Re: Bug in perlguts documentation?

[perl5.git] / pod / perlguts.pod

diff --git a/pod/perlguts.pod b/pod/perlguts.pod
index c069e88..3fea294 100644 (file)
--- a/pod/perlguts.pod
+++ b/pod/perlguts.pod
@@ -76,6 +76,10 @@ L<perlsec>).  This pointer may be NULL if that information is not
 important.  Note that this function requires you to specify the length of
 the format.
 
 important.  Note that this function requires you to specify the length of
 the format.
 

+STRLEN is an integer type (Size_t, usually defined as size_t in
+config.h) guaranteed to be large enough to represent the size of
+any string that perl can handle.
+

 The C<sv_set*()> functions are not generic enough to operate on values
 that have "magic".  See L<Magic Virtual Tables> later in this document.
 
 The C<sv_set*()> functions are not generic enough to operate on values
 that have "magic".  See L<Magic Virtual Tables> later in this document.
 


@@ -220,7 +224,7 @@ actually removing the characters, C<sv_chop> sets the flag C<OOK>
 (offset OK) to signal to other functions that the offset hack is in
 effect, and it puts the number of bytes chopped off into the IV field
 of the SV. It then moves the PV pointer (called C<SvPVX>) forward that
 (offset OK) to signal to other functions that the offset hack is in
 effect, and it puts the number of bytes chopped off into the IV field
 of the SV. It then moves the PV pointer (called C<SvPVX>) forward that

-many bytes, and adjusts C<SvCUR> and C<SvLEN>. 
+many bytes, and adjusts C<SvCUR> and C<SvLEN>.

 
 Hence, at this point, the start of the buffer that we allocated lives
 at C<SvPVX(sv) - SvIV(sv)> in memory and the PV pointer is pointing
 
 Hence, at this point, the start of the buffer that we allocated lives
 at C<SvPVX(sv) - SvIV(sv)> in memory and the PV pointer is pointing


@@ -243,6 +247,15 @@ portion of the string between the "real" and the "fake" beginnings is
 shown in parentheses, and the values of C<SvCUR> and C<SvLEN> reflect
 the fake beginning, not the real one.
 
 shown in parentheses, and the values of C<SvCUR> and C<SvLEN> reflect
 the fake beginning, not the real one.
 

+Something similar to the offset hack is perfomed on AVs to enable
+efficient shifting and splicing off the beginning of the array; while
+C<AvARRAY> points to the first element in the array that is visible from
+Perl, C<AvALLOC> points to the real start of the C array. These are
+usually the same, but a C<shift> operation can be carried out by
+increasing C<AvARRAY> by one and decreasing C<AvFILL> and C<AvLEN>.
+Again, the location of the real start of the C array only comes into
+play when freeing the array. See C<av_shift> in F<av.c>.
+

 =head2 What's Really Stored in an SV?
 
 Recall that the usual method of determining the type of scalar you have is
 =head2 What's Really Stored in an SV?
 
 Recall that the usual method of determining the type of scalar you have is


@@ -526,10 +539,11 @@ class.  SV is returned.
 
        SV* newSVrv(SV* rv, const char* classname);
 
 
        SV* newSVrv(SV* rv, const char* classname);
 

-Copies integer or double into an SV whose reference is C<rv>.  SV is blessed
+Copies integer, unsigned integer or double into an SV whose reference is C<rv>.  SV is blessed

 if C<classname> is non-null.
 
        SV* sv_setref_iv(SV* rv, const char* classname, IV iv);
 if C<classname> is non-null.
 
        SV* sv_setref_iv(SV* rv, const char* classname, IV iv);

+       SV* sv_setref_uv(SV* rv, const char* classname, UV uv);

        SV* sv_setref_nv(SV* rv, const char* classname, NV iv);
 
 Copies the pointer value (I<the address, not the string!>) into an SV whose
        SV* sv_setref_nv(SV* rv, const char* classname, NV iv);
 
 Copies the pointer value (I<the address, not the string!>) into an SV whose


@@ -557,7 +571,7 @@ is the function implementing the C<UNIVERSAL::isa> functionality.
 
        bool sv_derived_from(SV* sv, const char* name);
 
 
        bool sv_derived_from(SV* sv, const char* name);
 

-To check if you've got an object derived from a specific class you have 
+To check if you've got an object derived from a specific class you have

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


@@ -636,9 +650,11 @@ See L<perlcall> and L<perlxs> for more details on these macros.
 However, if you mortalize a variable twice, the reference count will
 later be decremented twice.
 
 However, if you mortalize a variable twice, the reference count will
 later be decremented twice.
 

-You should be careful about creating mortal variables.  Strange things
-can happen if you make the same value mortal within multiple contexts,
-or if you make a variable mortal multiple times.
+"Mortal" SVs are mainly used for SVs that are placed on perl's stack.
+For example an SV which is created just to pass a number to a called sub
+is made mortal to have it cleaned up automatically when stack is popped.
+Similarly results returned by XSUBs (which go in the stack) are often
+made mortal.

