SvOK(SV*)
The scalar C<undef> value is stored in an SV instance called C<PL_sv_undef>.
-Its address can be used whenever an C<SV*> is needed.
+
+Its address can be used whenever an C<SV*> is needed. Make sure that
+you don't try to compare a random sv with C<&PL_sv_undef>. For example
+when interfacing Perl code, it'll work correctly for:
+
+ foo(undef);
+
+But won't work when called as:
+
+ $x = undef;
+ foo($x);
+
+So to repeat always use SvOK() to check whether an sv is defined.
+
+Also you have to be careful when using C<&PL_sv_undef> as a value in
+AVs or HVs (see L<AVs, HVs and undefined values>).
There are also the two values C<PL_sv_yes> and C<PL_sv_no>, which contain
boolean TRUE and FALSE values, respectively. Like C<PL_sv_undef>, their
If these functions return a NULL value, the caller will usually have to
decrement the reference count of C<val> to avoid a memory leak.
+=head2 AVs, HVs and undefined values
+
+Sometimes you have to store undefined values in AVs or HVs. Although
+this may be a rare case, it can be tricky. That's because you're
+used to using C<&PL_sv_undef> if you need an undefined SV.
+
+For example, intuition tells you that this XS code:
+
+ AV *av = newAV();
+ av_store( av, 0, &PL_sv_undef );
+
+is equivalent to this Perl code:
+
+ my @av;
+ $av[0] = undef;
+
+Unfortunately, this isn't true. AVs use C<&PL_sv_undef> as a marker
+for indicating that an array element has not yet been initialized.
+Thus, C<exists $av[0]> would be true for the above Perl code, but
+false for the array generated by the XS code.
+
+Other problems can occur when storing C<&PL_sv_undef> in HVs:
+
+ hv_store( hv, "key", 3, &PL_sv_undef, 0 );
+
+This will indeed make the value C<undef>, but if you try to modify
+the value of C<key>, you'll get the following error:
+
+ Modification of non-creatable hash value attempted
+
+In perl 5.8.0, C<&PL_sv_undef> was also used to mark placeholders
+in restricted hashes. This caused such hash entries not to appear
+when iterating over the hash or when checking for the keys
+with the C<hv_exists> function.
+
+You can run into similar problems when you store C<&PL_sv_true> or
+C<&PL_sv_false> into AVs or HVs. Trying to modify such elements
+will give you the following error:
+
+ Modification of a read-only value attempted
+
+To make a long story short, you can use the special variables
+C<&PL_sv_undef>, C<&PL_sv_true> and C<&PL_sv_false> with AVs and
+HVs, but you have to make sure you know what you're doing.
+
+Generally, if you want to store an undefined value in an AV
+or HV, you should not use C<&PL_sv_undef>, but rather create a
+new undefined value using the C<newSV> function, for example:
+
+ av_store( av, 42, newSV(0) );
+ hv_store( hv, "foo", 3, newSV(0), 0 );
+
=head2 References
References are a special type of scalar that point to other data types
The C<name> and C<namlen> arguments are used to associate a string with
the magic, typically the name of a variable. C<namlen> is stored in the
-C<mg_len> field and if C<name> is non-null and C<namlen> E<gt>= 0 a malloc'd
-copy of the name is stored in C<mg_ptr> field.
+C<mg_len> field and if C<name> is non-null then either a C<savepvn> copy of
+C<name> or C<name> itself is stored in the C<mg_ptr> field, depending on
+whether C<namlen> is greater than zero or equal to zero respectively. As a
+special case, if C<(name && namlen == HEf_SVKEY)> then C<name> is assumed
+to contain an C<SV*> and is stored as-is with its REFCNT incremented.
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.
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.
+See also C<sv_magicext> in L<perlapi> for a more flexible way to add magic
+to an SV.
