.\" Automatically generated by Pod::Man v1.34, Pod::Parser v1.13 .\" .\" Standard preamble: .\" ======================================================================== .de Sh \" Subsection heading .br .if t .Sp .ne 5 .PP \fB\\$1\fR .PP .. .de Sp \" Vertical space (when we can't use .PP) .if t .sp .5v .if n .sp .. .de Vb \" Begin verbatim text .ft CW .nf .ne \\$1 .. .de Ve \" End verbatim text .ft R .fi .. .\" Set up some character translations and predefined strings. \*(-- will .\" give an unbreakable dash, \*(PI will give pi, \*(L" will give a left .\" double quote, and \*(R" will give a right double quote. | will give a .\" real vertical bar. \*(C+ will give a nicer C++. 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No user-serviceable parts. . \" fudge factors for nroff and troff .if n \{\ . ds #H 0 . ds #V .8m . ds #F .3m . ds #[ \f1 . ds #] \fP .\} .if t \{\ . ds #H ((1u-(\\\\n(.fu%2u))*.13m) . ds #V .6m . ds #F 0 . ds #[ \& . ds #] \& .\} . \" simple accents for nroff and troff .if n \{\ . ds ' \& . ds ` \& . ds ^ \& . ds , \& . ds ~ ~ . ds / .\} .if t \{\ . ds ' \\k:\h'-(\\n(.wu*8/10-\*(#H)'\'\h"|\\n:u" . ds ` \\k:\h'-(\\n(.wu*8/10-\*(#H)'\`\h'|\\n:u' . ds ^ \\k:\h'-(\\n(.wu*10/11-\*(#H)'^\h'|\\n:u' . ds , \\k:\h'-(\\n(.wu*8/10)',\h'|\\n:u' . ds ~ \\k:\h'-(\\n(.wu-\*(#H-.1m)'~\h'|\\n:u' . ds / \\k:\h'-(\\n(.wu*8/10-\*(#H)'\z\(sl\h'|\\n:u' .\} . \" troff and (daisy-wheel) nroff accents .ds : \\k:\h'-(\\n(.wu*8/10-\*(#H+.1m+\*(#F)'\v'-\*(#V'\z.\h'.2m+\*(#F'.\h'|\\n:u'\v'\*(#V' .ds 8 \h'\*(#H'\(*b\h'-\*(#H' .ds o \\k:\h'-(\\n(.wu+\w'\(de'u-\*(#H)/2u'\v'-.3n'\*(#[\z\(de\v'.3n'\h'|\\n:u'\*(#] .ds d- \h'\*(#H'\(pd\h'-\w'~'u'\v'-.25m'\f2\(hy\fP\v'.25m'\h'-\*(#H' .ds D- D\\k:\h'-\w'D'u'\v'-.11m'\z\(hy\v'.11m'\h'|\\n:u' .ds th \*(#[\v'.3m'\s+1I\s-1\v'-.3m'\h'-(\w'I'u*2/3)'\s-1o\s+1\*(#] .ds Th \*(#[\s+2I\s-2\h'-\w'I'u*3/5'\v'-.3m'o\v'.3m'\*(#] .ds ae a\h'-(\w'a'u*4/10)'e .ds Ae A\h'-(\w'A'u*4/10)'E . \" corrections for vroff .if v .ds ~ \\k:\h'-(\\n(.wu*9/10-\*(#H)'\s-2\u~\d\s+2\h'|\\n:u' .if v .ds ^ \\k:\h'-(\\n(.wu*10/11-\*(#H)'\v'-.4m'^\v'.4m'\h'|\\n:u' . \" for low resolution devices (crt and lpr) .if \n(.H>23 .if \n(.V>19 \ \{\ . ds : e . ds 8 ss . ds o a . ds d- d\h'-1'\(ga . ds D- D\h'-1'\(hy . ds th \o'bp' . ds Th \o'LP' . ds ae ae . ds Ae AE .\} .rm #[ #] #H #V #F C .\" ======================================================================== .\" .IX Title "PERLGUTS 1" .TH PERLGUTS 1 "2002-11-24" "perl v5.8.0" "Perl Programmers Reference Guide" .SH "NAME" perlguts \- Introduction to the Perl API .SH "DESCRIPTION" .IX Header "DESCRIPTION" This document attempts to describe how to use the Perl \s-1API\s0, as well as containing 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. .SH "Variables" .IX Header "Variables" .Sh "Datatypes" .IX Subsection "Datatypes" Perl has three typedefs that handle Perl's three main data types: .PP .Vb 3 \& SV Scalar Value \& AV Array Value \& HV Hash Value .Ve .PP Each typedef has specific routines that manipulate the various data types. .ie n .Sh "What is an ""\s-1IV\s0""?" .el .Sh "What is an ``\s-1IV\s0''?" .IX Subsection "What is an IV?" Perl uses a special typedef \s-1IV\s0 which is a simple signed integer type that is guaranteed to be large enough to hold a pointer (as well as an integer). Additionally, there is the \s-1UV\s0, which is simply an unsigned \s-1IV\s0. .PP Perl also uses two special typedefs, I32 and I16, which will always be at least 32\-bits and 16\-bits long, respectively. (Again, there are U32 and U16, as well.) They will usually be exactly 32 and 16 bits long, but on Crays they will both be 64 bits. .Sh "Working with SVs" .IX Subsection "Working with SVs" An \s-1SV\s0 can be created and loaded with one command. There are five types of values that can be loaded: an integer value (\s-1IV\s0), an unsigned integer value (\s-1UV\s0), a double (\s-1NV\s0), a string (\s-1PV\s0), and another scalar (\s-1SV\s0). .PP The seven routines are: .PP .Vb 7 \& SV* newSViv(IV); \& SV* newSVuv(UV); \& SV* newSVnv(double); \& SV* newSVpv(const char*, int); \& SV* newSVpvn(const char*, int); \& SV* newSVpvf(const char*, ...); \& SV* newSVsv(SV*); .Ve .PP If you require more complex initialisation you can create an empty \s-1SV\s0 with newSV(len). If \f(CW\*(C`len\*(C'\fR is 0 an empty \s-1SV\s0 of type \s-1NULL\s0 is returned, else an \&\s-1SV\s0 of type \s-1PV\s0 is returned with len + 1 (for the \s-1NUL\s0) bytes of storage allocated, accessible via SvPVX. In both cases the \s-1SV\s0 has value undef. .PP .Vb 2 \& SV* newSV(0); /* no storage allocated */ \& SV* newSV(10); /* 10 (+1) bytes of uninitialised storage allocated */ .Ve .PP To change the value of an *already\-existing* \s-1SV\s0, there are eight routines: .PP .Vb 8 \& 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_setpvf(SV*, const char*, ...); \& void sv_vsetpvfn(SV*, const char*, STRLEN, va_list *, SV **, I32, bool *); \& void sv_setsv(SV*, SV*); .Ve .PP Notice that you can choose to specify the length of the string to be assigned by using \f(CW\*(C`sv_setpvn\*(C'\fR, \f(CW\*(C`newSVpvn\*(C'\fR, or \f(CW\*(C`newSVpv\*(C'\fR, or you may allow Perl to calculate the length by using \f(CW\*(C`sv_setpv\*(C'\fR or by specifying 0 as the second argument to \f(CW\*(C`newSVpv\*(C'\fR. Be warned, though, that Perl will determine the string's length by using \f(CW\*(C`strlen\*(C'\fR, which depends on the string terminating with a \s-1NUL\s0 character. .PP The arguments of \f(CW\*(C`sv_setpvf\*(C'\fR are processed like \f(CW\*(C`sprintf\*(C'\fR, and the formatted output becomes the value. .PP \&\f(CW\*(C`sv_vsetpvfn\*(C'\fR is an analogue of \f(CW\*(C`vsprintf\*(C'\fR, but it allows you to specify either a pointer to a variable argument list or the address and length of an array of SVs. The last argument points to a boolean; on return, if that boolean is true, then locale-specific information has been used to format the string, and the string's contents are therefore untrustworthy (see perlsec). This pointer may be \s-1NULL\s0 if that information is not important. Note that this function requires you to specify the length of the format. .PP \&\s-1STRLEN\s0 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. .PP The \f(CW\*(C`sv_set*()\*(C'\fR functions are not generic enough to operate on values that have \*(L"magic\*(R". See \*(L"Magic Virtual Tables\*(R" later in this document. .PP All SVs that contain strings should be terminated with a \s-1NUL\s0 character. If it is not NUL-terminated there is a risk of core dumps and corruptions from code which passes the string to C functions or system calls which expect a NUL-terminated string. Perl's own functions typically add a trailing \s-1NUL\s0 for this reason. Nevertheless, you should be very careful when you pass a string stored in an \s-1SV\s0 to a C function or system call. .PP To access the actual value that an \s-1SV\s0 points to, you can use the macros: .PP .Vb 5 \& SvIV(SV*) \& SvUV(SV*) \& SvNV(SV*) \& SvPV(SV*, STRLEN len) \& SvPV_nolen(SV*) .Ve .PP which will automatically coerce the actual scalar type into an \s-1IV\s0, \s-1UV\s0, double, or string. .PP In the \f(CW\*(C`SvPV\*(C'\fR macro, the length of the string returned is placed into the variable \f(CW\*(C`len\*(C'\fR (this is a macro, so you do \fInot\fR use \f(CW&len\fR). If you do not care what the length of the data is, use the \f(CW\*(C`SvPV_nolen\*(C'\fR macro. Historically the \f(CW\*(C`SvPV\*(C'\fR macro with the global variable \f(CW\*(C`PL_na\*(C'\fR has been used in this case. But that can be quite inefficient because \f(CW\*(C`PL_na\*(C'\fR must be accessed in thread-local storage in threaded Perl. In any case, remember that Perl allows arbitrary strings of data that may both contain NULs and might not be terminated by a \s-1NUL\s0. .PP Also remember that C doesn't allow you to safely say \f(CW\*(C`foo(SvPV(s, len), len);\*(C'\fR. It might work with your compiler, but it won't work for everyone. Break this sort of statement up into separate assignments: .PP .Vb 5 \& SV *s; \& STRLEN len; \& char * ptr; \& ptr = SvPV(s, len); \& foo(ptr, len); .Ve .PP If you want to know if the scalar value is \s-1TRUE\s0, you can use: .PP .Vb 1 \& SvTRUE(SV*) .Ve .PP Although Perl will automatically grow strings for you, if you need to force Perl to allocate more memory for your \s-1SV\s0, you can use the macro .PP .Vb 1 \& SvGROW(SV*, STRLEN newlen) .Ve .PP which will determine if more memory needs to be allocated. If so, it will call the function \f(CW\*(C`sv_grow\*(C'\fR. Note that \f(CW\*(C`SvGROW\*(C'\fR can only increase, not decrease, the allocated memory of an \s-1SV\s0 and that it does not automatically add a byte for the a trailing \s-1NUL\s0 (perl's own string functions typically do \&\f(CW\*(C`SvGROW(sv, len + 1)\*(C'\fR). .PP If you have an \s-1SV\s0 and want to know what kind of data Perl thinks is stored in it, you can use the following macros to check the type of \s-1SV\s0 you have. .PP .Vb 3 \& SvIOK(SV*) \& SvNOK(SV*) \& SvPOK(SV*) .Ve .PP You can get and set the current