 
 To create a mortal variable, use the functions:
 
 
 To create a mortal variable, use the functions:
 


@@ -646,9 +662,28 @@ To create a mortal variable, use the functions:
     SV*  sv_2mortal(SV*)
     SV*  sv_mortalcopy(SV*)
 
     SV*  sv_2mortal(SV*)
     SV*  sv_mortalcopy(SV*)
 

-The first call creates a mortal SV, the second converts an existing
+The first call creates a mortal SV (with no value), the second converts an existing

 SV to a mortal SV (and thus defers a call to C<SvREFCNT_dec>), and the
 third creates a mortal copy of an existing SV.
 SV to a mortal SV (and thus defers a call to C<SvREFCNT_dec>), and the
 third creates a mortal copy of an existing SV.

+Because C<sv_newmortal> gives the new SV no value,it must normally be given one
+via C<sv_setpv>, C<sv_setiv> etc. :
+
+    SV *tmp = sv_newmortal();
+    sv_setiv(tmp, an_integer);
+
+As that is multiple C statements it is quite common so see this idiom instead:
+
+    SV *tmp = sv_2mortal(newSViv(an_integer));
+
+
+You should be careful about creating mortal variables.  Strange things
+can happen if you make the same value mortal within multiple contexts,
+or if you make a variable mortal multiple times. Thinking of "Mortalization"
+as deferred C<SvREFCNT_dec> should help to minimize such problems.
+For example if you are passing an SV which you I<know> has high enough REFCNT
+to survive its use on the stack you need not do any mortalization.
+If you are not sure then doing an C<SvREFCNT_inc> and C<sv_2mortal>, or
+making a C<sv_mortalcopy> is safer.

 
 The mortal routines are not just for SVs -- AVs and HVs can be
 made mortal by passing their address (type-casted to C<SV*>) to the
 
 The mortal routines are not just for SVs -- AVs and HVs can be
 made mortal by passing their address (type-casted to C<SV*>) to the


@@ -796,13 +831,17 @@ copy of the name is stored in C<mg_ptr> field.
 The sv_magic function uses C<how> to determine which, if any, predefined
 "Magic Virtual Table" should be assigned to the C<mg_virtual> field.
 See the "Magic Virtual Table" section below.  The C<how> argument is also
 The sv_magic function uses C<how> to determine which, if any, predefined
 "Magic Virtual Table" should be assigned to the C<mg_virtual> field.
 See the "Magic Virtual Table" section below.  The C<how> argument is also

-stored in the C<mg_type> field.
+stored in the C<mg_type> field. The value of C<how> should be chosen
+from the set of macros C<PERL_MAGIC_foo> found perl.h. Note that before
+these macros were added, perl internals used to directly use character
+literals, so you may occasionally come across old code or documentation
+referrring to 'U' magic rather than C<PERL_MAGIC_uvar> for example.

 
 The C<obj> argument is stored in the C<mg_obj> field of the C<MAGIC>
 structure.  If it is not the same as the C<sv> argument, the reference
 count of the C<obj> object is incremented.  If it is the same, or if
 
 The C<obj> argument is stored in the C<mg_obj> field of the C<MAGIC>
 structure.  If it is not the same as the C<sv> argument, the reference
 count of the C<obj> object is incremented.  If it is the same, or if

-the C<how> argument is "#", or if it is a NULL pointer, then C<obj> is
-merely stored, without the reference count being incremented.
+the C<how> argument is C<PERL_MAGIC_arylen>", or if it is a NULL pointer,
+then C<obj> is merely stored, without the reference count being incremented.

 
 There is also a function to add magic to an C<HV>:
 
 
 There is also a function to add magic to an C<HV>:
 


@@ -846,67 +885,76 @@ actions depending on which function is being called.
     svt_free            Free any extra storage associated with the SV.
 
 For instance, the MGVTBL structure called C<vtbl_sv> (which corresponds
     svt_free            Free any extra storage associated with the SV.
 
 For instance, the MGVTBL structure called C<vtbl_sv> (which corresponds

-to an C<mg_type> of '\0') contains:
+to an C<mg_type> of C<PERL_MAGIC_sv>) contains:

 
     { magic_get, magic_set, magic_len, 0, 0 }
 
 
     { magic_get, magic_set, magic_len, 0, 0 }
 

-Thus, when an SV is determined to be magical and of type '\0', if a get
-operation is being performed, the routine C<magic_get> is called.  All
-the various routines for the various magical types begin with C<magic_>.
-NOTE: the magic routines are not considered part of the Perl API, and may
-not be exported by the Perl library.
+Thus, when an SV is determined to be magical and of type C<PERL_MAGIC_sv>,
+if a get operation is being performed, the routine C<magic_get> is
+called.  All the various routines for the various magical types begin
+with C<magic_>.  NOTE: the magic routines are not considered part of
+the Perl API, and may not be exported by the Perl library.

 
 The current kinds of Magic Virtual Tables are:
 
 
 The current kinds of Magic Virtual Tables are:
 

