X-Git-Url: http://git.shadowcat.co.uk/gitweb/gitweb.cgi?a=blobdiff_plain;f=pod%2Fperlguts.pod;h=78a1dfc50e686876afec2ef571929f578ee4ec78;hb=038fcae385eee134ba22a23fbbf09eafafbe7927;hp=83ed06860444bcd1e3b4bddd7476e32630c7017d;hpb=2307c6d0ffdaa48a896531d5bc3bf64c2dee420d;p=p5sagit%2Fp5-mst-13.2.git diff --git a/pod/perlguts.pod b/pod/perlguts.pod index 83ed068..78a1dfc 100644 --- a/pod/perlguts.pod +++ b/pod/perlguts.pod @@ -5,7 +5,7 @@ perlguts - Introduction to the Perl API =head1 DESCRIPTION This document attempts to describe how to use the Perl API, as well as -containing some info on the basic workings of the Perl core. It is far +to provide some info on the basic workings of the Perl core. It is far from complete and probably contains many errors. Please refer any questions or comments to the author below. @@ -43,26 +43,31 @@ The seven routines are: SV* newSViv(IV); SV* newSVuv(UV); SV* newSVnv(double); - SV* newSVpv(const char*, int); - SV* newSVpvn(const char*, int); + SV* newSVpv(const char*, STRLEN); + SV* newSVpvn(const char*, STRLEN); SV* newSVpvf(const char*, ...); SV* newSVsv(SV*); -If you require more complex initialisation you can create an empty SV with -newSV(len). If C is 0 an empty SV of type NULL is returned, else an -SV of type PV is returned with len + 1 (for the NUL) bytes of storage -allocated, accessible via SvPVX. In both cases the SV has value undef. +C is an integer type (Size_t, usually defined as size_t in +F) guaranteed to be large enough to represent the size of +any string that perl can handle. + +In the unlikely case of a SV requiring more complex initialisation, you +can create an empty SV with newSV(len). If C is 0 an empty SV of +type NULL is returned, else an SV of type PV is returned with len + 1 (for +the NUL) bytes of storage allocated, accessible via SvPVX. In both cases +the SV has value undef. - SV* newSV(0); /* no storage allocated */ - SV* newSV(10); /* 10 (+1) bytes of uninitialised storage allocated */ + SV *sv = newSV(0); /* no storage allocated */ + SV *sv = newSV(10); /* 10 (+1) bytes of uninitialised storage allocated */ -To change the value of an *already-existing* SV, there are eight routines: +To change the value of an I SV, there are eight routines: void sv_setiv(SV*, IV); void sv_setuv(SV*, UV); void sv_setnv(SV*, double); void sv_setpv(SV*, const char*); - void sv_setpvn(SV*, const char*, int) + void sv_setpvn(SV*, const char*, STRLEN) void sv_setpvf(SV*, const char*, ...); void sv_vsetpvfn(SV*, const char*, STRLEN, va_list *, SV **, I32, bool *); void sv_setsv(SV*, SV*); @@ -86,10 +91,6 @@ L). This pointer may be NULL if that information is not 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 functions are not generic enough to operate on values that have "magic". See L later in this document. @@ -199,12 +200,14 @@ you can call: SvOK(SV*) -The scalar C value is stored in an SV instance called C. Its -address can be used whenever an C is needed. +The scalar C value is stored in an SV instance called C. +Its address can be used whenever an C is needed. +However, you have to be careful when using C<&PL_sv_undef> as a value in AVs +or HVs (see L). -There are also the two values C and C, which contain Boolean -TRUE and FALSE values, respectively. Like C, their addresses can -be used whenever an C is needed. +There are also the two values C and C, which contain +boolean TRUE and FALSE values, respectively. Like C, their +addresses can be used whenever an C is needed. Do not be fooled into thinking that C<(SV *) 0> is the same as C<&PL_sv_undef>. Take this code: @@ -218,8 +221,8 @@ Take this code: This code tries to return a new SV (which contains the value 42) if it should return a real value, or undef otherwise. Instead it has returned a NULL pointer which, somewhere down the line, will cause a segmentation violation, -bus error, or just weird results. Change the zero to C<&PL_sv_undef> in the first -line and all will be well. +bus error, or just weird results. Change the zero to C<&PL_sv_undef> in the +first line and all will be well. To free an SV that you've created, call C. Normally this call is not necessary (see L). @@ -488,6 +491,58 @@ reference count of the stored C, which is the caller's responsibility. If these functions return a NULL value, the caller will usually have to decrement the reference count of C 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 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, but if you try to modify +the value of C, 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 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 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 @@ -536,7 +591,7 @@ The most useful types that will be returned are: =head2 Blessed References and Class Objects -References are also used to support object-oriented programming. In the +References are also used to support object-oriented programming. In perl's OO lexicon, an object is simply a reference that has been blessed into a package (or class). Once blessed, the programmer may now use the reference to access the various methods in the class. @@ -545,8 +600,8 @@ A reference can be blessed into a package with the following function: SV* sv_bless(SV* sv, HV* stash); -The C argument must be a reference. The C argument specifies -which class the reference will belong to. See +The C argument must be a reference value. The C argument +specifies which class the reference will belong to. See L for information on converting class names into stashes. /* Still under construction */ @@ -685,9 +740,9 @@ later be decremented