length of the string stored in an \s-1SV\s0 with the following macros: .PP .Vb 2 \& SvCUR(SV*) \& SvCUR_set(SV*, I32 val) .Ve .PP You can also get a pointer to the end of the string stored in the \s-1SV\s0 with the macro: .PP .Vb 1 \& SvEND(SV*) .Ve .PP But note that these last three macros are valid only if \f(CW\*(C`SvPOK()\*(C'\fR is true. .PP If you want to append something to the end of string stored in an \f(CW\*(C`SV*\*(C'\fR, you can use the following functions: .PP .Vb 5 \& void sv_catpv(SV*, const char*); \& void sv_catpvn(SV*, const char*, STRLEN); \& void sv_catpvf(SV*, const char*, ...); \& void sv_vcatpvfn(SV*, const char*, STRLEN, va_list *, SV **, I32, bool); \& void sv_catsv(SV*, SV*); .Ve .PP The first function calculates the length of the string to be appended by using \f(CW\*(C`strlen\*(C'\fR. In the second, you specify the length of the string yourself. The third function processes its arguments like \f(CW\*(C`sprintf\*(C'\fR and appends the formatted output. The fourth function works like \f(CW\*(C`vsprintf\*(C'\fR. You can specify the address and length of an array of SVs instead of the va_list argument. The fifth function extends the string stored in the first \&\s-1SV\s0 with the string stored in the second \s-1SV\s0. It also forces the second \s-1SV\s0 to be interpreted as a string. .PP The \f(CW\*(C`sv_cat*()\*(C'\fR functions are not generic enough to operate on values that have \*(L"magic\*(R". See \*(L"Magic Virtual Tables\*(R" later in this document. .PP If you know the name of a scalar variable, you can get a pointer to its \s-1SV\s0 by using the following: .PP .Vb 1 \& SV* get_sv("package::varname", FALSE); .Ve .PP This returns \s-1NULL\s0 if the variable does not exist. .PP If you want to know if this variable (or any other \s-1SV\s0) is actually \f(CW\*(C`defined\*(C'\fR, you can call: .PP .Vb 1 \& SvOK(SV*) .Ve .PP The scalar \f(CW\*(C`undef\*(C'\fR value is stored in an \s-1SV\s0 instance called \f(CW\*(C`PL_sv_undef\*(C'\fR. Its address can be used whenever an \f(CW\*(C`SV*\*(C'\fR is needed. .PP There are also the two values \f(CW\*(C`PL_sv_yes\*(C'\fR and \f(CW\*(C`PL_sv_no\*(C'\fR, which contain Boolean \&\s-1TRUE\s0 and \s-1FALSE\s0 values, respectively. Like \f(CW\*(C`PL_sv_undef\*(C'\fR, their addresses can be used whenever an \f(CW\*(C`SV*\*(C'\fR is needed. .PP Do not be fooled into thinking that \f(CW\*(C`(SV *) 0\*(C'\fR is the same as \f(CW&PL_sv_undef\fR. Take this code: .PP .Vb 5 \& SV* sv = (SV*) 0; \& if (I-am-to-return-a-real-value) { \& sv = sv_2mortal(newSViv(42)); \& } \& sv_setsv(ST(0), sv); .Ve .PP This code tries to return a new \s-1SV\s0 (which contains the value 42) if it should return a real value, or undef otherwise. Instead it has returned a \s-1NULL\s0 pointer which, somewhere down the line, will cause a segmentation violation, bus error, or just weird results. Change the zero to \f(CW&PL_sv_undef\fR in the first line and all will be well. .PP To free an \s-1SV\s0 that you've created, call \f(CW\*(C`SvREFCNT_dec(SV*)\*(C'\fR. Normally this call is not necessary (see \*(L"Reference Counts and Mortality\*(R"). .Sh "Offsets" .IX Subsection "Offsets" Perl provides the function \f(CW\*(C`sv_chop\*(C'\fR to efficiently remove characters from the beginning of a string; you give it an \s-1SV\s0 and a pointer to somewhere inside the \s-1PV\s0, and it discards everything before the pointer. The efficiency comes by means of a little hack: instead of actually removing the characters, \f(CW\*(C`sv_chop\*(C'\fR sets the flag \f(CW\*(C`OOK\*(C'\fR (offset \s-1OK\s0) to signal to other functions that the offset hack is in effect, and it puts the number of bytes chopped off into the \s-1IV\s0 field of the \s-1SV\s0. It then moves the \s-1PV\s0 pointer (called \f(CW\*(C`SvPVX\*(C'\fR) forward that many bytes, and adjusts \f(CW\*(C`SvCUR\*(C'\fR and \f(CW\*(C`SvLEN\*(C'\fR. .PP Hence, at this point, the start of the buffer that we allocated lives at \f(CW\*(C`SvPVX(sv) \- SvIV(sv)\*(C'\fR in memory and the \s-1PV\s0 pointer is pointing into the middle of this allocated storage. .PP This is best demonstrated by example: .PP .Vb 8 \& % ./perl -Ilib -MDevel::Peek -le '$a="12345"; $a=~s/.//; Dump($a)' \& SV = PVIV(0x8128450) at 0x81340f0 \& REFCNT = 1 \& FLAGS = (POK,OOK,pPOK) \& IV = 1 (OFFSET) \& PV = 0x8135781 ( "1" . ) "2345"\e0 \& CUR = 4 \& LEN = 5 .Ve .PP Here the number of bytes chopped off (1) is put into \s-1IV\s0, and \&\f(CW\*(C`Devel::Peek::Dump\*(C'\fR helpfully reminds us that this is an offset. The portion of the string between the \*(L"real\*(R" and the \*(L"fake\*(R" beginnings is shown in parentheses, and the values of \f(CW\*(C`SvCUR\*(C'\fR and \f(CW\*(C`SvLEN\*(C'\fR reflect the fake beginning, not the real one. .PP Something similar to the offset hack is performed on AVs to enable efficient shifting and splicing off the beginning of the array; while \&\f(CW\*(C`AvARRAY\*(C'\fR points to the first element in the array that is visible from Perl, \f(CW\*(C`AvALLOC\*(C'\fR points to the real start of the C array. These are usually the same, but a \f(CW\*(C`shift\*(C'\fR operation can be carried out by increasing \f(CW\*(C`AvARRAY\*(C'\fR by one and decreasing \f(CW\*(C`AvFILL\*(C'\fR and \f(CW\*(C`AvLEN\*(C'\fR. Again, the location of the real start of the C array only comes into play when freeing the array. See \f(CW\*(C`av_shift\*(C'\fR in \fIav.c\fR. .Sh "What's Really Stored in an \s-1SV\s0?" .IX Subsection "What's Really Stored in an SV?" Recall that the usual method of determining the type of scalar you have is to use \f(CW\*(C`Sv*OK\*(C'\fR macros. Because a scalar can be both a number and a string, usually these macros will always return \s-1TRUE\s0 and calling the \f(CW\*(C`Sv*V\*(C'\fR macros will do the appropriate conversion of string to integer/double or integer/double to string. .PP If you \fIreally\fR need to know if you have an integer, double, or string pointer in an \s-1SV\s0, you can use the following three macros instead: .PP .Vb 3 \& SvIOKp(SV*) \& SvNOKp(SV*) \& SvPOKp(SV*) .Ve .PP These will tell you if you truly have an integer, double, or string pointer stored in your \s-1SV\s0. The \*(L"p\*(R" stands for private. .PP The are various ways in which the private and public flags may differ. For example, a tied \s-1SV\s0 may have a valid underlying value in the \s-1IV\s0 slot (so SvIOKp is true), but the data should be accessed via the \s-1FETCH\s0 routine rather than directly, so SvIOK is false. Another is when numeric conversion has occured and precision has been lost: only the private flag is set on 'lossy' values. So when an \s-1NV\s0 is converted to an \&\s-1IV\s0 with loss, SvIOKp, SvNOKp and SvNOK will be set, while SvIOK wont be. .PP In general, though, it's best to use the \f(CW\*(C`Sv*V\*(C'\fR macros. .Sh "Working with AVs" .IX Subsection "Working with AVs" There are two ways to create and load an \s-1AV\s0. The first method creates an empty \s-1AV:\s0 .PP .Vb 1 \& AV* newAV(); .Ve .PP The second method both creates the \s-1AV\s0 and initially populates it with SVs: .PP .Vb 1 \& AV* av_make(I32 num, SV **ptr); .Ve .PP The second argument points to an array containing \f(CW\*(C`num\*(C'\fR \f(CW\*(C`SV*\*(C'\fR's. Once the \&\s-1AV\s0 has been created, the SVs can be destroyed, if so desired. .PP Once the \s-1AV\s0 has been created, the following operations are possible on AVs: .PP .Vb 4 \& void av_push(AV*, SV*); \& SV* av_pop(AV*); \& SV* av_shift(AV*); \& void av_unshift(AV*, I32 num); .Ve .PP These should be familiar operations, with the exception of \f(CW\*(C`av_unshift\*(C'\fR. This routine adds \f(CW\*(C`num\*(C'\fR elements at the front of the array with the \f(CW\*(C`undef\*(C'\fR value. You must then use \f(CW\*(C`av_store\*(C'\fR (described below) to assign values to these new elements. .PP Here are some other functions: .PP .Vb 3 \& I32 av_len(AV*); \& SV** av_fetch(AV*, I32 key, I32 lval); \& SV** av_store(AV*, I32 key, SV* val); .Ve .PP The \f(CW\*(C`av_len\*(C'\fR function returns the highest index value in array (just like $#array in Perl). If the array is empty, \-1 is returned. The \&\f(CW\*(C`av_fetch\*(C'\fR function returns the value at index \f(CW\*(C`key\*(C'\fR, but if \f(CW\*(C`lval\*(C'\fR is non\-zero, then \f(CW\*(C`av_fetch\*(C'\fR will store an undef value at that index. The \f(CW\*(C`av_store\*(C'\fR function stores the value \f(CW\*(C`val\*(C'\fR at index \f(CW\*(C`key\*(C'\fR, and does not increment the reference count of \f(CW\*(C`val\*(C'\fR. Thus the caller is responsible for taking care of that, and if \f(CW\*(C`av_store\*(C'\fR returns \s-1NULL\s0, the caller will have to decrement the reference count to avoid a memory leak. Note that \&\f(CW\*(C`av_fetch\*(C'\fR and \f(CW\*(C`av_store\*(C'\fR both return \f(CW\*(C`SV**\*(C'\fR's, not \f(CW\*(C`SV*\*(C'\fR's as their return value. .PP .Vb 3 \& void av_clear(AV*); \& void av_undef(AV*); \& void av_extend(AV*, I32 key); .Ve .PP The \f(CW\*(C`av_clear\*(C'\fR function deletes all the elements in the AV* array, but does not actually