-    mg_type  MGVTBL              Type of magic
-    -------  ------              ----------------------------
-    \0       vtbl_sv             Special scalar variable
-    A        vtbl_amagic         %OVERLOAD hash
-    a        vtbl_amagicelem     %OVERLOAD hash element
-    c        (none)              Holds overload table (AMT) on stash
-    B        vtbl_bm             Boyer-Moore (fast string search)
-    D        vtbl_regdata        Regex match position data (@+ and @- vars)
-    d        vtbl_regdatum       Regex match position data element
-    E        vtbl_env            %ENV hash
-    e        vtbl_envelem        %ENV hash element
-    f        vtbl_fm             Formline ('compiled' format)
-    g        vtbl_mglob          m//g target / study()ed string
-    I        vtbl_isa            @ISA array
-    i        vtbl_isaelem        @ISA array element
-    k        vtbl_nkeys          scalar(keys()) lvalue
-    L        (none)              Debugger %_<filename 
-    l        vtbl_dbline         Debugger %_<filename element
-    o        vtbl_collxfrm       Locale transformation
-    P        vtbl_pack           Tied array or hash
-    p        vtbl_packelem       Tied array or hash element
-    q        vtbl_packelem       Tied scalar or handle
-    S        vtbl_sig            %SIG hash
-    s        vtbl_sigelem        %SIG hash element
-    t        vtbl_taint          Taintedness
-    U        vtbl_uvar           Available for use by extensions
-    v        vtbl_vec            vec() lvalue
-    x        vtbl_substr         substr() lvalue
-    y        vtbl_defelem        Shadow "foreach" iterator variable /
-                                  smart parameter vivification
-    *        vtbl_glob           GV (typeglob)
-    #        vtbl_arylen         Array length ($#ary)
-    .        vtbl_pos            pos() lvalue
-    ~        (none)              Available for use by extensions
+    mg_type
+    (old-style char and macro)   MGVTBL         Type of magic
+    --------------------------   ------         ----------------------------
+    \0 PERL_MAGIC_sv             vtbl_sv        Special scalar variable
+    A  PERL_MAGIC_overload       vtbl_amagic    %OVERLOAD hash
+    a  PERL_MAGIC_overload_elem  vtbl_amagicelem %OVERLOAD hash element
+    c  PERL_MAGIC_overload_table (none)         Holds overload table (AMT)
+                                               on stash
+    B  PERL_MAGIC_bm             vtbl_bm        Boyer-Moore (fast string search)
+    D  PERL_MAGIC_regdata        vtbl_regdata   Regex match position data
+                                               (@+ and @- vars)
+    d  PERL_MAGIC_regdatum       vtbl_regdatum  Regex match position data
+                                               element
+    E  PERL_MAGIC_env            vtbl_env       %ENV hash
+    e  PERL_MAGIC_envelem        vtbl_envelem   %ENV hash element
+    f  PERL_MAGIC_fm             vtbl_fm        Formline ('compiled' format)
+    g  PERL_MAGIC_regex_global   vtbl_mglob     m//g target / study()ed string
+    I  PERL_MAGIC_isa            vtbl_isa       @ISA array
+    i  PERL_MAGIC_isaelem        vtbl_isaelem   @ISA array element
+    k  PERL_MAGIC_nkeys          vtbl_nkeys     scalar(keys()) lvalue
+    L  PERL_MAGIC_dbfile         (none)         Debugger %_<filename
+    l  PERL_MAGIC_dbline         vtbl_dbline    Debugger %_<filename element
+    m  PERL_MAGIC_mutex          vtbl_mutex     ???
+    o  PERL_MAGIC_collxfrm       vtbl_collxfrm  Locale transformation
+    P  PERL_MAGIC_tied           vtbl_pack      Tied array or hash
+    p  PERL_MAGIC_tiedelem       vtbl_packelem  Tied array or hash element
+    q  PERL_MAGIC_tiedscalar     vtbl_packelem  Tied scalar or handle
+    r  PERL_MAGIC_qr             vtbl_qr        precompiled qr// regex
+    S  PERL_MAGIC_sig            vtbl_sig       %SIG hash
+    s  PERL_MAGIC_sigelem        vtbl_sigelem   %SIG hash element
+    t  PERL_MAGIC_taint          vtbl_taint     Taintedness
+    U  PERL_MAGIC_uvar           vtbl_uvar      Available for use by extensions
+    v  PERL_MAGIC_vec            vtbl_vec       vec() lvalue
+    x  PERL_MAGIC_substr         vtbl_substr    substr() lvalue
+    y  PERL_MAGIC_defelem        vtbl_defelem   Shadow "foreach" iterator
+                                               variable / smart parameter
+                                               vivification
+    *  PERL_MAGIC_glob           vtbl_glob      GV (typeglob)
+    #  PERL_MAGIC_arylen         vtbl_arylen    Array length ($#ary)
+    .  PERL_MAGIC_pos            vtbl_pos       pos() lvalue
+    <  PERL_MAGIC_backref        vtbl_backref   ???
+    ~  PERL_MAGIC_ext            (none)         Available for use by extensions

 
 When an uppercase and lowercase letter both exist in the table, then the
 uppercase letter is used to represent some kind of composite type (a list
 or a hash), and the lowercase letter is used to represent an element of
 
 When an uppercase and lowercase letter both exist in the table, then the
 uppercase letter is used to represent some kind of composite type (a list
 or a hash), and the lowercase letter is used to represent an element of

-that composite type.
-
-The '~' and 'U' magic types are defined specifically for use by
-extensions and will not be used by perl itself.  Extensions can use
-'~' magic to 'attach' private information to variables (typically
-objects).  This is especially useful because there is no way for
-normal perl code to corrupt this private information (unlike using
-extra elements of a hash object).
-
-Similarly, 'U' magic can be used much like tie() to call a C function
-any time a scalar's value is used or changed.  The C<MAGIC>'s
+that composite type. Some internals code makes use of this case
+relationship.
+
+The C<PERL_MAGIC_ext> and C<PERL_MAGIC_uvar> magic types are defined
+specifically for use by extensions and will not be used by perl itself.
+Extensions can use C<PERL_MAGIC_ext> magic to 'attach' private information
+to variables (typically objects).  This is especially useful because
+there is no way for normal perl code to corrupt this private information
+(unlike using extra elements of a hash object).
+
+Similarly, C<PERL_MAGIC_uvar> magic can be used much like tie() to call a
+C function any time a scalar's value is used or changed.  The C<MAGIC>'s

