, the second uses the string stored
 821  in the SV.  Remember that a stash is just a hash table, so you get back an
 822  C<HV*>.  The C<flags> flag will create a new package if it is set to GV_ADD.
 824  The name that C<gv_stash*v> wants is the name of the package whose symbol table
 825  you want.  The default package is called C<main>.  If you have multiply nested
 826  packages, pass their names to C<gv_stash*v>, separated by C<::> as in the Perl
 827  language itself.
 829  Alternately, if you have an SV that is a blessed reference, you can find
 830  out the stash pointer by using:
 832      HV*  SvSTASH(SvRV(SV*));
 834  then use the following to get the package name itself:
 836      char*  HvNAME(HV* stash);
 838  If you need to bless or re-bless an object you can use the following
 839  function:
 841      SV*  sv_bless(SV*, HV* stash)
 843  where the first argument, an C<SV*>, must be a reference, and the second
 844  argument is a stash.  The returned C<SV*> can now be used in the same way
 845  as any other SV.
 847  For more information on references and blessings, consult L<perlref>.
 849  =head2 Double-Typed SVs
 851  Scalar variables normally contain only one type of value, an integer,
 852  double, pointer, or reference.  Perl will automatically convert the
 853  actual scalar data from the stored type into the requested type.
 855  Some scalar variables contain more than one type of scalar data.  For
 856  example, the variable C<$!> contains either the numeric value of C<errno>
 857  or its string equivalent from either C<strerror> or C<sys_errlist[]>.
 859  To force multiple data values into an SV, you must do two things: use the
 860  C<sv_set*v> routines to add the additional scalar type, then set a flag
 861  so that Perl will believe it contains more than one type of data.  The
 862  four macros to set the flags are:
 864      SvIOK_on
 865      SvNOK_on
 866      SvPOK_on
 867      SvROK_on
 869  The particular macro you must use depends on which C<sv_set*v> routine
 870  you called first.  This is because every C<sv_set*v> routine turns on
 871  only the bit for the particular type of data being set, and turns off
 872  all the rest.
 874  For example, to create a new Perl variable called "dberror" that contains
 875  both the numeric and descriptive string error values, you could use the
 876  following code:
 878      extern int  dberror;
 879      extern char *dberror_list;
 881      SV* sv = get_sv("dberror", TRUE);
 882      sv_setiv(sv, (IV) dberror);
 883      sv_setpv(sv, dberror_list[dberror]);
 884      SvIOK_on(sv);
 886  If the order of C<sv_setiv> and C<sv_setpv> had been reversed, then the
 887  macro C<SvPOK_on> would need to be called instead of C<SvIOK_on>.
 889  =head2 Magic Variables
 891  [This section still under construction.  Ignore everything here.  Post no
 892  bills.  Everything not permitted is forbidden.]
 894  Any SV may be magical, that is, it has special features that a normal
 895  SV does not have.  These features are stored in the SV structure in a
 896  linked list of C<struct magic>'s, typedef'ed to C<MAGIC>.
 898      struct magic {
 899          MAGIC*      mg_moremagic;
 900          MGVTBL*     mg_virtual;
 901          U16         mg_private;
 902          char        mg_type;
 903          U8          mg_flags;
 904          I32         mg_len;
 905          SV*         mg_obj;
 906          char*       mg_ptr;
 907      };
 909  Note this is current as of patchlevel 0, and could change at any time.
 911  =head2 Assigning Magic
 913  Perl adds magic to an SV using the sv_magic function:
 915      void sv_magic(SV* sv, SV* obj, int how, const char* name, I32 namlen);
 917  The C<sv> argument is a pointer to the SV that is to acquire a new magical
 918  feature.
 920  If C<sv> is not already magical, Perl uses the C<SvUPGRADE> macro to
 921  convert C<sv> to type C<SVt_PVMG>. Perl then continues by adding new magic
 922  to the beginning of the linked list of magical features.  Any prior entry
 923  of the same type of magic is deleted.  Note that this can be overridden,
 924  and multiple instances of the same type of magic can be associated with an
 925  SV.
 927  The C<name> and C<namlen> arguments are used to associate a string with
 928  the magic, typically the name of a variable. C<namlen> is stored in the
 929  C<mg_len> field and if C<name> is non-null then either a C<savepvn> copy of
 930  C<name> or C<name> itself is stored in the C<mg_ptr> field, depending on
 931  whether C<namlen> is greater than zero or equal to zero respectively.  As a
 932  special case, if C<(name && namlen == HEf_SVKEY)> then C<name> is assumed
 933  to contain an C<SV*> and is stored as-is with its REFCNT incremented.
 935  The sv_magic function uses C<how> to determine which, if any, predefined
 936  "Magic Virtual Table" should be assigned to the C<mg_virtual> field.
 937  See the L<Magic Virtual Tables> section below.  The C<how> argument is also
 938  stored in the C<mg_type> field. The value of C<how> should be chosen
 939  from the set of macros C<PERL_MAGIC_foo> found in F<perl.h>. Note that before
 940  these macros were added, Perl internals used to directly use character
 941  literals, so you may occasionally come across old code or documentation
 942  referring to 'U' magic rather than C<PERL_MAGIC_uvar> for example.
 944  The C<obj> argument is stored in the C<mg_obj> field of the C<MAGIC>
 945  structure.  If it is not the same as the C<sv> argument, the reference
 946  count of the C<obj> object is incremented.  If it is the same, or if
 947  the C<how> argument is C<PERL_MAGIC_arylen>, or if it is a NULL pointer,
 948  then C<obj> is merely stored, without the reference count being incremented.
 950  See also C<sv_magicext> in L<perlapi> for a more flexible way to add magic
 951  to an SV.
 953  There is also a function to add magic to an C<HV>:
 955      void hv_magic(HV *hv, GV *gv, int how);
 957  This simply calls C<sv_magic> and coerces the C<gv> argument into an C<SV>.
 959  To remove the magic from an SV, call the function sv_unmagic:
 961      void sv_unmagic(SV *sv, int type);
 963  The C<type> argument should be equal to the C<how> value when the C<SV>
 964  was initially made magical.
 966  =head2 Magic Virtual Tables
 968  The C<mg_virtual> field in the C<MAGIC> structure is a pointer to an
 969  C<MGVTBL>, which is a structure of function pointers and stands for
 970  "Magic Virtual Table" to handle the various operations that might be
 971  applied to that variable.
 973  The C<MGVTBL> has five (or sometimes eight) pointers to the following
 974  routine types:
 976      int  (*svt_get)(SV* sv, MAGIC* mg);
 977      int  (*svt_set)(SV* sv, MAGIC* mg);
 978      U32  (*svt_len)(SV* sv, MAGIC* mg);
 979      int  (*svt_clear)(SV* sv, MAGIC* mg);
 980      int  (*svt_free)(SV* sv, MAGIC* mg);
 982      int  (*svt_copy)(SV *sv, MAGIC* mg, SV *nsv, const char *name, int namlen);
 983      int  (*svt_dup)(MAGIC *mg, CLONE_PARAMS *param);
 984      int  (*svt_local)(SV *nsv, MAGIC *mg);
 987  This MGVTBL structure is set at compile-time in F<perl.h> and there are
 988  currently 19 types (or 21 with overloading turned on).  These different
 989  structures contain pointers to various routines that perform additional
 990  actions depending on which function is being called.
 992      Function pointer    Action taken
 993      ----------------    ------------
 994      svt_get             Do something before the value of the SV is retrieved.
 995      svt_set             Do something after the SV is assigned a value.
 996      svt_len             Report on the SV's length.
 997      svt_clear           Clear something the SV represents.
 998      svt_free            Free any extra storage associated with the SV.
1000      svt_copy            copy tied variable magic to a tied element
1001      svt_dup             duplicate a magic structure during thread cloning
1002      svt_local           copy magic to local value during 'local'
1004  For instance, the MGVTBL structure called C<vtbl_sv> (which corresponds
1005  to an C<mg_type> of C<PERL_MAGIC_sv>) contains:
1007      { magic_get, magic_set, magic_len, 0, 0 }
1009  Thus, when an SV is determined to be magical and of type C<PERL_MAGIC_sv>,
1010  if a get operation is being performed, the routine C<magic_get> is
1011  called.  All the various routines for the various magical types begin
1012  with C<magic_>.  NOTE: the magic routines are not considered part of
1013  the Perl API, and may not be exported by the Perl library.
1015  The last three slots are a recent addition, and for source code
1016  compatibility they are only checked for if one of the three flags
1017  MGf_COPY, MGf_DUP or MGf_LOCAL is set in mg_flags. This means that most
1018  code can continue declaring a vtable as a 5-element value. These three are
1019  currently used exclusively by the threading code, and are highly subject
1020  to change.