delete the array itself. The \f(CW\*(C`av_undef\*(C'\fR function will delete all the elements in the array plus the array itself. The \&\f(CW\*(C`av_extend\*(C'\fR function extends the array so that it contains at least \f(CW\*(C`key+1\*(C'\fR elements. If \f(CW\*(C`key+1\*(C'\fR is less than the currently allocated length of the array, then nothing is done. .PP If you know the name of an array variable, you can get a pointer to its \s-1AV\s0 by using the following: .PP .Vb 1 \& AV* get_av("package::varname", FALSE); .Ve .PP This returns \s-1NULL\s0 if the variable does not exist. .PP See \*(L"Understanding the Magic of Tied Hashes and Arrays\*(R" for more information on how to use the array access functions on tied arrays. .Sh "Working with HVs" .IX Subsection "Working with HVs" To create an \s-1HV\s0, you use the following routine: .PP .Vb 1 \& HV* newHV(); .Ve .PP Once the \s-1HV\s0 has been created, the following operations are possible on HVs: .PP .Vb 2 \& SV** hv_store(HV*, const char* key, U32 klen, SV* val, U32 hash); \& SV** hv_fetch(HV*, const char* key, U32 klen, I32 lval); .Ve .PP The \f(CW\*(C`klen\*(C'\fR parameter is the length of the key being passed in (Note that you cannot pass 0 in as a value of \f(CW\*(C`klen\*(C'\fR to tell Perl to measure the length of the key). The \f(CW\*(C`val\*(C'\fR argument contains the \s-1SV\s0 pointer to the scalar being stored, and \f(CW\*(C`hash\*(C'\fR is the precomputed hash value (zero if you want \f(CW\*(C`hv_store\*(C'\fR to calculate it for you). The \f(CW\*(C`lval\*(C'\fR parameter indicates whether this fetch is actually a part of a store operation, in which case a new undefined value will be added to the \s-1HV\s0 with the supplied key and \f(CW\*(C`hv_fetch\*(C'\fR will return as if the value had already existed. .PP Remember that \f(CW\*(C`hv_store\*(C'\fR and \f(CW\*(C`hv_fetch\*(C'\fR return \f(CW\*(C`SV**\*(C'\fR's and not just \&\f(CW\*(C`SV*\*(C'\fR. To access the scalar value, you must first dereference the return value. However, you should check to make sure that the return value is not \s-1NULL\s0 before dereferencing it. .PP These two functions check if a hash table entry exists, and deletes it. .PP .Vb 2 \& bool hv_exists(HV*, const char* key, U32 klen); \& SV* hv_delete(HV*, const char* key, U32 klen, I32 flags); .Ve .PP If \f(CW\*(C`flags\*(C'\fR does not include the \f(CW\*(C`G_DISCARD\*(C'\fR flag then \f(CW\*(C`hv_delete\*(C'\fR will create and return a mortal copy of the deleted value. .PP And more miscellaneous functions: .PP .Vb 2 \& void hv_clear(HV*); \& void hv_undef(HV*); .Ve .PP Like their \s-1AV\s0 counterparts, \f(CW\*(C`hv_clear\*(C'\fR deletes all the entries in the hash table but does not actually delete the hash table. The \f(CW\*(C`hv_undef\*(C'\fR deletes both the entries and the hash table itself. .PP Perl keeps the actual data in linked list of structures with a typedef of \s-1HE\s0. These contain the actual key and value pointers (plus extra administrative overhead). The key is a string pointer; the value is an \f(CW\*(C`SV*\*(C'\fR. However, once you have an \f(CW\*(C`HE*\*(C'\fR, to get the actual key and value, use the routines specified below. .PP .Vb 16 \& I32 hv_iterinit(HV*); \& /* Prepares starting point to traverse hash table */ \& HE* hv_iternext(HV*); \& /* Get the next entry, and return a pointer to a \& structure that has both the key and value */ \& char* hv_iterkey(HE* entry, I32* retlen); \& /* Get the key from an HE structure and also return \& the length of the key string */ \& SV* hv_iterval(HV*, HE* entry); \& /* Return an SV pointer to the value of the HE \& structure */ \& SV* hv_iternextsv(HV*, char** key, I32* retlen); \& /* This convenience routine combines hv_iternext, \& hv_iterkey, and hv_iterval. The key and retlen \& arguments are return values for the key and its \& length. The value is returned in the SV* argument */ .Ve .PP If you know the name of a hash variable, you can get a pointer to its \s-1HV\s0 by using the following: .PP .Vb 1 \& HV* get_hv("package::varname", FALSE); .Ve .PP This returns \s-1NULL\s0 if the variable does not exist. .PP The hash algorithm is defined in the \f(CW\*(C`PERL_HASH(hash, key, klen)\*(C'\fR macro: .PP .Vb 4 \& hash = 0; \& while (klen--) \& hash = (hash * 33) + *key++; \& hash = hash + (hash >> 5); /* after 5.6 */ .Ve .PP The last step was added in version 5.6 to improve distribution of lower bits in the resulting hash value. .PP See \*(L"Understanding the Magic of Tied Hashes and Arrays\*(R" for more information on how to use the hash access functions on tied hashes. .Sh "Hash \s-1API\s0 Extensions" .IX Subsection "Hash API Extensions" Beginning with version 5.004, the following functions are also supported: .PP .Vb 2 \& HE* hv_fetch_ent (HV* tb, SV* key, I32 lval, U32 hash); \& HE* hv_store_ent (HV* tb, SV* key, SV* val, U32 hash); .Ve .PP .Vb 2 \& bool hv_exists_ent (HV* tb, SV* key, U32 hash); \& SV* hv_delete_ent (HV* tb, SV* key, I32 flags, U32 hash); .Ve .PP .Vb 1 \& SV* hv_iterkeysv (HE* entry); .Ve .PP Note that these functions take \f(CW\*(C`SV*\*(C'\fR keys, which simplifies writing of extension code that deals with hash structures. These functions also allow passing of \f(CW\*(C`SV*\*(C'\fR keys to \f(CW\*(C`tie\*(C'\fR functions without forcing you to stringify the keys (unlike the previous set of functions). .PP They also return and accept whole hash entries (\f(CW\*(C`HE*\*(C'\fR), making their use more efficient (since the hash number for a particular string doesn't have to be recomputed every time). See perlapi for detailed descriptions. .PP The following macros must always be used to access the contents of hash entries. Note that the arguments to these macros must be simple variables, since they may get evaluated more than once. See perlapi for detailed descriptions of these macros. .PP .Vb 6 \& HePV(HE* he, STRLEN len) \& HeVAL(HE* he) \& HeHASH(HE* he) \& HeSVKEY(HE* he) \& HeSVKEY_force(HE* he) \& HeSVKEY_set(HE* he, SV* sv) .Ve .PP These two lower level macros are defined, but must only be used when dealing with keys that are not \f(CW\*(C`SV*\*(C'\fRs: .PP .Vb 2 \& HeKEY(HE* he) \& HeKLEN(HE* he) .Ve .PP Note that both \f(CW\*(C`hv_store\*(C'\fR and \f(CW\*(C`hv_store_ent\*(C'\fR do not increment the reference count of the stored \f(CW\*(C`val\*(C'\fR, which is the caller's responsibility. If these functions return a \s-1NULL\s0 value, the caller will usually have to decrement the reference count of \f(CW\*(C`val\*(C'\fR to avoid a memory leak. .Sh "References" .IX Subsection "References" References are a special type of scalar that point to other data types (including references). .PP To create a reference, use either of the following functions: .PP .Vb 2 \& SV* newRV_inc((SV*) thing); \& SV* newRV_noinc((SV*) thing); .Ve .PP The \f(CW\*(C`thing\*(C'\fR argument can be any of an \f(CW\*(C`SV*\*(C'\fR, \f(CW\*(C`AV*\*(C'\fR, or \f(CW\*(C`HV*\*(C'\fR. The functions are identical except that \f(CW\*(C`newRV_inc\*(C'\fR increments the reference count of the \f(CW\*(C`thing\*(C'\fR, while \f(CW\*(C`newRV_noinc\*(C'\fR does not. For historical reasons, \f(CW\*(C`newRV\*(C'\fR is a synonym for \f(CW\*(C`newRV_inc\*(C'\fR. .PP Once you have a reference, you can use the following macro to dereference the reference: .PP .Vb 1 \& SvRV(SV*) .Ve .PP then call the appropriate routines, casting the returned \f(CW\*(C`SV*\*(C'\fR to either an \&\f(CW\*(C`AV*\*(C'\fR or \f(CW\*(C`HV*\*(C'\fR, if required. .PP To determine if an \s-1SV\s0 is a reference, you can use the following macro: .PP .Vb 1 \& SvROK(SV*) .Ve .PP To discover what type of value the reference refers to, use the following macro and then check the return value. .PP .Vb 1 \& SvTYPE(SvRV(SV*)) .Ve .PP The most useful types that will be returned are: .PP .Vb 9 \& SVt_IV Scalar \& SVt_NV Scalar \& SVt_PV Scalar \& SVt_RV Scalar \& SVt_PVAV Array \& SVt_PVHV Hash \& SVt_PVCV Code \& SVt_PVGV Glob (possible a file handle) \& SVt_PVMG Blessed or Magical Scalar .Ve .PP .Vb 1 \& See the sv.h header file for more details. .Ve .Sh "Blessed References and Class Objects" .IX Subsection "Blessed References and Class Objects" References are also used to support object-oriented programming. In the \&\s-1OO\s0 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. .PP A reference can be blessed into a package with the following function: .PP .Vb 1 \& SV* sv_bless(SV* sv, HV* stash); .Ve .PP The \f(CW\*(C`sv\*(C'\fR argument must be a reference. The \f(CW\*(C`stash\*(C'\fR argument specifies which class the reference will belong to. See \&\*(L"Stashes and Globs\*(R" for information on converting class names into stashes. .PP /* Still under construction */ .PP Upgrades rv to reference if not already one. Creates new \s-1SV\s0 for rv to point to. If \f(CW\*(C`classname\*(C'\fR is non\-null, the \s-1SV\s0 is blessed into the specified class. \s-1SV\s0 is returned. .PP .Vb 1 \& SV* newSVrv(SV* rv, const char* classname); .Ve .PP Copies integer, unsigned integer or double into an \s-1SV\s0 whose reference is \f(CW\*(C`rv\*(C'\fR. \s-1SV\s0 is blessed if \f(CW\*(C`classname\*(C'\fR is non\-null. .PP .Vb 3 \& 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); .Ve .PP Copies the pointer value (\fIthe address, not the string!