 C<mg_ptr> field points to a C<ufuncs> structure:
 
     struct ufuncs {
 C<mg_ptr> field points to a C<ufuncs> structure:
 
     struct ufuncs {


@@ -916,8 +964,8 @@ C<mg_ptr> field points to a C<ufuncs> structure:
     };
 
 When the SV is read from or written to, the C<uf_val> or C<uf_set>
     };
 
 When the SV is read from or written to, the C<uf_val> or C<uf_set>

-function will be called with C<uf_index> as the first arg and a
-pointer to the SV as the second.  A simple example of how to add 'U'
+function will be called with C<uf_index> as the first arg and a pointer to
+the SV as the second.  A simple example of how to add C<PERL_MAGIC_uvar>

 magic is shown below.  Note that the ufuncs structure is copied by
 sv_magic, so you can safely allocate it on the stack.
 
 magic is shown below.  Note that the ufuncs structure is copied by
 sv_magic, so you can safely allocate it on the stack.
 


@@ -930,14 +978,14 @@ sv_magic, so you can safely allocate it on the stack.
         uf.uf_val   = &my_get_fn;
         uf.uf_set   = &my_set_fn;
         uf.uf_index = 0;
         uf.uf_val   = &my_get_fn;
         uf.uf_set   = &my_set_fn;
         uf.uf_index = 0;

-        sv_magic(sv, 0, 'U', (char*)&uf, sizeof(uf));
+        sv_magic(sv, 0, PERL_MAGIC_uvar, (char*)&uf, sizeof(uf));

 
 

-Note that because multiple extensions may be using '~' or 'U' magic,
-it is important for extensions to take extra care to avoid conflict.
-Typically only using the magic on objects blessed into the same class
-as the extension is sufficient.  For '~' magic, it may also be
-appropriate to add an I32 'signature' at the top of the private data
-area and check that.
+Note that because multiple extensions may be using C<PERL_MAGIC_ext>
+or C<PERL_MAGIC_uvar> magic, it is important for extensions to take
+extra care to avoid conflict.  Typically only using the magic on
+objects blessed into the same class as the extension is sufficient.
+For C<PERL_MAGIC_ext> magic, it may also be appropriate to add an I32
+'signature' at the top of the private data area and check that.

 
 Also note that the C<sv_set*()> and C<sv_cat*()> functions described
 earlier do B<not> invoke 'set' magic on their targets.  This must
 
 Also note that the C<sv_set*()> and C<sv_cat*()> functions described
 earlier do B<not> invoke 'set' magic on their targets.  This must


@@ -967,7 +1015,8 @@ the mg_type field is changed to be the lowercase letter.
 
 =head2 Understanding the Magic of Tied Hashes and Arrays
 
 
 =head2 Understanding the Magic of Tied Hashes and Arrays
 

-Tied hashes and arrays are magical beasts of the 'P' magic type.
+Tied hashes and arrays are magical beasts of the C<PERL_MAGIC_tied>
+magic type.

 
 WARNING: As of the 5.004 release, proper usage of the array and hash
 access functions requires understanding a few caveats.  Some
 
 WARNING: As of the 5.004 release, proper usage of the array and hash
 access functions requires understanding a few caveats.  Some


@@ -998,7 +1047,7 @@ to do this.
         tie = newRV_noinc((SV*)newHV());
         stash = gv_stashpv("MyTie", TRUE);
         sv_bless(tie, stash);
         tie = newRV_noinc((SV*)newHV());
         stash = gv_stashpv("MyTie", TRUE);
         sv_bless(tie, stash);

-        hv_magic(hash, tie, 'P');
+        hv_magic(hash, (GV*)tie, PERL_MAGIC_tied);

         RETVAL = newRV_noinc(hash);
     OUTPUT:
         RETVAL
         RETVAL = newRV_noinc(hash);
     OUTPUT:
         RETVAL


@@ -1113,8 +1162,20 @@ and back.
 =item C<SAVEFREESV(SV *sv)>
 
 The refcount of C<sv> would be decremented at the end of
 =item C<SAVEFREESV(SV *sv)>
 
 The refcount of C<sv> would be decremented at the end of

-I<pseudo-block>. This is similar to C<sv_2mortal>, which should (?) be
-used instead.
+I<pseudo-block>.  This is similar to C<sv_2mortal> in that it is also a
+mechanism for doing a delayed C<SvREFCNT_dec>.  However, while C<sv_2mortal>
+extends the lifetime of C<sv> until the beginning of the next statement,
+C<SAVEFREESV> extends it until the end of the enclosing scope.  These
+lifetimes can be wildly different.
+
+Also compare C<SAVEMORTALIZESV>.
+
+=item C<SAVEMORTALIZESV(SV *sv)>
+
+Just like C<SAVEFREESV>, but mortalizes C<sv> at the end of the current
+scope instead of decrementing its reference count.  This usually has the
+effect of keeping C<sv> alive until the statement that called the currently
+live scope has finished executing.