twice. "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. +is made mortal to have it cleaned up automatically when it's popped off +the stack. Similarly, results returned by XSUBs (which are pushed on the +stack) are often made mortal. To create a mortal variable, use the functions: @@ -724,8 +779,8 @@ C or C routines. =head2 Stashes and Globs -A "stash" is a hash that contains all of the different objects that -are contained within a package. Each key of the stash is a symbol +A B is a hash that contains all variables that are defined +within a package. Each key of the stash is a symbol name (shared by all the different types of objects that have the same name), and each value in the hash table is a GV (Glob Value). This GV in turn contains references to the various objects of that name, @@ -738,11 +793,11 @@ including (but not limited to) the following: Format Subroutine -There is a single stash called "PL_defstash" that holds the items that exist -in the "main" package. To get at the items in other packages, append the -string "::" to the package name. The items in the "Foo" package are in -the stash "Foo::" in PL_defstash. The items in the "Bar::Baz" package are -in the stash "Baz::" in "Bar::"'s stash. +There is a single stash called C that holds the items that exist +in the C
package. To get at the items in other packages, append the +string "::" to the package name. The items in the C package are in +the stash C in PL_defstash. The items in the C package are +in the stash C in C's stash. To get the stash pointer for a particular package, use the function: @@ -863,9 +918,9 @@ copy of the name is stored in C field. The sv_magic function uses C to determine which, if any, predefined "Magic Virtual Table" should be assigned to the C field. -See the "Magic Virtual Table" section below. The C argument is also +See the L section below. The C argument is also stored in the C field. The value of C should be chosen -from the set of macros C found perl.h. Note that before +from the set of macros C found in F. Note that before these macros were added, Perl internals used to directly use character literals, so you may occasionally come across old code or documentation referring to 'U' magic rather than C for example. @@ -904,7 +959,7 @@ The C has five pointers to the following routine types: int (*svt_clear)(SV* sv, MAGIC* mg); int (*svt_free)(SV* sv, MAGIC* mg); -This MGVTBL structure is set at compile-time in C and there are +This MGVTBL structure is set at compile-time in F and there are currently 19 types (or 21 with overloading turned on). These different structures contain pointers to various routines that perform additional actions depending on which function is being called. @@ -1273,7 +1328,8 @@ Similar to C, but localize C<@gv> and C<%gv>. Duplicates the current value of C, on the exit from the current C/C I will restore the value of C -using the stored value. +using the stored value. It doesn't handle magic. Use C if +magic is affected. =item C @@ -1324,7 +1380,8 @@ where C is the macro that represents the local copy of the stack pointer, and C 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 C -macro. The values pushed will often need to be "mortal" (See L). +macro. The pushed values will often need to be "mortal" (See +L). PUSHs(sv_2mortal(newSViv(an_integer))) PUSHs(sv_2mortal(newSVpv("Some String",0))) @@ -1394,6 +1451,8 @@ consult L. =head2 Memory Allocation +=head3 Allocation + All memory meant to be used with the Perl API functions should be manipulated using the macros described in this section. The macros provide the necessary transparency between differences in the actual malloc implementation that is @@ -1404,12 +1463,12 @@ with Perl. It keeps pools of various sizes of unallocated memory in order to satisfy allocation requests more quickly. However, on some platforms, it may cause spurious malloc or free errors. +The following three macros are used to initially allocate memory : + New(x, pointer, number, type); Newc(x, pointer, number, type, cast); Newz(x, pointer, number, type); -These three macros are used to initially allocate memory. - The first argument C was a "magic cookie" that was used to keep track of who called the macro, to help when debugging memory problems. However, the current code makes no use of this feature (most Perl developers now @@ -1427,6 +1486,8 @@ argument. Unlike the C and C macros, the C macro calls C to zero out all the newly allocated memory. +=head3 Reallocation + Renew(pointer, number, type); Renewc(pointer, number, type, cast); Safefree(pointer) @@ -1436,6 +1497,8 @@ piece of memory no longer needed. The arguments to C and C match those of C and C with the exception of not needing the "magic cookie" argument. +=head3 Moving + Move(source, dest, number, type); Copy(source, dest, number, type); Zero(dest, number, type); @@ -1564,8 +1627,8 @@ is the same as in our example. =head2 Examining the tree -If you have your perl compiled for debugging (usually done with C<-D -optimize=-g> on C command line), you may examine the +If you have your perl compiled for debugging (usually done with +C<-DDEBUGGING> on the C command line), you may examine the compiled tree by specifying C<-Dx> on the Perl command line. The output takes several lines per node, and for C<$b+$c> it looks like this: @@ -1634,6 +1697,9 @@ complicate matters, if a C is actually a C op after optimization (see L) it will still have children in accordance with its former type. +Another way to examine the tree is to use a compiler back-end module, such +as L. + =head2 Compile pass 1: check routines The tree is created by the