\fR) into an \s-1SV\s0 whose reference is rv. \s-1SV\s0 is blessed if \f(CW\*(C`classname\*(C'\fR is non\-null. .PP .Vb 1 \& SV* sv_setref_pv(SV* rv, const char* classname, PV iv); .Ve .PP Copies string into an \s-1SV\s0 whose reference is \f(CW\*(C`rv\*(C'\fR. Set length to 0 to let Perl calculate the string length. \s-1SV\s0 is blessed if \f(CW\*(C`classname\*(C'\fR is non\-null. .PP .Vb 1 \& SV* sv_setref_pvn(SV* rv, const char* classname, PV iv, STRLEN length); .Ve .PP Tests whether the \s-1SV\s0 is blessed into the specified class. It does not check inheritance relationships. .PP .Vb 1 \& int sv_isa(SV* sv, const char* name); .Ve .PP Tests whether the \s-1SV\s0 is a reference to a blessed object. .PP .Vb 1 \& int sv_isobject(SV* sv); .Ve .PP Tests whether the \s-1SV\s0 is derived from the specified class. \s-1SV\s0 can be either a reference to a blessed object or a string containing a class name. This is the function implementing the \f(CW\*(C`UNIVERSAL::isa\*(C'\fR functionality. .PP .Vb 1 \& bool sv_derived_from(SV* sv, const char* name); .Ve .PP To check if you've got an object derived from a specific class you have to write: .PP .Vb 1 \& if (sv_isobject(sv) && sv_derived_from(sv, class)) { ... } .Ve .Sh "Creating New Variables" .IX Subsection "Creating New Variables" To create a new Perl variable with an undef value which can be accessed from your Perl script, use the following routines, depending on the variable type. .PP .Vb 3 \& SV* get_sv("package::varname", TRUE); \& AV* get_av("package::varname", TRUE); \& HV* get_hv("package::varname", TRUE); .Ve .PP Notice the use of \s-1TRUE\s0 as the second parameter. The new variable can now be set, using the routines appropriate to the data type. .PP There are additional macros whose values may be bitwise \s-1OR\s0'ed with the \&\f(CW\*(C`TRUE\*(C'\fR argument to enable certain extra features. Those bits are: .IP "\s-1GV_ADDMULTI\s0" 4 .IX Item "GV_ADDMULTI" Marks the variable as multiply defined, thus preventing the: .Sp .Vb 1 \& Name used only once: possible typo .Ve .Sp warning. .IP "\s-1GV_ADDWARN\s0" 4 .IX Item "GV_ADDWARN" Issues the warning: .Sp .Vb 1 \& Had to create unexpectedly .Ve .Sp if the variable did not exist before the function was called. .PP If you do not specify a package name, the variable is created in the current package. .Sh "Reference Counts and Mortality" .IX Subsection "Reference Counts and Mortality" Perl uses a reference count-driven garbage collection mechanism. SVs, AVs, or HVs (xV for short in the following) start their life with a reference count of 1. If the reference count of an xV ever drops to 0, then it will be destroyed and its memory made available for reuse. .PP This normally doesn't happen at the Perl level unless a variable is undef'ed or the last variable holding a reference to it is changed or overwritten. At the internal level, however, reference counts can be manipulated with the following macros: .PP .Vb 3 \& int SvREFCNT(SV* sv); \& SV* SvREFCNT_inc(SV* sv); \& void SvREFCNT_dec(SV* sv); .Ve .PP However, there is one other function which manipulates the reference count of its argument. The \f(CW\*(C`newRV_inc\*(C'\fR function, you will recall, creates a reference to the specified argument. As a side effect, it increments the argument's reference count. If this is not what you want, use \f(CW\*(C`newRV_noinc\*(C'\fR instead. .PP For example, imagine you want to return a reference from an \s-1XSUB\s0 function. Inside the \s-1XSUB\s0 routine, you create an \s-1SV\s0 which initially has a reference count of one. Then you call \f(CW\*(C`newRV_inc\*(C'\fR, passing it the just-created \s-1SV\s0. This returns the reference as a new \s-1SV\s0, but the reference count of the \&\s-1SV\s0 you passed to \f(CW\*(C`newRV_inc\*(C'\fR has been incremented to two. Now you return the reference from the \s-1XSUB\s0 routine and forget about the \s-1SV\s0. But Perl hasn't! Whenever the returned reference is destroyed, the reference count of the original \s-1SV\s0 is decreased to one and nothing happens. The \s-1SV\s0 will hang around without any way to access it until Perl itself terminates. This is a memory leak. .PP The correct procedure, then, is to use \f(CW\*(C`newRV_noinc\*(C'\fR instead of \&\f(CW\*(C`newRV_inc\*(C'\fR. Then, if and when the last reference is destroyed, the reference count of the \s-1SV\s0 will go to zero and it will be destroyed, stopping any memory leak. .PP There are some convenience functions available that can help with the destruction of xVs. These functions introduce the concept of \*(L"mortality\*(R". An xV that is mortal has had its reference count marked to be decremented, but not actually decremented, until \*(L"a short time later\*(R". Generally the term \*(L"short time later\*(R" means a single Perl statement, such as a call to an \s-1XSUB\s0 function. The actual determinant for when mortal xVs have their reference count decremented depends on two macros, \s-1SAVETMPS\s0 and \s-1FREETMPS\s0. See perlcall and perlxs for more details on these macros. .PP \&\*(L"Mortalization\*(R" then is at its simplest a deferred \f(CW\*(C`SvREFCNT_dec\*(C'\fR. However, if you mortalize a variable twice, the reference count will later be decremented twice. .PP \&\*(L"Mortal\*(R" SVs are mainly used for SVs that are placed on perl's stack. For example an \s-1SV\s0 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. .PP To create a mortal variable, use the functions: .PP .Vb 3 \& SV* sv_newmortal() \& SV* sv_2mortal(SV*) \& SV* sv_mortalcopy(SV*) .Ve .PP The first call creates a mortal \s-1SV\s0 (with no value), the second converts an existing \&\s-1SV\s0 to a mortal \s-1SV\s0 (and thus defers a call to \f(CW\*(C`SvREFCNT_dec\*(C'\fR), and the third creates a mortal copy of an existing \s-1SV\s0. Because \f(CW\*(C`sv_newmortal\*(C'\fR gives the new \s-1SV\s0 no value,it must normally be given one via \f(CW\*(C`sv_setpv\*(C'\fR, \f(CW\*(C`sv_setiv\*(C'\fR, etc. : .PP .Vb 2 \& SV *tmp = sv_newmortal(); \& sv_setiv(tmp, an_integer); .Ve .PP As that is multiple C statements it is quite common so see this idiom instead: .PP .Vb 1 \& SV *tmp = sv_2mortal(newSViv(an_integer)); .Ve .PP 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 \*(L"Mortalization\*(R" as deferred \f(CW\*(C`SvREFCNT_dec\*(C'\fR should help to minimize such problems. For example if you are passing an \s-1SV\s0 which you \fIknow\fR has high enough \s-1REFCNT\s0 to survive its use on the stack you need not do any mortalization. If you are not sure then doing an \f(CW\*(C`SvREFCNT_inc\*(C'\fR and \f(CW\*(C`sv_2mortal\*(C'\fR, or making a \f(CW\*(C`sv_mortalcopy\*(C'\fR is safer. .PP The mortal routines are not just for SVs \*(-- AVs and HVs can be made mortal by passing their address (type\-casted to \f(CW\*(C`SV*\*(C'\fR) to the \&\f(CW\*(C`sv_2mortal\*(C'\fR or \f(CW\*(C`sv_mortalcopy\*(C'\fR routines. .Sh "Stashes and Globs" .IX Subsection "Stashes and Globs" A \*(L"stash\*(R" is a hash that contains all of the different objects that are contained 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 \s-1GV\s0 (Glob Value). This \s-1GV\s0 in turn contains references to the various objects of that name, including (but not limited to) the following: .PP .Vb 6 \& Scalar Value \& Array Value \& Hash Value \& I/O Handle \& Format \& Subroutine .Ve .PP There is a single stash called \*(L"PL_defstash\*(R" that holds the items that exist in the \*(L"main\*(R" package. To get at the items in other packages, append the string \*(L"::\*(R" to the package name. The items in the \*(L"Foo\*(R" package are in the stash \*(L"Foo::\*(R" in PL_defstash. The items in the \*(L"Bar::Baz\*(R" package are in the stash \*(L"Baz::\*(R" in \*(L"Bar::\*(R"'s stash. .PP To get the stash pointer for a particular package, use the function: .PP .Vb 2 \& HV* gv_stashpv(const char* name, I32 create) \& HV* gv_stashsv(SV*, I32 create) .Ve .PP The first function takes a literal string, the second uses the string stored in the \s-1SV\s0. Remember that a stash is just a hash table, so you get back an \&\f(CW\*(C`HV*\*(C'\fR. The \f(CW\*(C`create\*(C'\fR flag will create a new package if it is set. .PP The name that \f(CW\*(C`gv_stash*v\*(C'\fR wants is the name of the package whose symbol table you want. The default package is called \f(CW\*(C`main\*(C'\fR. If you have multiply nested packages, pass their names to \f(CW\*(C`gv_stash*v\*(C'\fR, separated by \f(CW\*(C`::\*(C'\fR as in the Perl language itself. .PP Alternately, if you have an \s-1SV\s0 that is a blessed reference, you can find out the stash pointer by using: .PP .Vb 1 \& HV* SvSTASH(SvRV(SV*)); .Ve .PP then use the following to get the package name itself: .PP .Vb 1 \& char* HvNAME(HV* stash); .Ve .PP If you need to bless or re-bless an object you can use the following function: .PP .Vb 1 \& SV* sv_bless(SV*, HV* stash) .Ve .PP where the first argument, an \f(CW\*(C`SV*\*(C'\fR, must be a reference, and the second argument is a stash. The returned \f(CW\*(C`SV*\*(C'\fR can now be used in the same way as any other \s-1SV\s0. .PP For more information on references and blessings, consult perlref. .Sh "Double-Typed SVs" .IX Subsection "Double-Typed SVs" Scalar variables normally contain only one type of value, an integer, double, pointer, or reference. Perl will automatically convert the actual scalar data from the stored type into the requested type. .PP Some scalar variables contain more than one type of scalar data. For example, the variable \f(CW$!