 
 =item C<SAVEFREEOP(OP *op)>
 
 
 =item C<SAVEFREEOP(OP *op)>
 


@@ -1158,7 +1219,7 @@ at the end of I<pseudo-block>.
 
 The following API list contains functions, thus one needs to
 provide pointers to the modifiable data explicitly (either C pointers,
 
 The following API list contains functions, thus one needs to
 provide pointers to the modifiable data explicitly (either C pointers,

-or Perlish C<GV *>s).  Where the above macros take C<int>, a similar 
+or Perlish C<GV *>s).  Where the above macros take C<int>, a similar

 function takes C<int *>.
 
 =over 4
 function takes C<int *>.
 
 =over 4


@@ -1227,13 +1288,12 @@ extended using the macro:
 where C<SP> is the macro that represents the local copy of the stack pointer,
 and C<num> is the number of elements the stack should be extended by.
 
 where C<SP> is the macro that represents the local copy of the stack pointer,
 and C<num> is the number of elements the stack should be extended by.
 

-Now that there is room on the stack, values can be pushed on it using the
-macros to push IVs, doubles, strings, and SV pointers respectively:
+Now that there is room on the stack, values can be pushed on it using C<PUSHs>
+macro. The values pushed will often need to be "mortal" (See L</Reference Counts and Mortality>).

 
 

-    PUSHi(IV)
-    PUSHn(double)
-    PUSHp(char*, I32)
-    PUSHs(SV*)
+    PUSHs(sv_2mortal(newSViv(an_integer)))
+    PUSHs(sv_2mortal(newSVpv("Some String",0)))
+    PUSHs(sv_2mortal(newSVnv(3.141592)))

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


@@ -1241,16 +1301,17 @@ as in:
     ($standard_abbrev, $summer_abbrev) = POSIX::tzname;
 
 An alternate (and possibly simpler) method to pushing values on the stack is
     ($standard_abbrev, $summer_abbrev) = POSIX::tzname;
 
 An alternate (and possibly simpler) method to pushing values on the stack is

-to use the macros:
+to use the macro:

 
 

-    XPUSHi(IV)
-    XPUSHn(double)
-    XPUSHp(char*, I32)

     XPUSHs(SV*)
 
     XPUSHs(SV*)
 

-These macros automatically adjust the stack for you, if needed.  Thus, you
+This macro automatically adjust the stack for you, if needed.  Thus, you

 do not need to call C<EXTEND> to extend the stack.
 
 do not need to call C<EXTEND> to extend the stack.
 

+Despite their suggestions in earlier versions of this document the macros
+C<PUSHi>, C<PUSHn> and C<PUSHp> are I<not> suited to XSUBs which return
+multiple results, see L</Putting a C value on Perl stack>.
+

 For more information, consult L<perlxs> and L<perlxstut>.
 
 =head2 Calling Perl Routines from within C Programs
 For more information, consult L<perlxs> and L<perlxstut>.
 
 =head2 Calling Perl Routines from within C Programs


@@ -1350,21 +1411,6 @@ destination starting points.  Perl will move, copy, or zero out C<number>
 instances of the size of the C<type> data structure (using the C<sizeof>
 function).
 
 instances of the size of the C<type> data structure (using the C<sizeof>
 function).
 

-Here is a handy table of equivalents between ordinary C and Perl's
-memory abstraction layer:
-
-    Instead Of:                Use:
-
-    malloc                     New
-    calloc                     Newz
-    realloc                    Renew
-    memcopy                    Copy
-    memmove                    Move
-    free                       Safefree
-    strdup                     savepv
-    strndup                    savepvn (Hey, strndup doesn't exist!)
-    memcpy/*(struct foo *)    StructCopy
-

 =head2 PerlIO
 
 The most recent development releases of Perl has been experimenting with
 =head2 PerlIO
 
 The most recent development releases of Perl has been experimenting with


@@ -1394,6 +1440,23 @@ The macro to put this target on stack is C<PUSHTARG>, and it is
 directly used in some opcodes, as well as indirectly in zillions of
 others, which use it via C<(X)PUSH[pni]>.
 
 directly used in some opcodes, as well as indirectly in zillions of
 others, which use it via C<(X)PUSH[pni]>.
 

+Because the target is reused, you must be careful when pushing multiple
+values on the stack. The following code will not do what you think:
+
+    XPUSHi(10);
+    XPUSHi(20);
+
+This translates as "set C<TARG> to 10, push a pointer to C<TARG> onto
+the stack; set C<TARG> to 20, push a pointer to C<TARG> onto the stack".
+At the end of the operation, the stack does not contain the values 10
+and 20, but actually contains two pointers to C<TARG>, which we have set
+to 20. If you need to push multiple different values, use C<XPUSHs>,
+which bypasses C<TARG>.
+
+On a related note, if you do use C<(X)PUSH[npi]>, then you're going to
+need a C<dTARG> in your variable declarations so that the C<*PUSH*>
+macros can make use of the local variable C<TARG>.
+

 =head2 Scratchpads
 
 The question remains on when the SVs which are I<target>s for opcodes
 =head2 Scratchpads
 
 The question remains on when the SVs which are I<target>s for opcodes


@@ -1511,6 +1574,31 @@ The execution order is indicated by C<===E<gt>> marks, thus it is C<3
 4 5 6> (node C<6> is not included into above listing), i.e.,
 C<gvsv gvsv add whatever>.
 
 4 5 6> (node C<6> is not included into above listing), i.e.,
 C<gvsv gvsv add whatever>.
 