1022  The current kinds of Magic Virtual Tables are:
1024      mg_type
1025      (old-style char and macro)   MGVTBL          Type of magic
1026      --------------------------   ------          -------------
1027      \0 PERL_MAGIC_sv             vtbl_sv         Special scalar variable
1028      A  PERL_MAGIC_overload       vtbl_amagic     %OVERLOAD hash
1029      a  PERL_MAGIC_overload_elem  vtbl_amagicelem %OVERLOAD hash element
1030      c  PERL_MAGIC_overload_table (none)          Holds overload table (AMT)
1031                                                   on stash
1032      B  PERL_MAGIC_bm             vtbl_bm         Boyer-Moore (fast string search)
1033      D  PERL_MAGIC_regdata        vtbl_regdata    Regex match position data
1034                                                   (@+ and @- vars)
1035      d  PERL_MAGIC_regdatum       vtbl_regdatum   Regex match position data
1036                                                   element
1037      E  PERL_MAGIC_env            vtbl_env        %ENV hash
1038      e  PERL_MAGIC_envelem        vtbl_envelem    %ENV hash element
1039      f  PERL_MAGIC_fm             vtbl_fm         Formline ('compiled' format)
1040      g  PERL_MAGIC_regex_global   vtbl_mglob      m//g target / study()ed string
1041      H  PERL_MAGIC_hints          vtbl_sig        %^H hash
1042      h  PERL_MAGIC_hintselem      vtbl_hintselem  %^H hash element
1043      I  PERL_MAGIC_isa            vtbl_isa        @ISA array
1044      i  PERL_MAGIC_isaelem        vtbl_isaelem    @ISA array element
1045      k  PERL_MAGIC_nkeys          vtbl_nkeys      scalar(keys()) lvalue
1046      L  PERL_MAGIC_dbfile         (none)          Debugger %_<filename
1047      l  PERL_MAGIC_dbline         vtbl_dbline     Debugger %_<filename element
1048      o  PERL_MAGIC_collxfrm       vtbl_collxfrm   Locale collate transformation
1049      P  PERL_MAGIC_tied           vtbl_pack       Tied array or hash
1050      p  PERL_MAGIC_tiedelem       vtbl_packelem   Tied array or hash element
1051      q  PERL_MAGIC_tiedscalar     vtbl_packelem   Tied scalar or handle
1052      r  PERL_MAGIC_qr             vtbl_qr         precompiled qr// regex
1053      S  PERL_MAGIC_sig            vtbl_sig        %SIG hash
1054      s  PERL_MAGIC_sigelem        vtbl_sigelem    %SIG hash element
1055      t  PERL_MAGIC_taint          vtbl_taint      Taintedness
1056      U  PERL_MAGIC_uvar           vtbl_uvar       Available for use by extensions
1057      v  PERL_MAGIC_vec            vtbl_vec        vec() lvalue
1058      V  PERL_MAGIC_vstring        (none)          v-string scalars
1059      w  PERL_MAGIC_utf8           vtbl_utf8       UTF-8 length+offset cache
1060      x  PERL_MAGIC_substr         vtbl_substr     substr() lvalue
1061      y  PERL_MAGIC_defelem        vtbl_defelem    Shadow "foreach" iterator
1062                                                   variable / smart parameter
1063                                                   vivification
1064      #  PERL_MAGIC_arylen         vtbl_arylen     Array length ($#ary)
1065      .  PERL_MAGIC_pos            vtbl_pos        pos() lvalue
1066      <  PERL_MAGIC_backref        vtbl_backref    back pointer to a weak ref 
1067      ~  PERL_MAGIC_ext            (none)          Available for use by extensions
1068      :  PERL_MAGIC_symtab         (none)          hash used as symbol table
1069      %  PERL_MAGIC_rhash          (none)          hash used as restricted hash
1070      @  PERL_MAGIC_arylen_p       vtbl_arylen_p   pointer to $#a from @a
1073  When an uppercase and lowercase letter both exist in the table, then the
1074  uppercase letter is typically used to represent some kind of composite type
1075  (a list or a hash), and the lowercase letter is used to represent an element
1076  of that composite type. Some internals code makes use of this case
1077  relationship.  However, 'v' and 'V' (vec and v-string) are in no way related.
1079  The C<PERL_MAGIC_ext> and C<PERL_MAGIC_uvar> magic types are defined
1080  specifically for use by extensions and will not be used by perl itself.
1081  Extensions can use C<PERL_MAGIC_ext> magic to 'attach' private information
1082  to variables (typically objects).  This is especially useful because
1083  there is no way for normal perl code to corrupt this private information
1084  (unlike using extra elements of a hash object).
1086  Similarly, C<PERL_MAGIC_uvar> magic can be used much like tie() to call a
1087  C function any time a scalar's value is used or changed.  The C<MAGIC>'s
1088  C<mg_ptr> field points to a C<ufuncs> structure:
1090      struct ufuncs {
1091          I32 (*uf_val)(pTHX_ IV, SV*);
1092          I32 (*uf_set)(pTHX_ IV, SV*);
1093          IV uf_index;
1094      };
1096  When the SV is read from or written to, the C<uf_val> or C<uf_set>
1097  function will be called with C<uf_index> as the first arg and a pointer to
1098  the SV as the second.  A simple example of how to add C<PERL_MAGIC_uvar>
1099  magic is shown below.  Note that the ufuncs structure is copied by
1100  sv_magic, so you can safely allocate it on the stack.
1102      void
1103      Umagic(sv)
1104          SV *sv;
1105      PREINIT:
1106          struct ufuncs uf;
1107      CODE:
1108          uf.uf_val   = &my_get_fn;
1109          uf.uf_set   = &my_set_fn;
1110          uf.uf_index = 0;
1111          sv_magic(sv, 0, PERL_MAGIC_uvar, (char*)&uf, sizeof(uf));
1113  Attaching C<PERL_MAGIC_uvar> to arrays is permissible but has no effect.
1115  For hashes there is a specialized hook that gives control over hash
1116  keys (but not values).  This hook calls C<PERL_MAGIC_uvar> 'get' magic
1117  if the "set" function in the C<ufuncs> structure is NULL.  The hook
1118  is activated whenever the hash is accessed with a key specified as
1119  an C<SV> through the functions C<hv_store_ent>, C<hv_fetch_ent>,
1120  C<hv_delete_ent>, and C<hv_exists_ent>.  Accessing the key as a string
1121  through the functions without the C<..._ent> suffix circumvents the
1122  hook.  See L<Hash::Util::Fieldhash/Guts> for a detailed description.
1124  Note that because multiple extensions may be using C<PERL_MAGIC_ext>
1125  or C<PERL_MAGIC_uvar> magic, it is important for extensions to take
1126  extra care to avoid conflict.  Typically only using the magic on
1127  objects blessed into the same class as the extension is sufficient.
1128  For C<PERL_MAGIC_ext> magic, it may also be appropriate to add an I32
1129  'signature' at the top of the private data area and check that.
1131  Also note that the C<sv_set*()> and C<sv_cat*()> functions described
1132  earlier do B<not> invoke 'set' magic on their targets.  This must
1133  be done by the user either by calling the C<SvSETMAGIC()> macro after
1134  calling these functions, or by using one of the C<sv_set*_mg()> or
1135  C<sv_cat*_mg()> functions.  Similarly, generic C code must call the
1136  C<SvGETMAGIC()> macro to invoke any 'get' magic if they use an SV
1137  obtained from external sources in functions that don't handle magic.
1138  See L<perlapi> for a description of these functions.
1139  For example, calls to the C<sv_cat*()> functions typically need to be
1140  followed by C<SvSETMAGIC()>, but they don't need a prior C<SvGETMAGIC()>
1141  since their implementation handles 'get' magic.
1143  =head2 Finding Magic
1145      MAGIC* mg_find(SV*, int type); /* Finds the magic pointer of that type */
1147  This routine returns a pointer to the C<MAGIC> structure stored in the SV.
1148  If the SV does not have that magical feature, C<NULL> is returned.  Also,
1149  if the SV is not of type SVt_PVMG, Perl may core dump.
1151      int mg_copy(SV* sv, SV* nsv, const char* key, STRLEN klen);
1153  This routine checks to see what types of magic C<sv> has.  If the mg_type
1154  field is an uppercase letter, then the mg_obj is copied to C<nsv>, but
1155  the mg_type field is changed to be the lowercase letter.
1157  =head2 Understanding the Magic of Tied Hashes and Arrays
1159  Tied hashes and arrays are magical beasts of the C<PERL_MAGIC_tied>
1160  magic type.
1162  WARNING: As of the 5.004 release, proper usage of the array and hash
1163  access functions requires understanding a few caveats.  Some
1164  of these caveats are actually considered bugs in the API, to be fixed
1165  in later releases, and are bracketed with [MAYCHANGE] below. If
1166  you find yourself actually applying such information in this section, be
1167  aware that the behavior may change in the future, umm, without warning.
1169  The perl tie function associates a variable with an object that implements
1170  the various GET, SET, etc methods.  To perform the equivalent of the perl
1171  tie function from an XSUB, you must mimic this behaviour.  The code below
1172  carries out the necessary steps - firstly it creates a new hash, and then
1173  creates a second hash which it blesses into the class which will implement
1174  the tie methods. Lastly it ties the two hashes together, and returns a
1175  reference to the new tied hash.  Note that the code below does NOT call the
1176  TIEHASH method in the MyTie class -
1177  see L<Calling Perl Routines from within C Programs> for details on how
1178  to do this.
1180      SV*
1181      mytie()
1182      PREINIT:
1183          HV *hash;
1184          HV *stash;
1185          SV *tie;
1186      CODE:
1187          hash = newHV();
1188          tie = newRV_noinc((SV*)newHV());
1189          stash = gv_stashpv("MyTie", GV_ADD);
1190          sv_bless(tie, stash);
1191          hv_magic(hash, (GV*)tie, PERL_MAGIC_tied);
1192          RETVAL = newRV_noinc(hash);
1193      OUTPUT:
1194          RETVAL
1196  The C<av_store> function, when given a tied array argument, merely
1197  copies the magic of the array onto the value to be "stored", using
1198  C<mg_copy>.  It may also return NULL, indicating that the value did not
1199  actually need to be stored in the array.  [MAYCHANGE] After a call to
1200  C<av_store> on a tied array, the caller will usually need to call
1201  C<mg_set(val)> to actually invoke the perl level "STORE" method on the
1202  TIEARRAY object.  If C<av_store> did return NULL, a call to
1203  C<SvREFCNT_dec(val)> will also be usually necessary to avoid a memory
1204  leak. [/MAYCHANGE]
1206  The previous paragraph is applicable verbatim to tied hash access using the
1207  C<hv_store> and C<hv_store_ent> functions as well.
1209  C<av_fetch> and the corresponding hash functions C<hv_fetch> and
1210  C<hv_fetch_ent> actually return an undefined mortal value whose magic
1211  has been initialized using C<mg_copy>.  Note the value so returned does not
1212  need to be deallocated, as it is already mortal.  [MAYCHANGE] But you will
1213  need to call C<mg_get()> on the returned value in order to actually invoke
1214  the perl level "FETCH" method on the underlying TIE object.  Similarly,
1215  you may also call C<mg_set()> on the return value after possibly assigning
1216  a suitable value to it using C<sv_setsv>,  which will invoke the "STORE"
1217  method on the TIE object. [/MAYCHANGE]
1220  In other words, the array or hash fetch/store functions don't really
1221  fetch and store actual values in the case of tied arrays and hashes.  They
1222  merely call C<mg_copy> to attach magic to the values that were meant to be
1223  "stored" or "fetched".  Later calls to C<mg_get> and C<mg_set> actually
1224  do the job of invoking the TIE methods on the underlying objects.  Thus
1225  the magic mechanism currently implements a kind of lazy access to arrays
1226  and hashes.