\fR contains either the numeric value of \f(CW\*(C`errno\*(C'\fR or its string equivalent from either \f(CW\*(C`strerror\*(C'\fR or \f(CW\*(C`sys_errlist[]\*(C'\fR. .PP To force multiple data values into an \s-1SV\s0, you must do two things: use the \&\f(CW\*(C`sv_set*v\*(C'\fR routines to add the additional scalar type, then set a flag so that Perl will believe it contains more than one type of data. The four macros to set the flags are: .PP .Vb 4 \& SvIOK_on \& SvNOK_on \& SvPOK_on \& SvROK_on .Ve .PP The particular macro you must use depends on which \f(CW\*(C`sv_set*v\*(C'\fR routine you called first. This is because every \f(CW\*(C`sv_set*v\*(C'\fR routine turns on only the bit for the particular type of data being set, and turns off all the rest. .PP For example, to create a new Perl variable called \*(L"dberror\*(R" that contains both the numeric and descriptive string error values, you could use the following code: .PP .Vb 2 \& extern int dberror; \& extern char *dberror_list; .Ve .PP .Vb 4 \& SV* sv = get_sv("dberror", TRUE); \& sv_setiv(sv, (IV) dberror); \& sv_setpv(sv, dberror_list[dberror]); \& SvIOK_on(sv); .Ve .PP If the order of \f(CW\*(C`sv_setiv\*(C'\fR and \f(CW\*(C`sv_setpv\*(C'\fR had been reversed, then the macro \f(CW\*(C`SvPOK_on\*(C'\fR would need to be called instead of \f(CW\*(C`SvIOK_on\*(C'\fR. .Sh "Magic Variables" .IX Subsection "Magic Variables" [This section still under construction. Ignore everything here. Post no bills. Everything not permitted is forbidden.] .PP Any \s-1SV\s0 may be magical, that is, it has special features that a normal \&\s-1SV\s0 does not have. These features are stored in the \s-1SV\s0 structure in a linked list of \f(CW\*(C`struct magic\*(C'\fR's, typedef'ed to \f(CW\*(C`MAGIC\*(C'\fR. .PP .Vb 10 \& struct magic { \& MAGIC* mg_moremagic; \& MGVTBL* mg_virtual; \& U16 mg_private; \& char mg_type; \& U8 mg_flags; \& SV* mg_obj; \& char* mg_ptr; \& I32 mg_len; \& }; .Ve .PP Note this is current as of patchlevel 0, and could change at any time. .Sh "Assigning Magic" .IX Subsection "Assigning Magic" Perl adds magic to an \s-1SV\s0 using the sv_magic function: .PP .Vb 1 \& void sv_magic(SV* sv, SV* obj, int how, const char* name, I32 namlen); .Ve .PP The \f(CW\*(C`sv\*(C'\fR argument is a pointer to the \s-1SV\s0 that is to acquire a new magical feature. .PP If \f(CW\*(C`sv\*(C'\fR is not already magical, Perl uses the \f(CW\*(C`SvUPGRADE\*(C'\fR macro to convert \f(CW\*(C`sv\*(C'\fR to type \f(CW\*(C`SVt_PVMG\*(C'\fR. Perl then continues by adding new magic to the beginning of the linked list of magical features. Any prior entry of the same type of magic is deleted. Note that this can be overridden, and multiple instances of the same type of magic can be associated with an \&\s-1SV\s0. .PP The \f(CW\*(C`name\*(C'\fR and \f(CW\*(C`namlen\*(C'\fR arguments are used to associate a string with the magic, typically the name of a variable. \f(CW\*(C`namlen\*(C'\fR is stored in the \&\f(CW\*(C`mg_len\*(C'\fR field and if \f(CW\*(C`name\*(C'\fR is non-null and \f(CW\*(C`namlen\*(C'\fR >= 0 a malloc'd copy of the name is stored in \f(CW\*(C`mg_ptr\*(C'\fR field. .PP The sv_magic function uses \f(CW\*(C`how\*(C'\fR to determine which, if any, predefined \&\*(L"Magic Virtual Table\*(R" should be assigned to the \f(CW\*(C`mg_virtual\*(C'\fR field. See the \*(L"Magic Virtual Table\*(R" section below. The \f(CW\*(C`how\*(C'\fR argument is also stored in the \f(CW\*(C`mg_type\*(C'\fR field. The value of \f(CW\*(C`how\*(C'\fR should be chosen from the set of macros \f(CW\*(C`PERL_MAGIC_foo\*(C'\fR 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 referring to 'U' magic rather than \f(CW\*(C`PERL_MAGIC_uvar\*(C'\fR for example. .PP The \f(CW\*(C`obj\*(C'\fR argument is stored in the \f(CW\*(C`mg_obj\*(C'\fR field of the \f(CW\*(C`MAGIC\*(C'\fR structure. If it is not the same as the \f(CW\*(C`sv\*(C'\fR argument, the reference count of the \f(CW\*(C`obj\*(C'\fR object is incremented. If it is the same, or if the \f(CW\*(C`how\*(C'\fR argument is \f(CW\*(C`PERL_MAGIC_arylen\*(C'\fR, or if it is a \s-1NULL\s0 pointer, then \f(CW\*(C`obj\*(C'\fR is merely stored, without the reference count being incremented. .PP There is also a function to add magic to an \f(CW\*(C`HV\*(C'\fR: .PP .Vb 1 \& void hv_magic(HV *hv, GV *gv, int how); .Ve .PP This simply calls \f(CW\*(C`sv_magic\*(C'\fR and coerces the \f(CW\*(C`gv\*(C'\fR argument into an \f(CW\*(C`SV\*(C'\fR. .PP To remove the magic from an \s-1SV\s0, call the function sv_unmagic: .PP .Vb 1 \& void sv_unmagic(SV *sv, int type); .Ve .PP The \f(CW\*(C`type\*(C'\fR argument should be equal to the \f(CW\*(C`how\*(C'\fR value when the \f(CW\*(C`SV\*(C'\fR was initially made magical. .Sh "Magic Virtual Tables" .IX Subsection "Magic Virtual Tables" The \f(CW\*(C`mg_virtual\*(C'\fR field in the \f(CW\*(C`MAGIC\*(C'\fR structure is a pointer to an \&\f(CW\*(C`MGVTBL\*(C'\fR, which is a structure of function pointers and stands for \&\*(L"Magic Virtual Table\*(R" to handle the various operations that might be applied to that variable. .PP The \f(CW\*(C`MGVTBL\*(C'\fR has five pointers to the following routine types: .PP .Vb 5 \& int (*svt_get)(SV* sv, MAGIC* mg); \& int (*svt_set)(SV* sv, MAGIC* mg); \& U32 (*svt_len)(SV* sv, MAGIC* mg); \& int (*svt_clear)(SV* sv, MAGIC* mg); \& int (*svt_free)(SV* sv, MAGIC* mg); .Ve .PP This \s-1MGVTBL\s0 structure is set at compile-time in \f(CW\*(C`perl.h\*(C'\fR 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. .PP .Vb 7 \& Function pointer Action taken \& ---------------- ------------ \& svt_get Do something before the value of the SV is retrieved. \& svt_set Do something after the SV is assigned a value. \& svt_len Report on the SV's length. \& svt_clear Clear something the SV represents. \& svt_free Free any extra storage associated with the SV. .Ve .PP For instance, the \s-1MGVTBL\s0 structure called \f(CW\*(C`vtbl_sv\*(C'\fR (which corresponds to an \f(CW\*(C`mg_type\*(C'\fR of \f(CW\*(C`PERL_MAGIC_sv\*(C'\fR) contains: .PP .Vb 1 \& { magic_get, magic_set, magic_len, 0, 0 } .Ve .PP Thus, when an \s-1SV\s0 is determined to be magical and of type \f(CW\*(C`PERL_MAGIC_sv\*(C'\fR, if a get operation is being performed, the routine \f(CW\*(C`magic_get\*(C'\fR is called. All the various routines for the various magical types begin with \f(CW\*(C`magic_\*(C'\fR. \s-1NOTE:\s0 the magic routines are not considered part of the Perl \s-1API\s0, and may not be exported by the Perl library. .PP The current kinds of Magic Virtual Tables are: .PP .Vb 42 \& mg_type \& (old-style char and macro) MGVTBL Type of magic \& -------------------------- ------ ---------------------------- \& \e0 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 %_ $c ---> + ---> $a ---> assign-to .Ve .PP But with the actual compile tree for \f(CW\*(C`$a = $b + $c\*(C'\fR it is different: some nodes \fIoptimized away\fR. As a corollary, though the actual tree contains more nodes than our simplified example, the execution order is the same as in our example. .Sh "Examining the tree" .IX Subsection "Examining the tree" If you have your perl compiled for debugging (usually done with \f(CW\*(C`\-D optimize=\-g\*(C'\fR on \f(CW\*(C`Configure\*(C'\fR command line), you may examine the compiled tree by specifying \f(CW\*(C`\-Dx\*(C'\fR on the Perl command line. The output takes several lines per node, and for \f(CW\*(C`$b+$c\*(C'\fR it looks like this: .PP .Vb 23 \& 5 TYPE = add ===> 6 \& TARG = 1 \& FLAGS = (SCALAR,KIDS) \& { \& TYPE = null ===> (4) \& (was rv2sv) \& FLAGS = (SCALAR,KIDS) \& { \& 3 TYPE = gvsv ===> 4 \& FLAGS = (SCALAR) \& GV = main::b \& } \& } \& { \& TYPE = null ===> (5) \& (was rv2sv) \& FLAGS = (SCALAR,KIDS) \& { \& 4 TYPE = gvsv ===> 5 \& FLAGS = (SCALAR) \& GV = main::c \& } \& } .Ve .PP This tree has 5 nodes (one per \f(CW\*(C`TYPE\*(C'\fR specifier), only 3 of them are not optimized away (one per number in the left column). The immediate children of the given node correspond to \f(CW\*(C`{}\*(C'\fR pairs on the same level of indentation, thus this listing corresponds to the tree: .PP .Vb 5 \& add \& / \e \& null null \& | | \& gvsv gvsv .Ve .PP The execution order is indicated by \f(CW\*(C`===>\*(C'\fR marks, thus it is \f(CW\*(C`3 4 5 6\*(C'\fR (node \f(CW6\fR is not included into above listing), i.e., \&\f(CW\*(C`gvsv gvsv