+Each of these nodes represents an op, a fundamental operation inside the
+Perl core. The code which implements each operation can be found in the
+F<pp*.c> files; the function which implements the op with type C<gvsv>
+is C<pp_gvsv>, and so on. As the tree above shows, different ops have
+different numbers of children: C<add> is a binary operator, as one would
+expect, and so has two children. To accommodate the various different
+numbers of children, there are various types of op data structure, and
+they link together in different ways.
+
+The simplest type of op structure is C<OP>: this has no children. Unary
+operators, C<UNOP>s, have one child, and this is pointed to by the
+C<op_first> field. Binary operators (C<BINOP>s) have not only an
+C<op_first> field but also an C<op_last> field. The most complex type of
+op is a C<LISTOP>, which has any number of children. In this case, the
+first child is pointed to by C<op_first> and the last child by
+C<op_last>. The children in between can be found by iteratively
+following the C<op_sibling> pointer from the first child to the last.
+
+There are also two other op types: a C<PMOP> holds a regular expression,
+and has no children, and a C<LOOP> may or may not have children. If the
+C<op_children> field is non-zero, it behaves like a C<LISTOP>. To
+complicate matters, if a C<UNOP> is actually a C<null> op after
+optimization (see L</Compile pass 2: context propagation>) it will still
+have children in accordance with its former type.
+

 =head2 Compile pass 1: check routines
 
 The tree is created by the compiler while I<yacc> code feeds it
 =head2 Compile pass 1: check routines
 
 The tree is created by the compiler while I<yacc> code feeds it


@@ -1571,6 +1659,41 @@ additional complications for conditionals).  These optimizations are
 done in the subroutine peep().  Optimizations performed at this stage
 are subject to the same restrictions as in the pass 2.
 
 done in the subroutine peep().  Optimizations performed at this stage
 are subject to the same restrictions as in the pass 2.
 

+=head1 Examining internal data structures with the C<dump> functions
+
+To aid debugging, the source file F<dump.c> contains a number of
+functions which produce formatted output of internal data structures.
+
+The most commonly used of these functions is C<Perl_sv_dump>; it's used
+for dumping SVs, AVs, HVs, and CVs. The C<Devel::Peek> module calls
+C<sv_dump> to produce debugging output from Perl-space, so users of that
+module should already be familiar with its format.
+
+C<Perl_op_dump> can be used to dump an C<OP> structure or any of its
+derivatives, and produces output similiar to C<perl -Dx>; in fact,
+C<Perl_dump_eval> will dump the main root of the code being evaluated,
+exactly like C<-Dx>.
+
+Other useful functions are C<Perl_dump_sub>, which turns a C<GV> into an
+op tree, C<Perl_dump_packsubs> which calls C<Perl_dump_sub> on all the
+subroutines in a package like so: (Thankfully, these are all xsubs, so
+there is no op tree)
+
+    (gdb) print Perl_dump_packsubs(PL_defstash)
+
+    SUB attributes::bootstrap = (xsub 0x811fedc 0)
+
+    SUB UNIVERSAL::can = (xsub 0x811f50c 0)
+
+    SUB UNIVERSAL::isa = (xsub 0x811f304 0)
+
+    SUB UNIVERSAL::VERSION = (xsub 0x811f7ac 0)
+
+    SUB DynaLoader::boot_DynaLoader = (xsub 0x805b188 0)
+
+and C<Perl_dump_all>, which dumps all the subroutines in the stash and
+the op tree of the main root.
+

 =head1 How multiple interpreters and concurrency are supported
 
 =head2 Background and PERL_IMPLICIT_CONTEXT
 =head1 How multiple interpreters and concurrency are supported
 
 =head2 Background and PERL_IMPLICIT_CONTEXT


@@ -1587,8 +1710,8 @@ interpreter.
 Three macros control the major Perl build flavors: MULTIPLICITY,
 USE_THREADS and PERL_OBJECT.  The MULTIPLICITY build has a C structure
 that packages all the interpreter state, there is a similar thread-specific
 Three macros control the major Perl build flavors: MULTIPLICITY,
 USE_THREADS and PERL_OBJECT.  The MULTIPLICITY build has a C structure
 that packages all the interpreter state, there is a similar thread-specific

-data structure under USE_THREADS, and the PERL_OBJECT build has a C++
-class to maintain interpreter state.  In all three cases,
+data structure under USE_THREADS, and the (now deprecated) PERL_OBJECT
+build has a C++ class to maintain interpreter state.  In all three cases,

 PERL_IMPLICIT_CONTEXT is also normally defined, and enables the
 support for passing in a "hidden" first argument that represents all three
 data structures.
 PERL_IMPLICIT_CONTEXT is also normally defined, and enables the
 support for passing in a "hidden" first argument that represents all three
 data structures.


@@ -1601,13 +1724,13 @@ the Perl source (as it does in so many other situations) makes heavy
 use of macros and subroutine naming conventions.
 
 First problem: deciding which functions will be public API functions and
 use of macros and subroutine naming conventions.
 