1228  Currently (as of perl version 5.004), use of the hash and array access
1229  functions requires the user to be aware of whether they are operating on
1230  "normal" hashes and arrays, or on their tied variants.  The API may be
1231  changed to provide more transparent access to both tied and normal data
1232  types in future versions.
1233  [/MAYCHANGE]
1235  You would do well to understand that the TIEARRAY and TIEHASH interfaces
1236  are mere sugar to invoke some perl method calls while using the uniform hash
1237  and array syntax.  The use of this sugar imposes some overhead (typically
1238  about two to four extra opcodes per FETCH/STORE operation, in addition to
1239  the creation of all the mortal variables required to invoke the methods).
1240  This overhead will be comparatively small if the TIE methods are themselves
1241  substantial, but if they are only a few statements long, the overhead
1242  will not be insignificant.
1244  =head2 Localizing changes
1246  Perl has a very handy construction
1248    {
1249      local $var = 2;
1250      ...
1251    }
1253  This construction is I<approximately> equivalent to
1255    {
1256      my $oldvar = $var;
1257      $var = 2;
1258      ...
1259      $var = $oldvar;
1260    }
1262  The biggest difference is that the first construction would
1263  reinstate the initial value of $var, irrespective of how control exits
1264  the block: C<goto>, C<return>, C<die>/C<eval>, etc. It is a little bit
1265  more efficient as well.
1267  There is a way to achieve a similar task from C via Perl API: create a
1268  I<pseudo-block>, and arrange for some changes to be automatically
1269  undone at the end of it, either explicit, or via a non-local exit (via
1270  die()). A I<block>-like construct is created by a pair of
1271  C<ENTER>/C<LEAVE> macros (see L<perlcall/"Returning a Scalar">).
1272  Such a construct may be created specially for some important localized
1273  task, or an existing one (like boundaries of enclosing Perl
1274  subroutine/block, or an existing pair for freeing TMPs) may be
1275  used. (In the second case the overhead of additional localization must
1276  be almost negligible.) Note that any XSUB is automatically enclosed in
1277  an C<ENTER>/C<LEAVE> pair.
1279  Inside such a I<pseudo-block> the following service is available:
1281  =over 4
1283  =item C<SAVEINT(int i)>
1285  =item C<SAVEIV(IV i)>
1287  =item C<SAVEI32(I32 i)>
1289  =item C<SAVELONG(long i)>
1291  These macros arrange things to restore the value of integer variable
1292  C<i> at the end of enclosing I<pseudo-block>.
1294  =item C<SAVESPTR(s)>
1296  =item C<SAVEPPTR(p)>
1298  These macros arrange things to restore the value of pointers C<s> and
1299  C<p>. C<s> must be a pointer of a type which survives conversion to
1300  C<SV*> and back, C<p> should be able to survive conversion to C<char*>
1301  and back.
1303  =item C<SAVEFREESV(SV *sv)>
1305  The refcount of C<sv> would be decremented at the end of
1306  I<pseudo-block>.  This is similar to C<sv_2mortal> in that it is also a
1307  mechanism for doing a delayed C<SvREFCNT_dec>.  However, while C<sv_2mortal>
1308  extends the lifetime of C<sv> until the beginning of the next statement,
1309  C<SAVEFREESV> extends it until the end of the enclosing scope.  These
1310  lifetimes can be wildly different.
1312  Also compare C<SAVEMORTALIZESV>.
1314  =item C<SAVEMORTALIZESV(SV *sv)>
1316  Just like C<SAVEFREESV>, but mortalizes C<sv> at the end of the current
1317  scope instead of decrementing its reference count.  This usually has the
1318  effect of keeping C<sv> alive until the statement that called the currently
1319  live scope has finished executing.
1321  =item C<SAVEFREEOP(OP *op)>
1323  The C<OP *> is op_free()ed at the end of I<pseudo-block>.
1325  =item C<SAVEFREEPV(p)>
1327  The chunk of memory which is pointed to by C<p> is Safefree()ed at the
1328  end of I<pseudo-block>.
1330  =item C<SAVECLEARSV(SV *sv)>
1332  Clears a slot in the current scratchpad which corresponds to C<sv> at
1333  the end of I<pseudo-block>.
1335  =item C<SAVEDELETE(HV *hv, char *key, I32 length)>
1337  The key C<key> of C<hv> is deleted at the end of I<pseudo-block>. The
1338  string pointed to by C<key> is Safefree()ed.  If one has a I<key> in
1339  short-lived storage, the corresponding string may be reallocated like
1340  this:
1342    SAVEDELETE(PL_defstash, savepv(tmpbuf), strlen(tmpbuf));
1346  At the end of I<pseudo-block> the function C<f> is called with the
1347  only argument C<p>.
1349  =item C<SAVEDESTRUCTOR_X(DESTRUCTORFUNC_t f, void *p)>
1351  At the end of I<pseudo-block> the function C<f> is called with the
1352  implicit context argument (if any), and C<p>.
1354  =item C<SAVESTACK_POS()>
1356  The current offset on the Perl internal stack (cf. C<SP>) is restored
1357  at the end of I<pseudo-block>.
1359  =back
1361  The following API list contains functions, thus one needs to
1362  provide pointers to the modifiable data explicitly (either C pointers,
1363  or Perlish C<GV *>s).  Where the above macros take C<int>, a similar
1364  function takes C<int *>.
1366  =over 4
1368  =item C<SV* save_scalar(GV *gv)>
1370  Equivalent to Perl code C<local $gv>.
1372  =item C<AV* save_ary(GV *gv)>
1374  =item C<HV* save_hash(GV *gv)>
1376  Similar to C<save_scalar>, but localize C<@gv> and C<%gv>.
1378  =item C<void save_item(SV *item)>
1380  Duplicates the current value of C<SV>, on the exit from the current
1381  C<ENTER>/C<LEAVE> I<pseudo-block> will restore the value of C<SV>
1382  using the stored value. It doesn't handle magic. Use C<save_scalar> if
1383  magic is affected.
1385  =item C<void save_list(SV **sarg, I32 maxsarg)>
1387  A variant of C<save_item> which takes multiple arguments via an array
1388  C<sarg> of C<SV*> of length C<maxsarg>.
1390  =item C<SV* save_svref(SV **sptr)>
1392  Similar to C<save_scalar>, but will reinstate an C<SV *>.
1394  =item C<void save_aptr(AV **aptr)>
1396  =item C<void save_hptr(HV **hptr)>
1398  Similar to C<save_svref>, but localize C<AV *> and C<HV *>.
1400  =back
1402  The C<Alias> module implements localization of the basic types within the
1403  I<caller's scope>.  People who are interested in how to localize things in
1404  the containing scope should take a look there too.
1406  =head1 Subroutines
1408  =head2 XSUBs and the Argument Stack
1410  The XSUB mechanism is a simple way for Perl programs to access C subroutines.
1411  An XSUB routine will have a stack that contains the arguments from the Perl
1412  program, and a way to map from the Perl data structures to a C equivalent.
1414  The stack arguments are accessible through the C<ST(n)> macro, which returns
1415  the C<n>'th stack argument.  Argument 0 is the first argument passed in the
1416  Perl subroutine call.  These arguments are C<SV*>, and can be used anywhere
1417  an C<SV*> is used.
1419  Most of the time, output from the C routine can be handled through use of
1420  the RETVAL and OUTPUT directives.  However, there are some cases where the
1421  argument stack is not already long enough to handle all the return values.
1422  An example is the POSIX tzname() call, which takes no arguments, but returns
1423  two, the local time zone's standard and summer time abbreviations.
1425  To handle this situation, the PPCODE directive is used and the stack is
1426  extended using the macro:
1428      EXTEND(SP, num);
1430  where C<SP> is the macro that represents the local copy of the stack pointer,
1431  and C<num> is the number of elements the stack should be extended by.
1433  Now that there is room on the stack, values can be pushed on it using C<PUSHs>
1434  macro. The pushed values will often need to be "mortal" (See
1435  L</Reference Counts and Mortality>):
1437      PUSHs(sv_2mortal(newSViv(an_integer)))
1438      PUSHs(sv_2mortal(newSVuv(an_unsigned_integer)))
1439      PUSHs(sv_2mortal(newSVnv(a_double)))
1440      PUSHs(sv_2mortal(newSVpv("Some String",0)))
1442  And now the Perl program calling C<tzname>, the two values will be assigned
1443  as in:
1445      ($standard_abbrev, $summer_abbrev) = POSIX::tzname;
1447  An alternate (and possibly simpler) method to pushing values on the stack is
1448  to use the macro:
1450      XPUSHs(SV*)
1452  This macro automatically adjust the stack for you, if needed.  Thus, you
1453  do not need to call C<EXTEND> to extend the stack.