add whatever\*(C'\fR. .PP 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 \&\fIpp*.c\fR files; the function which implements the op with type \f(CW\*(C`gvsv\*(C'\fR is \f(CW\*(C`pp_gvsv\*(C'\fR, and so on. As the tree above shows, different ops have different numbers of children: \f(CW\*(C`add\*(C'\fR 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. .PP The simplest type of op structure is \f(CW\*(C`OP\*(C'\fR: this has no children. Unary operators, \f(CW\*(C`UNOP\*(C'\fRs, have one child, and this is pointed to by the \&\f(CW\*(C`op_first\*(C'\fR field. Binary operators (\f(CW\*(C`BINOP\*(C'\fRs) have not only an \&\f(CW\*(C`op_first\*(C'\fR field but also an \f(CW\*(C`op_last\*(C'\fR field. The most complex type of op is a \f(CW\*(C`LISTOP\*(C'\fR, which has any number of children. In this case, the first child is pointed to by \f(CW\*(C`op_first\*(C'\fR and the last child by \&\f(CW\*(C`op_last\*(C'\fR. The children in between can be found by iteratively following the \f(CW\*(C`op_sibling\*(C'\fR pointer from the first child to the last. .PP There are also two other op types: a \f(CW\*(C`PMOP\*(C'\fR holds a regular expression, and has no children, and a \f(CW\*(C`LOOP\*(C'\fR may or may not have children. If the \&\f(CW\*(C`op_children\*(C'\fR field is non\-zero, it behaves like a \f(CW\*(C`LISTOP\*(C'\fR. To complicate matters, if a \f(CW\*(C`UNOP\*(C'\fR is actually a \f(CW\*(C`null\*(C'\fR op after optimization (see \*(L"Compile pass 2: context propagation\*(R") it will still have children in accordance with its former type. .Sh "Compile pass 1: check routines" .IX Subsection "Compile pass 1: check routines" The tree is created by the compiler while \fIyacc\fR code feeds it the constructions it recognizes. Since \fIyacc\fR works bottom\-up, so does the first pass of perl compilation. .PP What makes this pass interesting for perl developers is that some optimization may be performed on this pass. This is optimization by so-called \*(L"check routines\*(R". The correspondence between node names and corresponding check routines is described in \fIopcode.pl\fR (do not forget to run \f(CW\*(C`make regen_headers\*(C'\fR if you modify this file). .PP A check routine is called when the node is fully constructed except for the execution-order thread. Since at this time there are no back-links to the currently constructed node, one can do most any operation to the top-level node, including freeing it and/or creating new nodes above/below it. .PP The check routine returns the node which should be inserted into the tree (if the top-level node was not modified, check routine returns its argument). .PP By convention, check routines have names \f(CW\*(C`ck_*\*(C'\fR. They are usually called from \f(CW\*(C`new*OP\*(C'\fR subroutines (or \f(CW\*(C`convert\*(C'\fR) (which in turn are called from \fIperly.y\fR). .Sh "Compile pass 1a: constant folding" .IX Subsection "Compile pass 1a: constant folding" Immediately after the check routine is called the returned node is checked for being compile-time executable. If it is (the value is judged to be constant) it is immediately executed, and a \fIconstant\fR node with the \*(L"return value\*(R" of the corresponding subtree is substituted instead. The subtree is deleted. .PP If constant folding was not performed, the execution-order thread is created. .Sh "Compile pass 2: context propagation" .IX Subsection "Compile pass 2: context propagation" When a context for a part of compile tree is known, it is propagated down through the tree. At this time the context can have 5 values (instead of 2 for runtime context): void, boolean, scalar, list, and lvalue. In contrast with the pass 1 this pass is processed from top to bottom: a node's context determines the context for its children. .PP Additional context-dependent optimizations are performed at this time. Since at this moment the compile tree contains back-references (via \&\*(L"thread\*(R" pointers), nodes cannot be \fIfree()\fRd now. To allow optimized-away nodes at this stage, such nodes are \fInull()\fRified instead of \fIfree()\fRing (i.e. their type is changed to \s-1OP_NULL\s0). .Sh "Compile pass 3: peephole optimization" .IX Subsection "Compile pass 3: peephole optimization" After the compile tree for a subroutine (or for an \f(CW\*(C`eval\*(C'\fR or a file) is created, an additional pass over the code is performed. This pass is neither top-down or bottom\-up, but in the execution order (with additional complications for conditionals). These optimizations are done in the subroutine \fIpeep()\fR. Optimizations performed at this stage are subject to the same restrictions as in the pass 2. .Sh "Pluggable runops" .IX Subsection "Pluggable runops" The compile tree is executed in a runops function. There are two runops functions in \fIrun.c\fR. \f(CW\*(C`Perl_runops_debug\*(C'\fR is used with \s-1DEBUGGING\s0 and \&\f(CW\*(C`Perl_runops_standard\*(C'\fR is used otherwise. For fine control over the execution of the compile tree it is possible to provide your own runops function. .PP It's probably best to copy one of the existing runops functions and change it to suit your needs. Then, in the \s-1BOOT\s0 section of your \s-1XS\s0 file, add the line: .PP .Vb 1 \& PL_runops = my_runops; .Ve .PP This function should be as efficient as possible to keep your programs running as fast as possible. .ie n .SH "Examining internal data structures with the ""dump"" functions" .el .SH "Examining internal data structures with the \f(CWdump\fP functions" .IX Header "Examining internal data structures with the dump functions" To aid debugging, the source file \fIdump.c\fR contains a number of functions which produce formatted output of internal data structures. .PP The most commonly used of these functions is \f(CW\*(C`Perl_sv_dump\*(C'\fR; it's used for dumping SVs, AVs, HVs, and CVs. The \f(CW\*(C`Devel::Peek\*(C'\fR module calls \&\f(CW\*(C`sv_dump\*(C'\fR to produce debugging output from Perl\-space, so users of that module should already be familiar with its format. .PP \&\f(CW\*(C`Perl_op_dump\*(C'\fR can be used to dump an \f(CW\*(C`OP\*(C'\fR structure or any of its derivatives, and produces output similar to \f(CW\*(C`perl \-Dx\*(C'\fR; in fact, \&\f(CW\*(C`Perl_dump_eval\*(C'\fR will dump the main root of the code being evaluated, exactly like \f(CW\*(C`\-Dx\*(C'\fR. .PP Other useful functions are \f(CW\*(C`Perl_dump_sub\*(C'\fR, which turns a \f(CW\*(C`GV\*(C'\fR into an op tree, \f(CW\*(C`Perl_dump_packsubs\*(C'\fR which calls \f(CW\*(C`Perl_dump_sub\*(C'\fR on all the subroutines in a package like so: (Thankfully, these are all xsubs, so there is no op tree) .PP .Vb 1 \& (gdb) print Perl_dump_packsubs(PL_defstash) .Ve .PP .Vb 1 \& SUB attributes::bootstrap = (xsub 0x811fedc 0) .Ve .PP .Vb 1 \& SUB UNIVERSAL::can = (xsub 0x811f50c 0) .Ve .PP .Vb 1 \& SUB UNIVERSAL::isa = (xsub 0x811f304 0) .Ve .PP .Vb 1 \& SUB UNIVERSAL::VERSION = (xsub 0x811f7ac 0) .Ve .PP .Vb 1 \& SUB DynaLoader::boot_DynaLoader = (xsub 0x805b188 0) .Ve .PP and \f(CW\*(C`Perl_dump_all\*(C'\fR, which dumps all the subroutines in the stash and the op tree of the main root. .SH "How multiple interpreters and concurrency are supported" .IX Header "How multiple interpreters and concurrency are supported" .Sh "Background and \s-1PERL_IMPLICIT_CONTEXT\s0" .IX Subsection "Background and PERL_IMPLICIT_CONTEXT" The Perl interpreter can be regarded as a closed box: it has an \s-1API\s0 for feeding it code or otherwise making it do things, but it also has functions for its own use. This smells a lot like an object, and there are ways for you to build Perl so that you can have multiple interpreters, with one interpreter represented either as a C structure, or inside a thread-specific structure. These structures contain all the context, the state of that interpreter. .PP Two macros control the major Perl build flavors: \s-1MULTIPLICITY\s0 and \&\s-1USE_5005THREADS\s0. The \s-1MULTIPLICITY\s0 build has a C structure that packages all the interpreter state, and there is a similar thread-specific data structure under \s-1USE_5005THREADS\s0. In both cases, \&\s-1PERL_IMPLICIT_CONTEXT\s0 is also normally defined, and enables the support for passing in a \*(L"hidden\*(R" first argument that represents all three data structures. .PP All this obviously requires a way for the Perl internal functions to be either subroutines taking some kind of structure as the first argument, or subroutines taking nothing as the first argument. To enable these two very different ways of building the interpreter, the Perl source (as it does in so many other situations) makes heavy use of macros and subroutine naming conventions. .PP First problem: deciding which functions will be public \s-1API\s0 functions and which will be private. All functions whose names begin \f(CW\*(C`S_\*(C'\fR are private (think \*(L"S\*(R" for \*(L"secret\*(R" or \*(L"static\*(R"). All other functions begin with \&\*(L"Perl_\*(R", but just because a function begins with \*(L"Perl_\*(R" does not mean it is part of