+
There is also a function to add magic to an C<HV>:
void hv_magic(HV *hv, GV *gv, int how);
Duplicates the current value of C<SV>, on the exit from the current
C<ENTER>/C<LEAVE> I<pseudo-block> will restore the value of C<SV>
-using the stored value.
+using the stored value. It doesn't handle magic. Use C<save_scalar> if
+magic is affected.
=item C<void save_list(SV **sarg, I32 maxsarg)>
Now that there is room on the stack, values can be pushed on it using C<PUSHs>
macro. The pushed values will often need to be "mortal" (See
-L</Reference Counts and Mortality>).
+L</Reference Counts and Mortality>):
PUSHs(sv_2mortal(newSViv(an_integer)))
+ PUSHs(sv_2mortal(newSVuv(an_unsigned_integer)))
+ PUSHs(sv_2mortal(newSVnv(a_double)))
PUSHs(sv_2mortal(newSVpv("Some String",0)))
- PUSHs(sv_2mortal(newSVnv(3.141592)))
And now the Perl program calling C<tzname>, the two values will be assigned
as in:
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>.
+C<(X)PUSH[iunp]> are I<not> suited to XSUBs which return multiple results.
+For that, either stick to the C<(X)PUSHs> macros shown above, or use the new
+C<m(X)PUSH[iunp]> macros instead; see L</Putting a C value on Perl stack>.
For more information, consult L<perlxs> and L<perlxstut>.
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]>.
+others, which use it via C<(X)PUSH[iunp]>.
Because the target is reused, you must be careful when pushing multiple
values on the stack. The following code will not do what you think:
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>.
+to 20.
+
+If you need to push multiple different values then you should either use
+the C<(X)PUSHs> macros, or else use the new C<m(X)PUSH[iunp]> macros,
+none of which make use of C<TARG>. The C<(X)PUSHs> macros simply push an
+SV* on the stack, which, as noted under L</XSUBs and the Argument Stack>,
+will often need to be "mortal". The new C<m(X)PUSH[iunp]> macros make
+this a little easier to achieve by creating a new mortal for you (via
+C<(X)PUSHmortal>), pushing that onto the stack (extending it if necessary
+in the case of the C<mXPUSH[iunp]> macros), and then setting its value.
+Thus, instead of writing this to "fix" the example above:
+
+ XPUSHs(sv_2mortal(newSViv(10)))
+ XPUSHs(sv_2mortal(newSViv(20)))
-On a related note, if you do use C<(X)PUSH[npi]>, then you're going to
+you can simply write:
+
+ mXPUSHi(10)
+ mXPUSHi(20)
+
+On a related note, if you do use C<(X)PUSH[iunp]>, 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>.
+macros can make use of the local variable C<TARG>. See also C<dTARGET>
+and C<dXSTARG>.
=head2 Scratchpads
=head2 Pluggable runops
The compile tree is executed in a runops function. There are two runops
-functions in F<run.c>. C<Perl_runops_debug> is used with DEBUGGING and
-C<Perl_runops_standard> is used otherwise. For fine control over the
-execution of the compile tree it is possible to provide your own runops
-function.
+functions, in F<run.c> and in F<dump.c>. C<Perl_runops_debug> is used
+with DEBUGGING and C<Perl_runops_standard> is used otherwise. For fine
+control over the execution of the compile tree it is possible to provide
+your own runops function.
It's probably best to copy one of the existing runops functions and
change it to suit your needs. Then, in the BOOT section of your XS
Inside the Perl core, you can get at the functions either with or
without the C<Perl_> prefix, thanks to a bunch of defines that live in
F<embed.h>. This header file is generated automatically from
-F<embed.pl>. F<embed.pl> also creates the prototyping header files for
-the internal functions, generates the documentation and a lot of other
-bits and pieces. It's important that when you add a new function to the
-core or change an existing one, you change the data in the table at the
-end of F<embed.pl> as well. Here's a sample entry from that table:
+F<embed.pl> and F<embed.fnc>. F<embed.pl> also creates the prototyping
+header files for the internal functions, generates the documentation
+and a lot of other bits and pieces. It's important that when you add
+a new function to the core or change an existing one, you change the
+data in the table in F<embed.fnc> as well. Here's a sample entry from
+that table:
Apd |SV** |av_fetch |AV* ar|I32 key|I32 lval
This function should not have a compatibility macro to define, say,
C<Perl_parse> to C<parse>. It must be called as C<Perl_parse>.