1455  Despite their suggestions in earlier versions of this document the macros
1456  C<(X)PUSH[iunp]> are I<not> suited to XSUBs which return multiple results.
1457  For that, either stick to the C<(X)PUSHs> macros shown above, or use the new
1458  C<m(X)PUSH[iunp]> macros instead; see L</Putting a C value on Perl stack>.
1460  For more information, consult L<perlxs> and L<perlxstut>.
1462  =head2 Calling Perl Routines from within C Programs
1464  There are four routines that can be used to call a Perl subroutine from
1465  within a C program.  These four are:
1467      I32  call_sv(SV*, I32);
1468      I32  call_pv(const char*, I32);
1469      I32  call_method(const char*, I32);
1470      I32  call_argv(const char*, I32, register char**);
1472  The routine most often used is C<call_sv>.  The C<SV*> argument
1473  contains either the name of the Perl subroutine to be called, or a
1474  reference to the subroutine.  The second argument consists of flags
1475  that control the context in which the subroutine is called, whether
1476  or not the subroutine is being passed arguments, how errors should be
1477  trapped, and how to treat return values.
1479  All four routines return the number of arguments that the subroutine returned
1480  on the Perl stack.
1482  These routines used to be called C<perl_call_sv>, etc., before Perl v5.6.0,
1483  but those names are now deprecated; macros of the same name are provided for
1484  compatibility.
1486  When using any of these routines (except C<call_argv>), the programmer
1487  must manipulate the Perl stack.  These include the following macros and
1488  functions:
1490      dSP
1491      SP
1492      PUSHMARK()
1493      PUTBACK
1494      SPAGAIN
1495      ENTER
1496      SAVETMPS
1497      FREETMPS
1498      LEAVE
1499      XPUSH*()
1500      POP*()
1502  For a detailed description of calling conventions from C to Perl,
1503  consult L<perlcall>.
1505  =head2 Memory Allocation
1507  =head3 Allocation
1509  All memory meant to be used with the Perl API functions should be manipulated
1510  using the macros described in this section.  The macros provide the necessary
1511  transparency between differences in the actual malloc implementation that is
1512  used within perl.
1514  It is suggested that you enable the version of malloc that is distributed
1515  with Perl.  It keeps pools of various sizes of unallocated memory in
1516  order to satisfy allocation requests more quickly.  However, on some
1517  platforms, it may cause spurious malloc or free errors.
1519  The following three macros are used to initially allocate memory :
1521      Newx(pointer, number, type);
1522      Newxc(pointer, number, type, cast);
1523      Newxz(pointer, number, type);
1525  The first argument C<pointer> should be the name of a variable that will
1526  point to the newly allocated memory.
1528  The second and third arguments C<number> and C<type> specify how many of
1529  the specified type of data structure should be allocated.  The argument
1530  C<type> is passed to C<sizeof>.  The final argument to C<Newxc>, C<cast>,
1531  should be used if the C<pointer> argument is different from the C<type>
1532  argument.
1534  Unlike the C<Newx> and C<Newxc> macros, the C<Newxz> macro calls C<memzero>
1535  to zero out all the newly allocated memory.
1537  =head3 Reallocation
1539      Renew(pointer, number, type);
1540      Renewc(pointer, number, type, cast);
1541      Safefree(pointer)
1543  These three macros are used to change a memory buffer size or to free a
1544  piece of memory no longer needed.  The arguments to C<Renew> and C<Renewc>
1545  match those of C<New> and C<Newc> with the exception of not needing the
1546  "magic cookie" argument.
1548  =head3 Moving
1550      Move(source, dest, number, type);
1551      Copy(source, dest, number, type);
1552      Zero(dest, number, type);
1554  These three macros are used to move, copy, or zero out previously allocated
1555  memory.  The C<source> and C<dest> arguments point to the source and
1556  destination starting points.  Perl will move, copy, or zero out C<number>
1557  instances of the size of the C<type> data structure (using the C<sizeof>
1558  function).
1560  =head2 PerlIO
1562  The most recent development releases of Perl has been experimenting with
1563  removing Perl's dependency on the "normal" standard I/O suite and allowing
1564  other stdio implementations to be used.  This involves creating a new
1565  abstraction layer that then calls whichever implementation of stdio Perl
1566  was compiled with.  All XSUBs should now use the functions in the PerlIO
1567  abstraction layer and not make any assumptions about what kind of stdio
1568  is being used.
1570  For a complete description of the PerlIO abstraction, consult L<perlapio>.
1572  =head2 Putting a C value on Perl stack
1574  A lot of opcodes (this is an elementary operation in the internal perl
1575  stack machine) put an SV* on the stack. However, as an optimization
1576  the corresponding SV is (usually) not recreated each time. The opcodes
1577  reuse specially assigned SVs (I<target>s) which are (as a corollary)
1578  not constantly freed/created.
1580  Each of the targets is created only once (but see
1581  L<Scratchpads and recursion> below), and when an opcode needs to put
1582  an integer, a double, or a string on stack, it just sets the
1583  corresponding parts of its I<target> and puts the I<target> on stack.
1585  The macro to put this target on stack is C<PUSHTARG>, and it is
1586  directly used in some opcodes, as well as indirectly in zillions of
1587  others, which use it via C<(X)PUSH[iunp]>.
1589  Because the target is reused, you must be careful when pushing multiple
1590  values on the stack. The following code will not do what you think:
1592      XPUSHi(10);
1593      XPUSHi(20);
1595  This translates as "set C<TARG> to 10, push a pointer to C<TARG> onto
1596  the stack; set C<TARG> to 20, push a pointer to C<TARG> onto the stack".
1597  At the end of the operation, the stack does not contain the values 10
1598  and 20, but actually contains two pointers to C<TARG>, which we have set
1599  to 20.
1601  If you need to push multiple different values then you should either use
1602  the C<(X)PUSHs> macros, or else use the new C<m(X)PUSH[iunp]> macros,
1603  none of which make use of C<TARG>.  The C<(X)PUSHs> macros simply push an
1604  SV* on the stack, which, as noted under L</XSUBs and the Argument Stack>,
1605  will often need to be "mortal".  The new C<m(X)PUSH[iunp]> macros make
1606  this a little easier to achieve by creating a new mortal for you (via
1607  C<(X)PUSHmortal>), pushing that onto the stack (extending it if necessary
1608  in the case of the C<mXPUSH[iunp]> macros), and then setting its value.
1609  Thus, instead of writing this to "fix" the example above:
1611      XPUSHs(sv_2mortal(newSViv(10)))
1612      XPUSHs(sv_2mortal(newSViv(20)))
1614  you can simply write:
1616      mXPUSHi(10)
1617      mXPUSHi(20)
1619  On a related note, if you do use C<(X)PUSH[iunp]>, then you're going to
1620  need a C<dTARG> in your variable declarations so that the C<*PUSH*>
1621  macros can make use of the local variable C<TARG>.  See also C<dTARGET>
1622  and C<dXSTARG>.
1624  =head2 Scratchpads
1626  The question remains on when the SVs which are I<target>s for opcodes
1627  are created. The answer is that they are created when the current unit --
1628  a subroutine or a file (for opcodes for statements outside of
1629  subroutines) -- is compiled. During this time a special anonymous Perl
1630  array is created, which is called a scratchpad for the current
1631  unit.
1633  A scratchpad keeps SVs which are lexicals for the current unit and are
1634  targets for opcodes. One can deduce that an SV lives on a scratchpad
1635  by looking on its flags: lexicals have C<SVs_PADMY> set, and
1636  I<target>s have C<SVs_PADTMP> set.
1638  The correspondence between OPs and I<target>s is not 1-to-1. Different
1639  OPs in the compile tree of the unit can use the same target, if this
1640  would not conflict with the expected life of the temporary.
1642  =head2 Scratchpads and recursion
1644  In fact it is not 100% true that a compiled unit contains a pointer to
1645  the scratchpad AV. In fact it contains a pointer to an AV of
1646  (initially) one element, and this element is the scratchpad AV. Why do
1647  we need an extra level of indirection?
1649  The answer is B<recursion>, and maybe B<threads>. Both
1650  these can create several execution pointers going into the same
1651  subroutine. For the subroutine-child not write over the temporaries
1652  for the subroutine-parent (lifespan of which covers the call to the
1653  child), the parent and the child should have different
1654  scratchpads. (I<And> the lexicals should be separate anyway!)
1656  So each subroutine is born with an array of scratchpads (of length 1).
1657  On each entry to the subroutine it is checked that the current
1658  depth of the recursion is not more than the length of this array, and
1659  if it is, new scratchpad is created and pushed into the array.
1661  The I<target>s on this scratchpad are C<undef>s, but they are already
1662  marked with correct flags.
1664  =head1 Compiled code
1666  =head2 Code tree
1668  Here we describe the internal form your code is converted to by
1669  Perl. Start with a simple example:
1671    $a = $b + $c;
1673  This is converted to a tree similar to this one:
1675               assign-to
1676             /           \
1677            +             $a
1678          /   \
1679        $b     $c
1681  (but slightly more complicated).  This tree reflects the way Perl
1682  parsed your code, but has nothing to do with the execution order.
1683  There is an additional "thread" going through the nodes of the tree
1684  which shows the order of execution of the nodes.  In our simplified
1685  example above it looks like:
1687       $b ---> $c ---> + ---> $a ---> assign-to
1689  But with the actual compile tree for C<$a = $b + $c> it is different:
1690  some nodes I<optimized away>.  As a corollary, though the actual tree
1691  contains more nodes than our simplified example, the execution order
1692  is the same as in our example.