the \s-1API\s0. (See \*(L"Internal Functions\*(R".) The easiest way to be \fBsure\fR a function is part of the \s-1API\s0 is to find its entry in perlapi. If it exists in perlapi, it's part of the \s-1API\s0. If it doesn't, and you think it should be (i.e., you need it for your extension), send mail via perlbug explaining why you think it should be. .PP Second problem: there must be a syntax so that the same subroutine declarations and calls can pass a structure as their first argument, or pass nothing. To solve this, the subroutines are named and declared in a particular way. Here's a typical start of a static function used within the Perl guts: .PP .Vb 2 \& STATIC void \& S_incline(pTHX_ char *s) .Ve .PP \&\s-1STATIC\s0 becomes \*(L"static\*(R" in C, and may be #define'd to nothing in some configurations in future. .PP A public function (i.e. part of the internal \s-1API\s0, but not necessarily sanctioned for use in extensions) begins like this: .PP .Vb 2 \& void \& Perl_sv_setsv(pTHX_ SV* dsv, SV* ssv) .Ve .PP \&\f(CW\*(C`pTHX_\*(C'\fR is one of a number of macros (in perl.h) that hide the details of the interpreter's context. \s-1THX\s0 stands for \*(L"thread\*(R", \*(L"this\*(R", or \*(L"thingy\*(R", as the case may be. (And no, George Lucas is not involved. :\-) The first character could be 'p' for a \fBp\fRrototype, 'a' for \fBa\fRrgument, or 'd' for \fBd\fReclaration, so we have \f(CW\*(C`pTHX\*(C'\fR, \f(CW\*(C`aTHX\*(C'\fR and \f(CW\*(C`dTHX\*(C'\fR, and their variants. .PP When Perl is built without options that set \s-1PERL_IMPLICIT_CONTEXT\s0, 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 \&\s-1PERL_IMPLICIT_CONTEXT\s0 is not defined, pTHX_ will be ignored, and the subroutine is not prototyped to take the extra argument. The form of the macro without the trailing underscore is used when there are no additional explicit arguments. .PP When a core function calls another, it must pass the context. This is normally hidden via macros. Consider \f(CW\*(C`sv_setsv\*(C'\fR. It expands into something like this: .PP .Vb 6 \& ifdef PERL_IMPLICIT_CONTEXT \& define sv_setsv(a,b) Perl_sv_setsv(aTHX_ a, b) \& /* can't do this for vararg functions, see below */ \& else \& define sv_setsv Perl_sv_setsv \& endif .Ve .PP This works well, and means that \s-1XS\s0 authors can gleefully write: .PP .Vb 1 \& sv_setsv(foo, bar); .Ve .PP and still have it work under all the modes Perl could have been compiled with. .PP This doesn't work so cleanly for varargs functions, though, as macros imply that the number of arguments is known in advance. Instead we either need to spell them out fully, passing \f(CW\*(C`aTHX_\*(C'\fR as the first argument (the Perl core tends to do this with functions like Perl_warner), or use a context-free version. .PP The context-free version of Perl_warner is called Perl_warner_nocontext, and does not take the extra argument. Instead it does dTHX; to get the context from thread-local storage. We \&\f(CW\*(C`#define warner Perl_warner_nocontext\*(C'\fR so that extensions get source compatibility at the expense of performance. (Passing an arg is cheaper than grabbing it from thread-local storage.) .PP You can ignore [pad]THXx when browsing the Perl headers/sources. Those are strictly for use within the core. Extensions and embedders need only be aware of [pad]THX. .Sh "So what happened to dTHR?" .IX Subsection "So what happened to dTHR?" \&\f(CW\*(C`dTHR\*(C'\fR was introduced in perl 5.005 to support the older thread model. The older thread model now uses the \f(CW\*(C`THX\*(C'\fR mechanism to pass context pointers around, so \f(CW\*(C`dTHR\*(C'\fR 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. .Sh "How do I use all this in extensions?" .IX Subsection "How do I use all this in extensions?" When Perl is built with \s-1PERL_IMPLICIT_CONTEXT\s0, extensions that call any functions in the Perl \s-1API\s0 will need to pass the initial context argument somehow. The kicker is that you will need to write it in such a way that the extension still compiles when Perl hasn't been built with \s-1PERL_IMPLICIT_CONTEXT\s0 enabled. .PP There are three ways to do this. First, the easy but inefficient way, which is also the default, in order to maintain source compatibility with extensions: whenever \s-1XSUB\s0.h is #included, it redefines the aTHX and aTHX_ macros to call a function that will return the context. Thus, something like: .PP .Vb 1 \& sv_setsv(asv, bsv); .Ve .PP in your extension will translate to this when \s-1PERL_IMPLICIT_CONTEXT\s0 is in effect: .PP .Vb 1 \& Perl_sv_setsv(Perl_get_context(), asv, bsv); .Ve .PP or to this otherwise: .PP .Vb 1 \& Perl_sv_setsv(asv, bsv); .Ve .PP You have to do nothing new in your extension to get this; since the Perl library provides \fIPerl_get_context()\fR, it will all just work. .PP The second, more efficient way is to use the following template for your Foo.xs: .PP .Vb 4 \& #define PERL_NO_GET_CONTEXT /* we want efficiency */ \& #include "EXTERN.h" \& #include "perl.h" \& #include "XSUB.h" .Ve .PP .Vb 1 \& static my_private_function(int arg1, int arg2); .Ve .PP .Vb 6 \& static SV * \& my_private_function(int arg1, int arg2) \& { \& dTHX; /* fetch context */ \& ... call many Perl API functions ... \& } .Ve .PP .Vb 1 \& [... etc ...] .Ve .PP .Vb 1 \& MODULE = Foo PACKAGE = Foo .Ve .PP .Vb 1 \& /* typical XSUB */ .Ve .PP .Vb 5 \& void \& my_xsub(arg) \& int arg \& CODE: \& my_private_function(arg, 10); .Ve .PP Note that the only two changes from the normal way of writing an extension is the addition of a \f(CW\*(C`#define PERL_NO_GET_CONTEXT\*(C'\fR before including the Perl headers, followed by a \f(CW\*(C`dTHX;\*(C'\fR declaration at the start of every function that will call the Perl \s-1API\s0. (You'll know which functions need this, because the C compiler will complain that there's an undeclared identifier in those functions.) No changes are needed for the XSUBs themselves, because the \s-1\fIXS\s0()\fR macro is correctly defined to pass in the implicit context if needed. .PP The third, even more efficient way is to ape how it is done within the Perl guts: .PP .Vb 4 \& #define PERL_NO_GET_CONTEXT /* we want efficiency */ \& #include "EXTERN.h" \& #include "perl.h" \& #include "XSUB.h" .Ve .PP .Vb 2 \& /* pTHX_ only needed for functions that call Perl API */ \& static my_private_function(pTHX_ int arg1, int arg2); .Ve .PP .Vb 6 \& static SV * \& my_private_function(pTHX_ int arg1, int arg2) \& { \& /* dTHX; not needed here, because THX is an argument */ \& ... call Perl API functions ... \& } .Ve .PP .Vb 1 \& [... etc ...] .Ve .PP .Vb 1 \& MODULE = Foo PACKAGE = Foo .Ve .PP .Vb 1 \& /* typical XSUB */ .Ve .PP .Vb 5 \& void \& my_xsub(arg) \& int arg \& CODE: \& my_private_function(aTHX_ arg, 10); .Ve .PP This implementation never has to fetch the context using a function call, since it is always passed as an extra argument. Depending on your needs for simplicity or efficiency, you may mix the previous two approaches freely. .PP Never add a comma after \f(CW\*(C`pTHX\*(C'\fR 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. .Sh "Should I do anything special if I call perl from multiple threads?" .IX Subsection "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 (\s-1TLS\s0) slot is initialized correctly in each of those threads. .PP The \f(CW\*(C`perl_alloc\*(C'\fR and \f(CW\*(C`perl_clone\*(C'\fR \s-1API\s0 functions will automatically set the \s-1TLS\s0 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 \s-1TLS\s0 slot of the thread before calling any functions in the Perl \s-1API\s0 on that particular interpreter. This is done by calling the \f(CW\*(C`PERL_SET_CONTEXT\*(C'\fR macro in that thread as the first thing you do: .PP .Vb 2 \& /* do this before doing anything else with some_perl */ \& PERL_SET_CONTEXT(some_perl); .Ve .PP .Vb 1 \& ... other Perl API calls on some_perl go here ... .Ve .Sh "Future Plans and \s-1PERL_IMPLICIT_SYS\s0" .IX Subsection "Future Plans and PERL_IMPLICIT_SYS" Just as \s-1PERL_IMPLICIT_CONTEXT\s0 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 \&\s-1PERL_IMPLICIT_SYS\s0 macro. Currently it only works with \s-1USE_ITHREADS\s0 and \s-1USE_5005THREADS\s0 on Windows (see inside iperlsys.h). .PP This allows the ability to provide an extra pointer (called the \*(L"host\*(R" environment) for all the system calls. This makes it possible for all the system stuff to maintain their own state, broken down into seven C structures. These are thin wrappers around the usual system calls (see win32/perllib.c) for the default perl executable, but for a more ambitious host (like the one that would do \fIfork()\fR emulation) all the extra work needed to pretend that different interpreters are actually different \*(L"processes\*(R", would be done here. .PP The Perl engine/interpreter and the host are orthogonal entities. There could be one or more interpreters in a process, and one or more \*(L"hosts\*(R", with free association between them. .SH "Internal Functions" .IX Header "Internal Functions" All of Perl's internal