-=item j
-
-This function is not a member of C<CPerlObj>. If you don't know
-what this means, don't use it.
-
=item x
This function isn't exported out of the Perl core.
+=item m
+
+This is implemented as a macro.
+
+=item X
+
+This function is explicitly exported.
+
+=item E
+
+This function is visible to extensions included in the Perl core.
+
+=item b
+
+Binary backward compatibility; this function is a macro but also has
+a C<Perl_> implementation (which is exported).
+
=back
-If you edit F<embed.pl>, you will need to run C<make regen_headers> to
-force a rebuild of F<embed.h> and other auto-generated files.
+If you edit F<embed.pl> or F<embed.fnc>, you will need to run
+C<make regen_headers> to force a rebuild of F<embed.h> and other
+auto-generated files.
=head2 Formatted Printing of IVs, UVs, and NVs
AV *av = ...;
UV uv = PTR2UV(av);
+=head2 Exception Handling
+
+There are a couple of macros to do very basic exception handling in XS
+modules. You have to define C<NO_XSLOCKS> before including F<XSUB.h> to
+be able to use these macros:
+
+ #define NO_XSLOCKS
+ #include "XSUB.h"
+
+You can use these macros if you call code that may croak, but you need
+to do some cleanup before giving control back to Perl. For example:
+
+ dXCPT; /* set up neccessary variables */
+
+ XCPT_TRY_START {
+ code_that_may_croak();
+ } XCPT_TRY_END
+
+ XCPT_CATCH
+ {
+ /* do cleanup here */
+ XCPT_RETHROW;
+ }
+
+Note that you always have to rethrow an exception that has been
+caught. Using these macros, it is not possible to just catch the
+exception and ignore it. If you have to ignore the exception, you
+have to use the C<call_*> function.
+
+The advantage of using the above macros is that you don't have
+to setup an extra function for C<call_*>, and that using these
+macros is faster than using C<call_*>.
+
=head2 Source Documentation
There's an effort going on to document the internal functions and
Please try and supply some documentation if you add functions to the
Perl core.
+=head2 Backwards compatibility
+
+The Perl API changes over time. New functions are added or the interfaces
+of existing functions are changed. The C<Devel::PPPort> module tries to
+provide compatibility code for some of these changes, so XS writers don't
+have to code it themselves when supporting multiple versions of Perl.
+
+C<Devel::PPPort> generates a C header file F<ppport.h> that can also
+be run as a Perl script. To generate F<ppport.h>, run:
+
+ perl -MDevel::PPPort -eDevel::PPPort::WriteFile
+
+Besides checking existing XS code, the script can also be used to retrieve
+compatibility information for various API calls using the C<--api-info>
+command line switch. For example:
+
+ % perl ppport.h --api-info=sv_magicext
+
+For details, see C<perldoc ppport.h>.
+
=head1 Unicode Support
Perl 5.6.0 introduced Unicode support. It's important for porters and XS
To fix this, some people formed Unicode, Inc. and
produced a new character set containing all the characters you can
possibly think of and more. There are several ways of representing these
-characters, and the one Perl uses is called UTF8. UTF8 uses
+characters, and the one Perl uses is called UTF-8. UTF-8 uses
a variable number of bytes to represent a character, instead of just
one. You can learn more about Unicode at http://www.unicode.org/
-=head2 How can I recognise a UTF8 string?
+=head2 How can I recognise a UTF-8 string?