1694  =head2 Examining the tree
1696  If you have your perl compiled for debugging (usually done with
1697  C<-DDEBUGGING> on the C<Configure> command line), you may examine the
1698  compiled tree by specifying C<-Dx> on the Perl command line.  The
1699  output takes several lines per node, and for C<$b+$c> it looks like
1700  this:
1702      5           TYPE = add  ===> 6
1703                  TARG = 1
1704                  FLAGS = (SCALAR,KIDS)
1705                  {
1706                      TYPE = null  ===> (4)
1707                        (was rv2sv)
1708                      FLAGS = (SCALAR,KIDS)
1709                      {
1710      3                   TYPE = gvsv  ===> 4
1711                          FLAGS = (SCALAR)
1712                          GV = main::b
1713                      }
1714                  }
1715                  {
1716                      TYPE = null  ===> (5)
1717                        (was rv2sv)
1718                      FLAGS = (SCALAR,KIDS)
1719                      {
1720      4                   TYPE = gvsv  ===> 5
1721                          FLAGS = (SCALAR)
1722                          GV = main::c
1723                      }
1724                  }
1726  This tree has 5 nodes (one per C<TYPE> specifier), only 3 of them are
1727  not optimized away (one per number in the left column).  The immediate
1728  children of the given node correspond to C<{}> pairs on the same level
1729  of indentation, thus this listing corresponds to the tree:
1731                     add
1732                   /     \
1733                 null    null
1734                  |       |
1735                 gvsv    gvsv
1737  The execution order is indicated by C<===E<gt>> marks, thus it is C<3
1738  4 5 6> (node C<6> is not included into above listing), i.e.,
1739  C<gvsv gvsv add whatever>.
1741  Each of these nodes represents an op, a fundamental operation inside the
1742  Perl core. The code which implements each operation can be found in the
1743  F<pp*.c> files; the function which implements the op with type C<gvsv>
1744  is C<pp_gvsv>, and so on. As the tree above shows, different ops have
1745  different numbers of children: C<add> is a binary operator, as one would
1746  expect, and so has two children. To accommodate the various different
1747  numbers of children, there are various types of op data structure, and
1748  they link together in different ways.
1750  The simplest type of op structure is C<OP>: this has no children. Unary
1751  operators, C<UNOP>s, have one child, and this is pointed to by the
1752  C<op_first> field. Binary operators (C<BINOP>s) have not only an
1753  C<op_first> field but also an C<op_last> field. The most complex type of
1754  op is a C<LISTOP>, which has any number of children. In this case, the
1755  first child is pointed to by C<op_first> and the last child by
1756  C<op_last>. The children in between can be found by iteratively
1757  following the C<op_sibling> pointer from the first child to the last.
1759  There are also two other op types: a C<PMOP> holds a regular expression,
1760  and has no children, and a C<LOOP> may or may not have children. If the
1761  C<op_children> field is non-zero, it behaves like a C<LISTOP>. To
1762  complicate matters, if a C<UNOP> is actually a C<null> op after
1763  optimization (see L</Compile pass 2: context propagation>) it will still
1764  have children in accordance with its former type.
1766  Another way to examine the tree is to use a compiler back-end module, such
1767  as L<B::Concise>.
1769  =head2 Compile pass 1: check routines
1771  The tree is created by the compiler while I<yacc> code feeds it
1772  the constructions it recognizes. Since I<yacc> works bottom-up, so does
1773  the first pass of perl compilation.
1775  What makes this pass interesting for perl developers is that some
1776  optimization may be performed on this pass.  This is optimization by
1777  so-called "check routines".  The correspondence between node names
1778  and corresponding check routines is described in F<opcode.pl> (do not
1779  forget to run C<make regen_headers> if you modify this file).
1781  A check routine is called when the node is fully constructed except
1782  for the execution-order thread.  Since at this time there are no
1783  back-links to the currently constructed node, one can do most any
1784  operation to the top-level node, including freeing it and/or creating
1785  new nodes above/below it.
1787  The check routine returns the node which should be inserted into the
1788  tree (if the top-level node was not modified, check routine returns
1789  its argument).
1791  By convention, check routines have names C<ck_*>. They are usually
1792  called from C<new*OP> subroutines (or C<convert>) (which in turn are
1793  called from F<perly.y>).
1795  =head2 Compile pass 1a: constant folding
1797  Immediately after the check routine is called the returned node is
1798  checked for being compile-time executable.  If it is (the value is
1799  judged to be constant) it is immediately executed, and a I<constant>
1800  node with the "return value" of the corresponding subtree is
1801  substituted instead.  The subtree is deleted.
1803  If constant folding was not performed, the execution-order thread is
1804  created.
1806  =head2 Compile pass 2: context propagation
1808  When a context for a part of compile tree is known, it is propagated
1809  down through the tree.  At this time the context can have 5 values
1810  (instead of 2 for runtime context): void, boolean, scalar, list, and
1811  lvalue.  In contrast with the pass 1 this pass is processed from top
1812  to bottom: a node's context determines the context for its children.
1814  Additional context-dependent optimizations are performed at this time.
1815  Since at this moment the compile tree contains back-references (via
1816  "thread" pointers), nodes cannot be free()d now.  To allow
1817  optimized-away nodes at this stage, such nodes are null()ified instead
1818  of free()ing (i.e. their type is changed to OP_NULL).
1820  =head2 Compile pass 3: peephole optimization
1822  After the compile tree for a subroutine (or for an C<eval> or a file)
1823  is created, an additional pass over the code is performed. This pass
1824  is neither top-down or bottom-up, but in the execution order (with
1825  additional complications for conditionals).  These optimizations are
1826  done in the subroutine peep().  Optimizations performed at this stage
1827  are subject to the same restrictions as in the pass 2.
1829  =head2 Pluggable runops
1831  The compile tree is executed in a runops function.  There are two runops
1832  functions, in F<run.c> and in F<dump.c>.  C<Perl_runops_debug> is used
1833  with DEBUGGING and C<Perl_runops_standard> is used otherwise.  For fine
1834  control over the execution of the compile tree it is possible to provide
1835  your own runops function.
1837  It's probably best to copy one of the existing runops functions and
1838  change it to suit your needs.  Then, in the BOOT section of your XS
1839  file, add the line:
1841    PL_runops = my_runops;
1843  This function should be as efficient as possible to keep your programs
1844  running as fast as possible.
1846  =head1 Examining internal data structures with the C<dump> functions
1848  To aid debugging, the source file F<dump.c> contains a number of
1849  functions which produce formatted output of internal data structures.
1851  The most commonly used of these functions is C<Perl_sv_dump>; it's used
1852  for dumping SVs, AVs, HVs, and CVs. The C<Devel::Peek> module calls
1853  C<sv_dump> to produce debugging output from Perl-space, so users of that
1854  module should already be familiar with its format.
1856  C<Perl_op_dump> can be used to dump an C<OP> structure or any of its
1857  derivatives, and produces output similar to C<perl -Dx>; in fact,
1858  C<Perl_dump_eval> will dump the main root of the code being evaluated,
1859  exactly like C<-Dx>.
1861  Other useful functions are C<Perl_dump_sub>, which turns a C<GV> into an
1862  op tree, C<Perl_dump_packsubs> which calls C<Perl_dump_sub> on all the
1863  subroutines in a package like so: (Thankfully, these are all xsubs, so
1864  there is no op tree)
1866      (gdb) print Perl_dump_packsubs(PL_defstash)
1868      SUB attributes::bootstrap = (xsub 0x811fedc 0)
1870      SUB UNIVERSAL::can = (xsub 0x811f50c 0)
1872      SUB UNIVERSAL::isa = (xsub 0x811f304 0)
1874      SUB UNIVERSAL::VERSION = (xsub 0x811f7ac 0)
1876      SUB DynaLoader::boot_DynaLoader = (xsub 0x805b188 0)
1878  and C<Perl_dump_all>, which dumps all the subroutines in the stash and
1879  the op tree of the main root.
1881  =head1 How multiple interpreters and concurrency are supported
1883  =head2 Background and PERL_IMPLICIT_CONTEXT
1885  The Perl interpreter can be regarded as a closed box: it has an API
1886  for feeding it code or otherwise making it do things, but it also has
1887  functions for its own use.  This smells a lot like an object, and
1888  there are ways for you to build Perl so that you can have multiple
1889  interpreters, with one interpreter represented either as a C structure,
1890  or inside a thread-specific structure.  These structures contain all
1891  the context, the state of that interpreter.
1893  One macro controls the major Perl build flavor: MULTIPLICITY. The
1894  MULTIPLICITY build has a C structure that packages all the interpreter
1895  state. With multiplicity-enabled perls, PERL_IMPLICIT_CONTEXT is also
1896  normally defined, and enables the support for passing in a "hidden" first
1897  argument that represents all three data structures. MULTIPLICITY makes
1898  mutli-threaded perls possible (with the ithreads threading model, related
1899  to the macro USE_ITHREADS.)
1901  Two other "encapsulation" macros are the PERL_GLOBAL_STRUCT and
1902  PERL_GLOBAL_STRUCT_PRIVATE (the latter turns on the former, and the
1903  former turns on MULTIPLICITY.)  The PERL_GLOBAL_STRUCT causes all the
1904  internal variables of Perl to be wrapped inside a single global struct,
1905  struct perl_vars, accessible as (globals) &PL_Vars or PL_VarsPtr or
1906  the function  Perl_GetVars().  The PERL_GLOBAL_STRUCT_PRIVATE goes
1907  one step further, there is still a single struct (allocated in main()
1908  either from heap or from stack) but there are no global data symbols
1909  pointing to it.  In either case the global struct should be initialised
1910  as the very first thing in main() using Perl_init_global_struct() and
1911  correspondingly tear it down after perl_free() using Perl_free_global_struct(),
1912  please see F<miniperlmain.c> for usage details.  You may also need
1913  to use C<dVAR> in your coding to "declare the global variables"
1914  when you are using them.  dTHX does this for you automatically.
1916  To see whether you have non-const data you can use a BSD-compatible C<nm>:
1918    nm libperl.a | grep -v ' [TURtr] '
1920  If this displays any C<D> or C<d> symbols, you have non-const data.
1922  For backward compatibility reasons defining just PERL_GLOBAL_STRUCT
1923  doesn't actually hide all symbols inside a big global struct: some
1924  PerlIO_xxx vtables are left visible.  The PERL_GLOBAL_STRUCT_PRIVATE
1925  then hides everything (see how the PERLIO_FUNCS_DECL is used).