compiler while I code feeds it @@ -1697,10 +1763,10 @@ are subject to the same restrictions as in the pass 2. =head2 Pluggable runops The compile tree is executed in a runops function. There are two runops -functions in F. C is used with DEBUGGING and -C 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 and in F. C is used +with DEBUGGING and C 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 @@ -2012,19 +2078,20 @@ more "hosts", with free association between them. =head1 Internal Functions All of Perl's internal functions which will be exposed to the outside -world are be prefixed by C so that they will not conflict with XS +world are prefixed by C so that they will not conflict with XS functions or functions used in a program in which Perl is embedded. Similarly, all global variables begin with C. (By convention, -static functions start with C) +static functions start with C.) Inside the Perl core, you can get at the functions either with or without the C prefix, thanks to a bunch of defines that live in F. This header file is generated automatically from -F. F 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 as well. Here's a sample entry from that table: +F and F. F 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 as well. Here's a sample entry from +that table: Apd |SV** |av_fetch |AV* ar|I32 key|I32 lval @@ -2083,19 +2150,32 @@ or disappear without notice. This function should not have a compatibility macro to define, say, C to C. It must be called as C. -=item j - -This function is not a member of C. 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 implementation (which is exported). + =back -If you edit F, you will need to run C to -force a rebuild of F and other auto-generated files. +If you edit F or F, you will need to run +C to force a rebuild of F and other +auto-generated files. =head2 Formatted Printing of IVs, UVs, and NVs @@ -2190,34 +2270,34 @@ to one character. To fix this, some people formed Unicode, Inc. and produced a new character set containing all the characters you can possibly think of and more. There are several ways of representing these -characters, and the one Perl uses is called 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. Unfortunately, the non-Unicode string C 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 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 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; this continues up to character 191, which is C. Now we've run out of bits (191 is binary C<10111111>) so we move on; 192 is C. 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 macro: char *utf = "\305\233\340\240\201"; @@ -2227,12 +2307,12 @@ how long the first character in it is with the C macro: 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, 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): @@ -2241,7 +2321,7 @@ 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 */ @@ -2249,7 +2329,7 @@ whether the byte can be encoded as a single byte even in UTF-8): You can also see in that example that we use C to get the value of the character; the inverse function C 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 */ @@ -2259,14 +2339,14 @@ for putting a UV into UTF8: *utf8++ = uv; You B 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, and you skip -that character, you can never match a C 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, and you skip +that character, you can never match a C 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 @@ -2283,8 +2363,8 @@ C, C and other string handling operations will have 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 flag is separate to the PV value; you need be sure you don't accidentally knock it off while you're @@ -2301,7 +2381,7 @@ manipulating SVs. More specifically, you cannot expect to do this: The C 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); @@ -2310,17 +2390,17 @@ old SV has the UTF8 flag set, and act accordingly: SvUTF8_on(nsv); In fact, your C 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 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); @@ -2334,7 +2414,7 @@ If you do this in a binary operator, you will actually change one of the strings that came into the operator, and, while it shouldn't be noticeable by the end user, it can cause problems. -Instead, C will give you a UTF8-encoded B of its +Instead, C will give you a UTF-8-encoded B 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 to go the other way, but naturally, this will fail if @@ -2349,27 +2429,27 @@ Not really. Just remember these things: =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 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 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 use C to get at the value, +If a string is UTF-8, B use C to get at the value, unless C in which case you can use C<*s>. =item * -When writing a character C to a UTF8 string, B use +When writing a character C to a UTF-8 string, B use C, unless C in which case you can use C<*s = uv>. =item * -Mixing UTF8 and non-UTF8 strings is tricky. Use C 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 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 is one of those. =back @@ -2428,11 +2508,6 @@ Andreas Koenig, Paul Hudson, Ilya Zakharevich, Paul Marquess, Neil Bowers, Matthew Green, Tim Bunce, Spider Boardman, Ulrich Pfeifer, Stephen McCamant, and Gurusamy Sarathy. -API Listing originally by Dean Roehrich Eroehrich@cray.comE. - -Modifications to autogenerate the API listing (L) by Benjamin -Stuhl. - =head1 SEE ALSO perlapi(1), perlintern(1), perlxs(1), perlembed(1)