-You can't. This is because UTF8 data is stored in bytes just like
-non-UTF8 data. The Unicode character 200, (C<0xC8> for you hex types)
+You can't. This is because UTF-8 data is stored in bytes just like
+non-UTF-8 data. The Unicode character 200, (C<0xC8> for you hex types)
capital E with a grave accent, is represented by the two bytes
C<v196.172>. Unfortunately, the non-Unicode string C<chr(196).chr(172)>
has that byte sequence as well. So you can't tell just by looking - this
is what makes Unicode input an interesting problem.
The API function C<is_utf8_string> can help; it'll tell you if a string
-contains only valid UTF8 characters. However, it can't do the work for
+contains only valid UTF-8 characters. However, it can't do the work for
you. On a character-by-character basis, C<is_utf8_char> will tell you
-whether the current character in a string is valid UTF8.
+whether the current character in a string is valid UTF-8.
-=head2 How does UTF8 represent Unicode characters?
+=head2 How does UTF-8 represent Unicode characters?
-As mentioned above, UTF8 uses a variable number of bytes to store a
+As mentioned above, UTF-8 uses a variable number of bytes to store a
character. Characters with values 1...128 are stored in one byte, just
like good ol' ASCII. Character 129 is stored as C<v194.129>; this
continues up to character 191, which is C<v194.191>. Now we've run out of
bits (191 is binary C<10111111>) so we move on; 192 is C<v195.128>. And
so it goes on, moving to three bytes at character 2048.
-Assuming you know you're dealing with a UTF8 string, you can find out
+Assuming you know you're dealing with a UTF-8 string, you can find out
how long the first character in it is with the C<UTF8SKIP> macro:
char *utf = "\305\233\340\240\201";
utf += len;
len = UTF8SKIP(utf); /* len is 3 here */
-Another way to skip over characters in a UTF8 string is to use
+Another way to skip over characters in a UTF-8 string is to use
C<utf8_hop>, which takes a string and a number of characters to skip
over. You're on your own about bounds checking, though, so don't use it
lightly.
-All bytes in a multi-byte UTF8 character will have the high bit set,
+All bytes in a multi-byte UTF-8 character will have the high bit set,
so you can test if you need to do something special with this
character like this (the UTF8_IS_INVARIANT() is a macro that tests
whether the byte can be encoded as a single byte even in UTF-8):
UV uv; /* Note: a UV, not a U8, not a char */
if (!UTF8_IS_INVARIANT(*utf))
- /* Must treat this as UTF8 */
+ /* Must treat this as UTF-8 */
uv = utf8_to_uv(utf);
else
/* OK to treat this character as a byte */
You can also see in that example that we use C<utf8_to_uv> to get the
value of the character; the inverse function C<uv_to_utf8> is available
-for putting a UV into UTF8:
+for putting a UV into UTF-8:
if (!UTF8_IS_INVARIANT(uv))
/* Must treat this as UTF8 */
*utf8++ = uv;
You B<must> convert characters to UVs using the above functions if
-you're ever in a situation where you have to match UTF8 and non-UTF8
-characters. You may not skip over UTF8 characters in this case. If you
-do this, you'll lose the ability to match hi-bit non-UTF8 characters;
-for instance, if your UTF8 string contains C<v196.172>, and you skip
-that character, you can never match a C<chr(200)> in a non-UTF8 string.
+you're ever in a situation where you have to match UTF-8 and non-UTF-8
+characters. You may not skip over UTF-8 characters in this case. If you
+do this, you'll lose the ability to match hi-bit non-UTF-8 characters;
+for instance, if your UTF-8 string contains C<v196.172>, and you skip
+that character, you can never match a C<chr(200)> in a non-UTF-8 string.
So don't do that!
-=head2 How does Perl store UTF8 strings?
+=head2 How does Perl store UTF-8 strings?