1927  All this obviously requires a way for the Perl internal functions to be
1928  either subroutines taking some kind of structure as the first
1929  argument, or subroutines taking nothing as the first argument.  To
1930  enable these two very different ways of building the interpreter,
1931  the Perl source (as it does in so many other situations) makes heavy
1932  use of macros and subroutine naming conventions.
1934  First problem: deciding which functions will be public API functions and
1935  which will be private.  All functions whose names begin C<S_> are private
1936  (think "S" for "secret" or "static").  All other functions begin with
1937  "Perl_", but just because a function begins with "Perl_" does not mean it is
1938  part of the API. (See L</Internal Functions>.) The easiest way to be B<sure> a
1939  function is part of the API is to find its entry in L<perlapi>.
1940  If it exists in L<perlapi>, it's part of the API.  If it doesn't, and you
1941  think it should be (i.e., you need it for your extension), send mail via
1942  L<perlbug> explaining why you think it should be.
1944  Second problem: there must be a syntax so that the same subroutine
1945  declarations and calls can pass a structure as their first argument,
1946  or pass nothing.  To solve this, the subroutines are named and
1947  declared in a particular way.  Here's a typical start of a static
1948  function used within the Perl guts:
1950    STATIC void
1951    S_incline(pTHX_ char *s)
1953  STATIC becomes "static" in C, and may be #define'd to nothing in some
1954  configurations in future.
1956  A public function (i.e. part of the internal API, but not necessarily
1957  sanctioned for use in extensions) begins like this:
1959    void
1960    Perl_sv_setiv(pTHX_ SV* dsv, IV num)
1962  C<pTHX_> is one of a number of macros (in perl.h) that hide the
1963  details of the interpreter's context.  THX stands for "thread", "this",
1964  or "thingy", as the case may be.  (And no, George Lucas is not involved. :-)
1965  The first character could be 'p' for a B<p>rototype, 'a' for B<a>rgument,
1966  or 'd' for B<d>eclaration, so we have C<pTHX>, C<aTHX> and C<dTHX>, and
1967  their variants.
1969  When Perl is built without options that set PERL_IMPLICIT_CONTEXT, there is no
1970  first argument containing the interpreter's context.  The trailing underscore
1971  in the pTHX_ macro indicates that the macro expansion needs a comma
1972  after the context argument because other arguments follow it.  If
1973  PERL_IMPLICIT_CONTEXT is not defined, pTHX_ will be ignored, and the
1974  subroutine is not prototyped to take the extra argument.  The form of the
1975  macro without the trailing underscore is used when there are no additional
1976  explicit arguments.
1978  When a core function calls another, it must pass the context.  This
1979  is normally hidden via macros.  Consider C<sv_setiv>.  It expands into
1980  something like this:
1982      #ifdef PERL_IMPLICIT_CONTEXT
1983        #define sv_setiv(a,b)      Perl_sv_setiv(aTHX_ a, b)
1984        /* can't do this for vararg functions, see below */
1985      #else
1986        #define sv_setiv           Perl_sv_setiv
1987      #endif
1989  This works well, and means that XS authors can gleefully write:
1991      sv_setiv(foo, bar);
1993  and still have it work under all the modes Perl could have been
1994  compiled with.
1996  This doesn't work so cleanly for varargs functions, though, as macros
1997  imply that the number of arguments is known in advance.  Instead we
1998  either need to spell them out fully, passing C<aTHX_> as the first
1999  argument (the Perl core tends to do this with functions like
2000  Perl_warner), or use a context-free version.
2002  The context-free version of Perl_warner is called
2003  Perl_warner_nocontext, and does not take the extra argument.  Instead
2004  it does dTHX; to get the context from thread-local storage.  We
2005  C<#define warner Perl_warner_nocontext> so that extensions get source
2006  compatibility at the expense of performance.  (Passing an arg is
2007  cheaper than grabbing it from thread-local storage.)
2009  You can ignore [pad]THXx when browsing the Perl headers/sources.
2010  Those are strictly for use within the core.  Extensions and embedders
2011  need only be aware of [pad]THX.
2013  =head2 So what happened to dTHR?
2015  C<dTHR> was introduced in perl 5.005 to support the older thread model.
2016  The older thread model now uses the C<THX> mechanism to pass context
2017  pointers around, so C<dTHR> is not useful any more.  Perl 5.6.0 and
2018  later still have it for backward source compatibility, but it is defined
2019  to be a no-op.
2021  =head2 How do I use all this in extensions?
2023  When Perl is built with PERL_IMPLICIT_CONTEXT, extensions that call
2024  any functions in the Perl API will need to pass the initial context
2025  argument somehow.  The kicker is that you will need to write it in
2026  such a way that the extension still compiles when Perl hasn't been
2027  built with PERL_IMPLICIT_CONTEXT enabled.
2029  There are three ways to do this.  First, the easy but inefficient way,
2030  which is also the default, in order to maintain source compatibility
2031  with extensions: whenever XSUB.h is #included, it redefines the aTHX
2032  and aTHX_ macros to call a function that will return the context.
2033  Thus, something like:
2035          sv_setiv(sv, num);
2037  in your extension will translate to this when PERL_IMPLICIT_CONTEXT is
2038  in effect:
2040          Perl_sv_setiv(Perl_get_context(), sv, num);
2042  or to this otherwise:
2044          Perl_sv_setiv(sv, num);
2046  You have to do nothing new in your extension to get this; since
2047  the Perl library provides Perl_get_context(), it will all just
2048  work.
2050  The second, more efficient way is to use the following template for
2051  your Foo.xs:
2053          #define PERL_NO_GET_CONTEXT     /* we want efficiency */
2054          #include "EXTERN.h"
2055          #include "perl.h"
2056          #include "XSUB.h"
2058          STATIC void my_private_function(int arg1, int arg2);
2060          STATIC void
2061          my_private_function(int arg1, int arg2)
2062          {
2063              dTHX;       /* fetch context */
2064              ... call many Perl API functions ...
2065          }
2067          [... etc ...]
2069          MODULE = Foo            PACKAGE = Foo
2071          /* typical XSUB */
2073          void
2074          my_xsub(arg)
2075                  int arg
2076              CODE:
2077                  my_private_function(arg, 10);
2079  Note that the only two changes from the normal way of writing an
2080  extension is the addition of a C<#define PERL_NO_GET_CONTEXT> before
2081  including the Perl headers, followed by a C<dTHX;> declaration at
2082  the start of every function that will call the Perl API.  (You'll
2083  know which functions need this, because the C compiler will complain
2084  that there's an undeclared identifier in those functions.)  No changes
2085  are needed for the XSUBs themselves, because the XS() macro is
2086  correctly defined to pass in the implicit context if needed.
2088  The third, even more efficient way is to ape how it is done within
2089  the Perl guts:
2092          #define PERL_NO_GET_CONTEXT     /* we want efficiency */
2093          #include "EXTERN.h"
2094          #include "perl.h"
2095          #include "XSUB.h"
2097          /* pTHX_ only needed for functions that call Perl API */
2098          STATIC void my_private_function(pTHX_ int arg1, int arg2);
2100          STATIC void
2101          my_private_function(pTHX_ int arg1, int arg2)
2102          {
2103              /* dTHX; not needed here, because THX is an argument */
2104              ... call Perl API functions ...
2105          }
2107          [... etc ...]
2109          MODULE = Foo            PACKAGE = Foo
2111          /* typical XSUB */
2113          void
2114          my_xsub(arg)
2115                  int arg
2116              CODE:
2117                  my_private_function(aTHX_ arg, 10);
2119  This implementation never has to fetch the context using a function
2120  call, since it is always passed as an extra argument.  Depending on
2121  your needs for simplicity or efficiency, you may mix the previous
2122  two approaches freely.
2124  Never add a comma after C<pTHX> yourself--always use the form of the
2125  macro with the underscore for functions that take explicit arguments,
2126  or the form without the argument for functions with no explicit arguments.
2128  If one is compiling Perl with the C<-DPERL_GLOBAL_STRUCT> the C<dVAR>
2129  definition is needed if the Perl global variables (see F<perlvars.h>
2130  or F<globvar.sym>) are accessed in the function and C<dTHX> is not
2131  used (the C<dTHX> includes the C<dVAR> if necessary).  One notices
2132  the need for C<dVAR> only with the said compile-time define, because
2133  otherwise the Perl global variables are visible as-is.
2135  =head2 Should I do anything special if I call perl from multiple threads?
2137  If you create interpreters in one thread and then proceed to call them in
2138  another, you need to make sure perl's own Thread Local Storage (TLS) slot is
2139  initialized correctly in each of those threads.
2141  The C<perl_alloc> and C<perl_clone> API functions will automatically set
2142  the TLS slot to the interpreter they created, so that there is no need to do
2143  anything special if the interpreter is always accessed in the same thread that
2144  created it, and that thread did not create or call any other interpreters
2145  afterwards.  If that is not the case, you have to set the TLS slot of the
2146  thread before calling any functions in the Perl API on that particular
2147  interpreter.  This is done by calling the C<PERL_SET_CONTEXT> macro in that
2148  thread as the first thing you do:
2150      /* do this before doing anything else with some_perl */
2151      PERL_SET_CONTEXT(some_perl);
2153      ... other Perl API calls on some_perl go here ...