 First problem: deciding which functions will be public API functions and

-which will be private.  All functions whose names begin C<S_> are private 
+which will be private.  All functions whose names begin C<S_> are private

 (think "S" for "secret" or "static").  All other functions begin with
 "Perl_", but just because a function begins with "Perl_" does not mean it is
 (think "S" for "secret" or "static").  All other functions begin with
 "Perl_", but just because a function begins with "Perl_" does not mean it is

-part of the API. (See L</Internal Functions>.) The easiest way to be B<sure> a 
-function is part of the API is to find its entry in L<perlapi>.  
-If it exists in L<perlapi>, it's part of the API.  If it doesn't, and you 
-think it should be (i.e., you need it for your extension), send mail via 
+part of the API. (See L</Internal Functions>.) The easiest way to be B<sure> a
+function is part of the API is to find its entry in L<perlapi>.
+If it exists in L<perlapi>, it's part of the API.  If it doesn't, and you
+think it should be (i.e., you need it for your extension), send mail via

 L<perlbug> explaining why you think it should be.
 
 Second problem: there must be a syntax so that the same subroutine
 L<perlbug> explaining why you think it should be.
 
 Second problem: there must be a syntax so that the same subroutine


@@ -1631,10 +1754,11 @@ C<pTHX_> is one of a number of macros (in perl.h) that hide the
 details of the interpreter's context.  THX stands for "thread", "this",
 or "thingy", as the case may be.  (And no, George Lucas is not involved. :-)
 The first character could be 'p' for a B<p>rototype, 'a' for B<a>rgument,
 details of the interpreter's context.  THX stands for "thread", "this",
 or "thingy", as the case may be.  (And no, George Lucas is not involved. :-)
 The first character could be 'p' for a B<p>rototype, 'a' for B<a>rgument,

-or 'd' for B<d>eclaration.
+or 'd' for B<d>eclaration, so we have C<pTHX>, C<aTHX> and C<dTHX>, and
+their variants.

 
 

-When Perl is built without PERL_IMPLICIT_CONTEXT, there is no first
-argument containing the interpreter's context.  The trailing underscore
+When Perl is built without options that set PERL_IMPLICIT_CONTEXT, there is no
+first argument containing the interpreter's context.  The trailing underscore

 in the pTHX_ macro indicates that the macro expansion needs a comma
 after the context argument because other arguments follow it.  If
 PERL_IMPLICIT_CONTEXT is not defined, pTHX_ will be ignored, and the
 in the pTHX_ macro indicates that the macro expansion needs a comma
 after the context argument because other arguments follow it.  If
 PERL_IMPLICIT_CONTEXT is not defined, pTHX_ will be ignored, and the


@@ -1643,14 +1767,14 @@ macro without the trailing underscore is used when there are no additional
 explicit arguments.
 
 When a core function calls another, it must pass the context.  This
 explicit arguments.
 
 When a core function calls another, it must pass the context.  This

-is normally hidden via macros.  Consider C<sv_setsv>.  It expands
+is normally hidden via macros.  Consider C<sv_setsv>.  It expands into

 something like this:
 
     ifdef PERL_IMPLICIT_CONTEXT
 something like this:
 
     ifdef PERL_IMPLICIT_CONTEXT

-      define sv_setsv(a,b)     Perl_sv_setsv(aTHX_ a, b)
+      define sv_setsv(a,b)      Perl_sv_setsv(aTHX_ a, b)

       /* can't do this for vararg functions, see below */
     else
       /* can't do this for vararg functions, see below */
     else

-      define sv_setsv          Perl_sv_setsv
+      define sv_setsv           Perl_sv_setsv

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


@@ -1670,8 +1794,8 @@ Under PERL_OBJECT in the core, that will translate to either:
                                        # see objXSUB.h
 
 Under PERL_OBJECT in extensions (aka PERL_CAPI), or under
                                        # see objXSUB.h
 
 Under PERL_OBJECT in extensions (aka PERL_CAPI), or under

-MULTIPLICITY/USE_THREADS w/ PERL_IMPLICIT_CONTEXT in both core
-and extensions, it will be:
+MULTIPLICITY/USE_THREADS with PERL_IMPLICIT_CONTEXT in both core
+and extensions, it will become:

 
     Perl_sv_setsv(aTHX_ foo, bar);     # the canonical Perl "API"
                                        # for all build flavors
 
     Perl_sv_setsv(aTHX_ foo, bar);     # the canonical Perl "API"
                                        # for all build flavors


@@ -1693,6 +1817,14 @@ You can ignore [pad]THX[xo] when browsing the Perl headers/sources.
 Those are strictly for use within the core.  Extensions and embedders
 need only be aware of [pad]THX.
 
 Those are strictly for use within the core.  Extensions and embedders
 need only be aware of [pad]THX.
 

+=head2 So what happened to dTHR?
+
+C<dTHR> was introduced in perl 5.005 to support the older thread model.
+The older thread model now uses the C<THX> mechanism to pass context
+pointers around, so C<dTHR> is not useful any more.  Perl 5.6.0 and
+later still have it for backward source compatibility, but it is defined
+to be a no-op.
+

 =head2 How do I use all this in extensions?
 
 When Perl is built with PERL_IMPLICIT_CONTEXT, extensions that call
 =head2 How do I use all this in extensions?
 
 When Perl is built with PERL_IMPLICIT_CONTEXT, extensions that call


@@ -1725,31 +1857,31 @@ work.
 The second, more efficient way is to use the following template for
 your Foo.xs:
 
 The second, more efficient way is to use the following template for
 your Foo.xs:
 

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

 
         static my_private_function(int arg1, int arg2);
 
 
         static my_private_function(int arg1, int arg2);
 

-       static SV *
-       my_private_function(int arg1, int arg2)
-       {
-           dTHX;       /* fetch context */
-           ... call many Perl API functions ...
-       }
+        static SV *
+        my_private_function(int arg1, int arg2)
+        {
+            dTHX;       /* fetch context */
+            ... call many Perl API functions ...
+        }

 
         [... etc ...]
 