Currently, Perl deals with Unicode strings and non-Unicode strings
slightly differently. If a string has been identified as being UTF-8
undesirable results.
The problem comes when you have, for instance, a string that isn't
-flagged is UTF8, and contains a byte sequence that could be UTF8 -
-especially when combining non-UTF8 and UTF8 strings.
+flagged is UTF-8, and contains a byte sequence that could be UTF-8 -
+especially when combining non-UTF-8 and UTF-8 strings.
Never forget that the C<SVf_UTF8> flag is separate to the PV value; you
need be sure you don't accidentally knock it off while you're
The C<char*> string does not tell you the whole story, and you can't
copy or reconstruct an SV just by copying the string value. Check if the
-old SV has the UTF8 flag set, and act accordingly:
+old SV has the UTF-8 flag set, and act accordingly:
p = SvPV(sv, len);
frobnicate(p);
SvUTF8_on(nsv);
In fact, your C<frobnicate> function should be made aware of whether or
-not it's dealing with UTF8 data, so that it can handle the string
+not it's dealing with UTF-8 data, so that it can handle the string
appropriately.
Since just passing an SV to an XS function and copying the data of
-the SV is not enough to copy the UTF8 flags, even less right is just
+the SV is not enough to copy the UTF-8 flags, even less right is just
passing a C<char *> to an XS function.
-=head2 How do I convert a string to UTF8?
+=head2 How do I convert a string to UTF-8?
-If you're mixing UTF8 and non-UTF8 strings, you might find it necessary
-to upgrade one of the strings to UTF8. If you've got an SV, the easiest
+If you're mixing UTF-8 and non-UTF-8 strings, you might find it necessary
+to upgrade one of the strings to UTF-8. If you've got an SV, the easiest
way to do this is:
sv_utf8_upgrade(sv);
strings that came into the operator, and, while it shouldn't be noticeable
by the end user, it can cause problems.
-Instead, C<bytes_to_utf8> will give you a UTF8-encoded B<copy> of its
+Instead, C<bytes_to_utf8> will give you a UTF-8-encoded B<copy> of its
string argument. This is useful for having the data available for
comparisons and so on, without harming the original SV. There's also
C<utf8_to_bytes> to go the other way, but naturally, this will fail if
=item *
-There's no way to tell if a string is UTF8 or not. You can tell if an SV
-is UTF8 by looking at is C<SvUTF8> flag. Don't forget to set the flag if
-something should be UTF8. Treat the flag as part of the PV, even though
+There's no way to tell if a string is UTF-8 or not. You can tell if an SV
+is UTF-8 by looking at is C<SvUTF8> flag. Don't forget to set the flag if
+something should be UTF-8. Treat the flag as part of the PV, even though
it's not - if you pass on the PV to somewhere, pass on the flag too.
=item *
-If a string is UTF8, B<always> use C<utf8_to_uv> to get at the value,
+If a string is UTF-8, B<always> use C<utf8_to_uv> to get at the value,
unless C<UTF8_IS_INVARIANT(*s)> in which case you can use C<*s>.
=item *
-When writing a character C<uv> to a UTF8 string, B<always> use
+When writing a character C<uv> to a UTF-8 string, B<always> use
C<uv_to_utf8>, unless C<UTF8_IS_INVARIANT(uv))> in which case
you can use C<*s = uv>.
=item *
-Mixing UTF8 and non-UTF8 strings is tricky. Use C<bytes_to_utf8> to get
-a new string which is UTF8 encoded. There are tricks you can use to
-delay deciding whether you need to use a UTF8 string until you get to a
+Mixing UTF-8 and non-UTF-8 strings is tricky. Use C<bytes_to_utf8> to get
+a new string which is UTF-8 encoded. There are tricks you can use to
+delay deciding whether you need to use a UTF-8 string until you get to a
high character - C<HALF_UPGRADE> is one of those.
=back
projects / p5sagit/p5-mst-13.2.git / blobdiff