2155  =head2 Future Plans and PERL_IMPLICIT_SYS
2157  Just as PERL_IMPLICIT_CONTEXT provides a way to bundle up everything
2158  that the interpreter knows about itself and pass it around, so too are
2159  there plans to allow the interpreter to bundle up everything it knows
2160  about the environment it's running on.  This is enabled with the
2161  PERL_IMPLICIT_SYS macro.  Currently it only works with USE_ITHREADS on
2162  Windows.
2164  This allows the ability to provide an extra pointer (called the "host"
2165  environment) for all the system calls.  This makes it possible for
2166  all the system stuff to maintain their own state, broken down into
2167  seven C structures.  These are thin wrappers around the usual system
2168  calls (see win32/perllib.c) for the default perl executable, but for a
2169  more ambitious host (like the one that would do fork() emulation) all
2170  the extra work needed to pretend that different interpreters are
2171  actually different "processes", would be done here.
2173  The Perl engine/interpreter and the host are orthogonal entities.
2174  There could be one or more interpreters in a process, and one or
2175  more "hosts", with free association between them.
2177  =head1 Internal Functions
2179  All of Perl's internal functions which will be exposed to the outside
2180  world are prefixed by C<Perl_> so that they will not conflict with XS
2181  functions or functions used in a program in which Perl is embedded.
2182  Similarly, all global variables begin with C<PL_>. (By convention,
2183  static functions start with C<S_>.)
2185  Inside the Perl core, you can get at the functions either with or
2186  without the C<Perl_> prefix, thanks to a bunch of defines that live in
2187  F<embed.h>. This header file is generated automatically from
2188  F<embed.pl> and F<embed.fnc>. F<embed.pl> also creates the prototyping
2189  header files for the internal functions, generates the documentation
2190  and a lot of other bits and pieces. It's important that when you add
2191  a new function to the core or change an existing one, you change the
2192  data in the table in F<embed.fnc> as well. Here's a sample entry from
2193  that table:
2195      Apd |SV**   |av_fetch   |AV* ar|I32 key|I32 lval
2197  The second column is the return type, the third column the name. Columns
2198  after that are the arguments. The first column is a set of flags:
2200  =over 3
2202  =item A
2204  This function is a part of the public API. All such functions should also
2205  have 'd', very few do not.
2207  =item p
2209  This function has a C<Perl_> prefix; i.e. it is defined as
2210  C<Perl_av_fetch>.
2212  =item d
2214  This function has documentation using the C<apidoc> feature which we'll
2215  look at in a second.  Some functions have 'd' but not 'A'; docs are good.
2217  =back
2219  Other available flags are:
2221  =over 3
2223  =item s
2225  This is a static function and is defined as C<STATIC S_whatever>, and
2226  usually called within the sources as C<whatever(...)>.
2228  =item n
2230  This does not need a interpreter context, so the definition has no
2231  C<pTHX>, and it follows that callers don't use C<aTHX>.  (See
2232  L<perlguts/Background and PERL_IMPLICIT_CONTEXT>.)
2234  =item r
2236  This function never returns; C<croak>, C<exit> and friends.
2238  =item f
2240  This function takes a variable number of arguments, C<printf> style.
2241  The argument list should end with C<...>, like this:
2243      Afprd   |void   |croak          |const char* pat|...
2245  =item M
2247  This function is part of the experimental development API, and may change
2248  or disappear without notice.
2250  =item o
2252  This function should not have a compatibility macro to define, say,
2253  C<Perl_parse> to C<parse>. It must be called as C<Perl_parse>.
2255  =item x
2257  This function isn't exported out of the Perl core.
2259  =item m
2261  This is implemented as a macro.
2263  =item X
2265  This function is explicitly exported.
2267  =item E
2269  This function is visible to extensions included in the Perl core.
2271  =item b
2273  Binary backward compatibility; this function is a macro but also has
2274  a C<Perl_> implementation (which is exported).
2276  =item others
2278  See the comments at the top of C<embed.fnc> for others.
2280  =back
2282  If you edit F<embed.pl> or F<embed.fnc>, you will need to run
2283  C<make regen_headers> to force a rebuild of F<embed.h> and other
2284  auto-generated files.
2286  =head2 Formatted Printing of IVs, UVs, and NVs
2288  If you are printing IVs, UVs, or NVS instead of the stdio(3) style
2289  formatting codes like C<%d>, C<%ld>, C<%f>, you should use the
2290  following macros for portability
2292          IVdf            IV in decimal
2293          UVuf            UV in decimal
2294          UVof            UV in octal
2295          UVxf            UV in hexadecimal
2296          NVef            NV %e-like
2297          NVff            NV %f-like
2298          NVgf            NV %g-like
2300  These will take care of 64-bit integers and long doubles.
2301  For example:
2303          printf("IV is %"IVdf"\n", iv);
2305  The IVdf will expand to whatever is the correct format for the IVs.
2307  If you are printing addresses of pointers, use UVxf combined
2308  with PTR2UV(), do not use %lx or %p.
2310  =head2 Pointer-To-Integer and Integer-To-Pointer
2312  Because pointer size does not necessarily equal integer size,
2313  use the follow macros to do it right.
2315          PTR2UV(pointer)
2316          PTR2IV(pointer)
2317          PTR2NV(pointer)
2318          INT2PTR(pointertotype, integer)
2320  For example:
2322          IV  iv = ...;
2323          SV *sv = INT2PTR(SV*, iv);
2325  and
2327          AV *av = ...;
2328          UV  uv = PTR2UV(av);
2330  =head2 Exception Handling
2332  There are a couple of macros to do very basic exception handling in XS
2333  modules. You have to define C<NO_XSLOCKS> before including F<XSUB.h> to
2334  be able to use these macros:
2336          #define NO_XSLOCKS
2337          #include "XSUB.h"
2339  You can use these macros if you call code that may croak, but you need
2340  to do some cleanup before giving control back to Perl. For example:
2342          dXCPT;    /* set up necessary variables */
2344          XCPT_TRY_START {
2345            code_that_may_croak();
2346          } XCPT_TRY_END
2348          XCPT_CATCH
2349          {
2350            /* do cleanup here */
2351            XCPT_RETHROW;
2352          }
2354  Note that you always have to rethrow an exception that has been
2355  caught. Using these macros, it is not possible to just catch the
2356  exception and ignore it. If you have to ignore the exception, you
2357  have to use the C<call_*> function.
2359  The advantage of using the above macros is that you don't have
2360  to setup an extra function for C<call_*>, and that using these
2361  macros is faster than using C<call_*>.
2363  =head2 Source Documentation
2365  There's an effort going on to document the internal functions and
2366  automatically produce reference manuals from them - L<perlapi> is one
2367  such manual which details all the functions which are available to XS
2368  writers. L<perlintern> is the autogenerated manual for the functions
2369  which are not part of the API and are supposedly for internal use only.
2371  Source documentation is created by putting POD comments into the C
2372  source, like this:
2374   /*
2375   =for apidoc sv_setiv
2377   Copies an integer into the given SV.  Does not handle 'set' magic.  See
2378   C<sv_setiv_mg>.
2380   =cut
2381   */
2383  Please try and supply some documentation if you add functions to the
2384  Perl core.
2386  =head2 Backwards compatibility
2388  The Perl API changes over time. New functions are added or the interfaces
2389  of existing functions are changed. The C<Devel::PPPort> module tries to
2390  provide compatibility code for some of these changes, so XS writers don't
2391  have to code it themselves when supporting multiple versions of Perl.
2393  C<Devel::PPPort> generates a C header file F<ppport.h> that can also
2394  be run as a Perl script. To generate F<ppport.h>, run:
2396      perl -MDevel::PPPort -eDevel::PPPort::WriteFile
2398  Besides checking existing XS code, the script can also be used to retrieve
2399  compatibility information for various API calls using the C<--api-info>
2400  command line switch. For example:
2402    % perl ppport.h --api-info=sv_magicext
2404  For details, see C<perldoc ppport.h>.
2406  =head1 Unicode Support
2408  Perl 5.6.0 introduced Unicode support. It's important for porters and XS
2409  writers to understand this support and make sure that the code they
2410  write does not corrupt Unicode data.
2412  =head2 What B<is> Unicode, anyway?
2414  In the olden, less enlightened times, we all used to use ASCII. Most of
2415  us did, anyway. The big problem with ASCII is that it's American. Well,
2416  no, that's not actually the problem; the problem is that it's not
2417  particularly useful for people who don't use the Roman alphabet. What
2418  used to happen was that particular languages would stick their own
2419  alphabet in the upper range of the sequence, between 128 and 255. Of
2420  course, we then ended up with plenty of variants that weren't quite
2421  ASCII, and the whole point of it being a standard was lost.
2423  Worse still, if you've got a language like Chinese or
2424  Japanese that has hundreds or thousands of characters, then you really
2425  can't fit them into a mere 256, so they had to forget about ASCII
2426  altogether, and build their own systems using pairs of numbers to refer
2427  to one character.
2429  To fix this, some people formed Unicode, Inc. and
2430  produced a new character set containing all the characters you can
2431  possibly think of and more. There are several ways of representing these
2432  characters, and the one Perl uses is called UTF-8. UTF-8 uses
2433  a variable number of bytes to represent a character. You can learn more
2434  about Unicode and Perl's Unicode model in L<perlunicode>.
2436  =head2 How can I recognise a UTF-8 string?
2438  You can't. This is because UTF-8 data is stored in bytes just like
2439  non-UTF-8 data. The Unicode character 200, (C<0xC8> for you hex types)
2440  capital E with a grave accent, is represented by the two bytes
2441  C<v196.172>. Unfortunately, the non-Unicode string C<chr(196).chr(172)>
2442  has that byte sequence as well. So you can't tell just by looking - this
2443  is what makes Unicode input an interesting problem.
2445  In general, you either have to know what you're dealing with, or you
2446  have to guess.  The API function C<is_utf8_string> can help; it'll tell
2447  you if a string contains only valid UTF-8 characters. However, it can't
2448  do the work for you. On a character-by-character basis, C<is_utf8_char>
2449  will tell you whether the current character in a string is valid UTF-8. 