 
         [... etc ...]
 

-       MODULE = Foo            PACKAGE = Foo
+        MODULE = Foo            PACKAGE = Foo

 
 

-       /* typical XSUB */
+        /* typical XSUB */

 
 

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

 
 Note that the only two changes from the normal way of writing an
 extension is the addition of a C<#define PERL_NO_GET_CONTEXT> before
 
 Note that the only two changes from the normal way of writing an
 extension is the addition of a C<#define PERL_NO_GET_CONTEXT> before


@@ -1764,32 +1896,32 @@ The third, even more efficient way is to ape how it is done within
 the Perl guts:
 
 
 the Perl guts:
 
 

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

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

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

 
         [... etc ...]
 
 
         [... etc ...]
 

-       MODULE = Foo            PACKAGE = Foo
+        MODULE = Foo            PACKAGE = Foo

 
 

-       /* typical XSUB */
+        /* typical XSUB */

 
 

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

 
 This implementation never has to fetch the context using a function
 call, since it is always passed as an extra argument.  Depending on
 
 This implementation never has to fetch the context using a function
 call, since it is always passed as an extra argument.  Depending on


@@ -1800,15 +1932,34 @@ Never add a comma after C<pTHX> yourself--always use the form of the
 macro with the underscore for functions that take explicit arguments,
 or the form without the argument for functions with no explicit arguments.
 
 macro with the underscore for functions that take explicit arguments,
 or the form without the argument for functions with no explicit arguments.
 

+=head2 Should I do anything special if I call perl from multiple threads?
+
+If you create interpreters in one thread and then proceed to call them in
+another, you need to make sure perl's own Thread Local Storage (TLS) slot is
+initialized correctly in each of those threads.
+
+The C<perl_alloc> and C<perl_clone> API functions will automatically set
+the TLS slot to the interpreter they created, so that there is no need to do
+anything special if the interpreter is always accessed in the same thread that
+created it, and that thread did not create or call any other interpreters
+afterwards.  If that is not the case, you have to set the TLS slot of the
+thread before calling any functions in the Perl API on that particular
+interpreter.  This is done by calling the C<PERL_SET_CONTEXT> macro in that
+thread as the first thing you do:
+
+       /* do this before doing anything else with some_perl */
+       PERL_SET_CONTEXT(some_perl);
+
+       ... other Perl API calls on some_perl go here ...
+

 =head2 Future Plans and PERL_IMPLICIT_SYS
 
 Just as PERL_IMPLICIT_CONTEXT provides a way to bundle up everything
 that the interpreter knows about itself and pass it around, so too are
 there plans to allow the interpreter to bundle up everything it knows
 about the environment it's running on.  This is enabled with the
 =head2 Future Plans and PERL_IMPLICIT_SYS
 
 Just as PERL_IMPLICIT_CONTEXT provides a way to bundle up everything
 that the interpreter knows about itself and pass it around, so too are
 there plans to allow the interpreter to bundle up everything it knows
 about the environment it's running on.  This is enabled with the

-PERL_IMPLICIT_SYS macro.  Currently it only works with PERL_OBJECT,
-but is mostly there for MULTIPLICITY and USE_THREADS (see inside
-iperlsys.h).
+PERL_IMPLICIT_SYS macro.  Currently it only works with PERL_OBJECT
+and USE_THREADS on Windows (see inside iperlsys.h).

 
 This allows the ability to provide an extra pointer (called the "host"
 environment) for all the system calls.  This makes it possible for
 
 This allows the ability to provide an extra pointer (called the "host"
 environment) for all the system calls.  This makes it possible for


@@ -1868,7 +2019,8 @@ Other available flags are:
 
 =item s
 
 
 =item s
 

-This is a static function and is defined as C<S_whatever>.
+This is a static function and is defined as C<S_whatever>, and usually
+called within the sources as C<whatever(...)>.

 
 =item n
 
 
 =item n
 


@@ -1886,9 +2038,9 @@ The argument list should end with C<...>, like this:
 
     Afprd   |void   |croak          |const char* pat|...
 
 
     Afprd   |void   |croak          |const char* pat|...
 

-=item m
+=item M

 
 

-This function is part of the experimental development API, and may change 
+This function is part of the experimental development API, and may change

 or disappear without notice.
 
 =item o
 or disappear without notice.
 
 =item o


@@ -1916,18 +2068,18 @@ If you are printing IVs, UVs, or NVS instead of the stdio(3) style
 formatting codes like C<%d>, C<%ld>, C<%f>, you should use the
 following macros for portability
 
 formatting codes like C<%d>, C<%ld>, C<%f>, you should use the
 following macros for portability
 

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

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

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

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


@@ -1939,20 +2091,20 @@ with PTR2UV(), do not use %lx or %p.
 Because pointer size does not necessarily equal integer size,
 use the follow macros to do it right.
 
 Because pointer size does not necessarily equal integer size,
 use the follow macros to do it right.
 

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

 
 For example:
 
 
 For example:
 

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

 
 and
 
 
 and
 

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

 
 =head2 Source Documentation
 
 
 =head2 Source Documentation