2451  =head2 How does UTF-8 represent Unicode characters?
2453  As mentioned above, UTF-8 uses a variable number of bytes to store a
2454  character. Characters with values 0...127 are stored in one byte, just
2455  like good ol' ASCII. Character 128 is stored as C<v194.128>; this
2456  continues up to character 191, which is C<v194.191>. Now we've run out of
2457  bits (191 is binary C<10111111>) so we move on; 192 is C<v195.128>. And
2458  so it goes on, moving to three bytes at character 2048.
2460  Assuming you know you're dealing with a UTF-8 string, you can find out
2461  how long the first character in it is with the C<UTF8SKIP> macro:
2463      char *utf = "\305\233\340\240\201";
2464      I32 len;
2466      len = UTF8SKIP(utf); /* len is 2 here */
2467      utf += len;
2468      len = UTF8SKIP(utf); /* len is 3 here */
2470  Another way to skip over characters in a UTF-8 string is to use
2471  C<utf8_hop>, which takes a string and a number of characters to skip
2472  over. You're on your own about bounds checking, though, so don't use it
2473  lightly.
2475  All bytes in a multi-byte UTF-8 character will have the high bit set,
2476  so you can test if you need to do something special with this
2477  character like this (the UTF8_IS_INVARIANT() is a macro that tests
2478  whether the byte can be encoded as a single byte even in UTF-8):
2480      U8 *utf;
2481      UV uv;    /* Note: a UV, not a U8, not a char */
2483      if (!UTF8_IS_INVARIANT(*utf))
2484          /* Must treat this as UTF-8 */
2485          uv = utf8_to_uv(utf);
2486      else
2487          /* OK to treat this character as a byte */
2488          uv = *utf;
2490  You can also see in that example that we use C<utf8_to_uv> to get the
2491  value of the character; the inverse function C<uv_to_utf8> is available
2492  for putting a UV into UTF-8:
2494      if (!UTF8_IS_INVARIANT(uv))
2495          /* Must treat this as UTF8 */
2496          utf8 = uv_to_utf8(utf8, uv);
2497      else
2498          /* OK to treat this character as a byte */
2499          *utf8++ = uv;
2501  You B<must> convert characters to UVs using the above functions if
2502  you're ever in a situation where you have to match UTF-8 and non-UTF-8
2503  characters. You may not skip over UTF-8 characters in this case. If you
2504  do this, you'll lose the ability to match hi-bit non-UTF-8 characters;
2505  for instance, if your UTF-8 string contains C<v196.172>, and you skip
2506  that character, you can never match a C<chr(200)> in a non-UTF-8 string.
2507  So don't do that!
2509  =head2 How does Perl store UTF-8 strings?
2511  Currently, Perl deals with Unicode strings and non-Unicode strings
2512  slightly differently. A flag in the SV, C<SVf_UTF8>, indicates that the
2513  string is internally encoded as UTF-8. Without it, the byte value is the
2514  codepoint number and vice versa (in other words, the string is encoded
2515  as iso-8859-1). You can check and manipulate this flag with the
2516  following macros:
2518      SvUTF8(sv)
2519      SvUTF8_on(sv)
2520      SvUTF8_off(sv)
2522  This flag has an important effect on Perl's treatment of the string: if
2523  Unicode data is not properly distinguished, regular expressions,
2524  C<length>, C<substr> and other string handling operations will have
2525  undesirable results.
2527  The problem comes when you have, for instance, a string that isn't
2528  flagged as UTF-8, and contains a byte sequence that could be UTF-8 -
2529  especially when combining non-UTF-8 and UTF-8 strings.
2531  Never forget that the C<SVf_UTF8> flag is separate to the PV value; you
2532  need be sure you don't accidentally knock it off while you're
2533  manipulating SVs. More specifically, you cannot expect to do this:
2535      SV *sv;
2536      SV *nsv;
2537      STRLEN len;
2538      char *p;
2540      p = SvPV(sv, len);
2541      frobnicate(p);
2542      nsv = newSVpvn(p, len);
2544  The C<char*> string does not tell you the whole story, and you can't
2545  copy or reconstruct an SV just by copying the string value. Check if the
2546  old SV has the UTF8 flag set, and act accordingly:
2548      p = SvPV(sv, len);
2549      frobnicate(p);
2550      nsv = newSVpvn(p, len);
2551      if (SvUTF8(sv))
2552          SvUTF8_on(nsv);
2554  In fact, your C<frobnicate> function should be made aware of whether or
2555  not it's dealing with UTF-8 data, so that it can handle the string
2556  appropriately.
2558  Since just passing an SV to an XS function and copying the data of
2559  the SV is not enough to copy the UTF8 flags, even less right is just
2560  passing a C<char *> to an XS function.
2562  =head2 How do I convert a string to UTF-8?
2564  If you're mixing UTF-8 and non-UTF-8 strings, it is necessary to upgrade
2565  one of the strings to UTF-8. If you've got an SV, the easiest way to do
2566  this is:
2568      sv_utf8_upgrade(sv);
2570  However, you must not do this, for example:
2572      if (!SvUTF8(left))
2573          sv_utf8_upgrade(left);
2575  If you do this in a binary operator, you will actually change one of the
2576  strings that came into the operator, and, while it shouldn't be noticeable
2577  by the end user, it can cause problems in deficient code.
2579  Instead, C<bytes_to_utf8> will give you a UTF-8-encoded B<copy> of its
2580  string argument. This is useful for having the data available for
2581  comparisons and so on, without harming the original SV. There's also
2582  C<utf8_to_bytes> to go the other way, but naturally, this will fail if
2583  the string contains any characters above 255 that can't be represented
2584  in a single byte.
2586  =head2 Is there anything else I need to know?
2588  Not really. Just remember these things:
2590  =over 3
2592  =item *
2594  There's no way to tell if a string is UTF-8 or not. You can tell if an SV
2595  is UTF-8 by looking at is C<SvUTF8> flag. Don't forget to set the flag if
2596  something should be UTF-8. Treat the flag as part of the PV, even though
2597  it's not - if you pass on the PV to somewhere, pass on the flag too.
2599  =item *
2601  If a string is UTF-8, B<always> use C<utf8_to_uv> to get at the value,
2602  unless C<UTF8_IS_INVARIANT(*s)> in which case you can use C<*s>.
2604  =item *
2606  When writing a character C<uv> to a UTF-8 string, B<always> use
2607  C<uv_to_utf8>, unless C<UTF8_IS_INVARIANT(uv))> in which case
2608  you can use C<*s = uv>.
2610  =item *
2612  Mixing UTF-8 and non-UTF-8 strings is tricky. Use C<bytes_to_utf8> to get
2613  a new string which is UTF-8 encoded. There are tricks you can use to
2614  delay deciding whether you need to use a UTF-8 string until you get to a
2615  high character - C<HALF_UPGRADE> is one of those.
2617  =back
2619  =head1 Custom Operators
2621  Custom operator support is a new experimental feature that allows you to
2622  define your own ops. This is primarily to allow the building of
2623  interpreters for other languages in the Perl core, but it also allows
2624  optimizations through the creation of "macro-ops" (ops which perform the
2625  functions of multiple ops which are usually executed together, such as
2626  C<gvsv, gvsv, add>.)
2628  This feature is implemented as a new op type, C<OP_CUSTOM>. The Perl
2629  core does not "know" anything special about this op type, and so it will
2630  not be involved in any optimizations. This also means that you can
2631  define your custom ops to be any op structure - unary, binary, list and
2632  so on - you like.
2634  It's important to know what custom operators won't do for you. They
2635  won't let you add new syntax to Perl, directly. They won't even let you
2636  add new keywords, directly. In fact, they won't change the way Perl
2637  compiles a program at all. You have to do those changes yourself, after
2638  Perl has compiled the program. You do this either by manipulating the op
2639  tree using a C<CHECK> block and the C<B::Generate> module, or by adding
2640  a custom peephole optimizer with the C<optimize> module.
2642  When you do this, you replace ordinary Perl ops with custom ops by
2643  creating ops with the type C<OP_CUSTOM> and the C<pp_addr> of your own
2644  PP function. This should be defined in XS code, and should look like
2645  the PP ops in C<pp_*.c>. You are responsible for ensuring that your op
2646  takes the appropriate number of values from the stack, and you are
2647  responsible for adding stack marks if necessary.
2649  You should also "register" your op with the Perl interpreter so that it
2650  can produce sensible error and warning messages. Since it is possible to
2651  have multiple custom ops within the one "logical" op type C<OP_CUSTOM>,
2652  Perl uses the value of C<< o->op_ppaddr >> as a key into the
2653  C<PL_custom_op_descs> and C<PL_custom_op_names> hashes. This means you
2654  need to enter a name and description for your op at the appropriate
2655  place in the C<PL_custom_op_names> and C<PL_custom_op_descs> hashes.
2657  Forthcoming versions of C<B::Generate> (version 1.0 and above) should
2658  directly support the creation of custom ops by name.
2660  =head1 AUTHORS
2662  Until May 1997, this document was maintained by Jeff Okamoto
2663  E<lt>okamoto@corp.hp.comE<gt>.  It is now maintained as part of Perl
2664  itself by the Perl 5 Porters E<lt>perl5-porters@perl.orgE<gt>.
2666  With lots of help and suggestions from Dean Roehrich, Malcolm Beattie,
2667  Andreas Koenig, Paul Hudson, Ilya Zakharevich, Paul Marquess, Neil
2668  Bowers, Matthew Green, Tim Bunce, Spider Boardman, Ulrich Pfeifer,
2669  Stephen McCamant, and Gurusamy Sarathy.
2671  =head1 SEE ALSO
2673  perlapi(1), perlintern(1), perlxs(1), perlembed(1)

Generated: Tue Mar 17 22:47:18 2015 Cross-referenced by PHPXref 0.7.1