functions which will be exposed to the outside world are be prefixed by \f(CW\*(C`Perl_\*(C'\fR so that they will not conflict with \s-1XS\s0 functions or functions used in a program in which Perl is embedded. Similarly, all global variables begin with \f(CW\*(C`PL_\*(C'\fR. (By convention, static functions start with \f(CW\*(C`S_\*(C'\fR) .PP Inside the Perl core, you can get at the functions either with or without the \f(CW\*(C`Perl_\*(C'\fR prefix, thanks to a bunch of defines that live in \&\fIembed.h\fR. This header file is generated automatically from \&\fIembed.pl\fR. \fIembed.pl\fR 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 \fIembed.pl\fR as well. Here's a sample entry from that table: .PP .Vb 1 \& Apd |SV** |av_fetch |AV* ar|I32 key|I32 lval .Ve .PP The second column is the return type, the third column the name. Columns after that are the arguments. The first column is a set of flags: .IP "A" 3 .IX Item "A" This function is a part of the public \s-1API\s0. .IP "p" 3 .IX Item "p" This function has a \f(CW\*(C`Perl_\*(C'\fR prefix; ie, it is defined as \f(CW\*(C`Perl_av_fetch\*(C'\fR .IP "d" 3 .IX Item "d" This function has documentation using the \f(CW\*(C`apidoc\*(C'\fR feature which we'll look at in a second. .PP Other available flags are: .IP "s" 3 .IX Item "s" This is a static function and is defined as \f(CW\*(C`S_whatever\*(C'\fR, and usually called within the sources as \f(CW\*(C`whatever(...)\*(C'\fR. .IP "n" 3 .IX Item "n" This does not use \f(CW\*(C`aTHX_\*(C'\fR and \f(CW\*(C`pTHX\*(C'\fR to pass interpreter context. (See \&\*(L"Background and \s-1PERL_IMPLICIT_CONTEXT\s0\*(R" in perlguts.) .IP "r" 3 .IX Item "r" This function never returns; \f(CW\*(C`croak\*(C'\fR, \f(CW\*(C`exit\*(C'\fR and friends. .IP "f" 3 .IX Item "f" This function takes a variable number of arguments, \f(CW\*(C`printf\*(C'\fR style. The argument list should end with \f(CW\*(C`...\*(C'\fR, like this: .Sp .Vb 1 \& Afprd |void |croak |const char* pat|... .Ve .IP "M" 3 .IX Item "M" This function is part of the experimental development \s-1API\s0, and may change or disappear without notice. .IP "o" 3 This function should not have a compatibility macro to define, say, \&\f(CW\*(C`Perl_parse\*(C'\fR to \f(CW\*(C`parse\*(C'\fR. It must be called as \f(CW\*(C`Perl_parse\*(C'\fR. .IP "j" 3 .IX Item "j" This function is not a member of \f(CW\*(C`CPerlObj\*(C'\fR. If you don't know what this means, don't use it. .IP "x" 3 .IX Item "x" This function isn't exported out of the Perl core. .PP If you edit \fIembed.pl\fR, you will need to run \f(CW\*(C`make regen_headers\*(C'\fR to force a rebuild of \fIembed.h\fR and other auto-generated files. .Sh "Formatted Printing of IVs, UVs, and NVs" .IX Subsection "Formatted Printing of IVs, UVs, and NVs" If you are printing IVs, UVs, or \s-1NVS\s0 instead of the \fIstdio\fR\|(3) style formatting codes like \f(CW%d\fR, \f(CW%ld\fR, \f(CW%f\fR, you should use the following macros for portability .PP .Vb 7 \& 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 .Ve .PP These will take care of 64\-bit integers and long doubles. For example: .PP .Vb 1 \& printf("IV is %"IVdf"\en", iv); .Ve .PP The IVdf will expand to whatever is the correct format for the IVs. .PP If you are printing addresses of pointers, use UVxf combined with \s-1\fIPTR2UV\s0()\fR, do not use \f(CW%lx\fR or \f(CW%p\fR. .Sh "Pointer-To-Integer and Integer-To-Pointer" .IX Subsection "Pointer-To-Integer and Integer-To-Pointer" Because pointer size does not necessarily equal integer size, use the follow macros to do it right. .PP .Vb 4 \& PTR2UV(pointer) \& PTR2IV(pointer) \& PTR2NV(pointer) \& INT2PTR(pointertotype, integer) .Ve .PP For example: .PP .Vb 2 \& IV iv = ...; \& SV *sv = INT2PTR(SV*, iv); .Ve .PP and .PP .Vb 2 \& AV *av = ...; \& UV uv = PTR2UV(av); .Ve .Sh "Source Documentation" .IX Subsection "Source Documentation" There's an effort going on to document the internal functions and automatically produce reference manuals from them \- perlapi is one such manual which details all the functions which are available to \s-1XS\s0 writers. perlintern is the autogenerated manual for the functions which are not part of the \s-1API\s0 and are supposedly for internal use only. .PP Source documentation is created by putting \s-1POD\s0 comments into the C source, like this: .PP .Vb 2 \& /* \& =for apidoc sv_setiv .Ve .PP .Vb 2 \& Copies an integer into the given SV. Does not handle 'set' magic. See \& C. .Ve .PP .Vb 2 \& =cut \& */ .Ve .PP Please try and supply some documentation if you add functions to the Perl core. .SH "Unicode Support" .IX Header "Unicode Support" Perl 5.6.0 introduced Unicode support. It's important for porters and \s-1XS\s0 writers to understand this support and make sure that the code they write does not corrupt Unicode data. .Sh "What \fBis\fP Unicode, anyway?" .IX Subsection "What is Unicode, anyway?" In the olden, less enlightened times, we all used to use \s-1ASCII\s0. Most of us did, anyway. The big problem with \s-1ASCII\s0 is that it's American. Well, no, that's not actually the problem; the problem is that it's not particularly useful for people who don't use the Roman alphabet. What used to happen was that particular languages would stick their own alphabet in the upper range of the sequence, between 128 and 255. Of course, we then ended up with plenty of variants that weren't quite \&\s-1ASCII\s0, and the whole point of it being a standard was lost. .PP Worse still, if you've got a language like Chinese or Japanese that has hundreds or thousands of characters, then you really can't fit them into a mere 256, so they had to forget about \s-1ASCII\s0 altogether, and build their own systems using pairs of numbers to refer to one character. .PP 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 \s-1UTF8\s0. \s-1UTF8\s0 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/ .Sh "How can I recognise a \s-1UTF8\s0 string?" .IX Subsection "How can I recognise a UTF8 string?" You can't. This is because \s-1UTF8\s0 data is stored in bytes just like non\-UTF8 data. The Unicode character 200, (\f(CW0xC8\fR for you hex types) capital E with a grave accent, is represented by the two bytes \&\f(CW\*(C`v196.172\*(C'\fR. Unfortunately, the non-Unicode string \f(CW\*(C`chr(196).chr(172)\*(C'\fR has that byte sequence as well. So you can't tell just by looking \- this is what makes Unicode input an interesting problem. .PP The \s-1API\s0 function \f(CW\*(C`is_utf8_string\*(C'\fR can help; it'll tell you if a string contains only valid \s-1UTF8\s0 characters. However, it can't do the work for you. On a character-by-character basis, \f(CW\*(C`is_utf8_char\*(C'\fR will tell you whether the current character in a string is valid \s-1UTF8\s0. .Sh "How does \s-1UTF8\s0 represent Unicode characters?" .IX Subsection "How does UTF8 represent Unicode characters?" As mentioned above, \s-1UTF8\s0 uses a variable number of bytes to store a character. Characters with values 1...128 are stored in one byte, just like good ol' \s-1ASCII\s0. Character 129 is stored as \f(CW\*(C`v194.129\*(C'\fR; this continues up to character 191, which is \f(CW\*(C`v194.191\*(C'\fR. Now we've run out of bits (191 is binary \f(CW10111111\fR) so we move on; 192 is \f(CW\*(C`v195.128\*(C'\fR. And so it goes on, moving to three bytes at character 2048. .PP Assuming you know you're dealing with a \s-1UTF8\s0 string, you can find out how long the first character in it is with the \f(CW\*(C`UTF8SKIP\*(C'\fR macro: .PP .Vb 2 \& char *utf = "\e305\e233\e340\e240\e201"; \& I32 len; .Ve .PP .Vb 3 \& len = UTF8SKIP(utf); /* len is 2 here */ \& utf += len; \& len = UTF8SKIP(utf); /* len is 3 here */ .Ve .PP Another way to skip over characters in a \s-1UTF8\s0 string is to use \&\f(CW\*(C`utf8_hop\*(C'\fR, 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. .PP All bytes in a multi-byte \s-1UTF8\s0 character will have the high bit set, so you can test if you need to do something special with this character like this: .PP .Vb 1 \& UV uv; .Ve .PP .Vb 6 \& if (utf & 0x80) \& /* Must treat this as UTF8 */ \& uv = utf8_to_uv(utf); \& else \& /* OK to treat this character as a byte */ \& uv = *utf; .Ve .PP You can also see in that example that we use \f(CW\*(C`utf8_to_uv\*(C'\fR to get the value of the character; the inverse function \f(CW\*(C`uv_to_utf8\*(C'\fR is available for putting a \s-1UV\s0 into \s-1UTF8:\s0 .PP .Vb 6 \& if (uv > 0x80) \& /* Must treat this as UTF8 */ \& utf8 = uv_to_utf8(utf8, uv); \& else \& /* OK to treat this character as a byte */ \& *utf8++ = uv; .Ve .PP You \fBmust\fR convert characters to UVs using the above functions if you're ever in a situation where you have to match \s-1UTF8\s0 and non\-UTF8 characters. You may not skip over \s-1UTF8\s0 characters in this case. If you do this, you'll lose the ability to match hi-bit non\-UTF8 characters; for instance, if your \s-1UTF8\s0 string contains \f(CW\*(C`v196.172\*(C'\fR, and you skip that character, you can never match a \f(CW\*(C`chr(200)\*(C'\fR in a non\-UTF8 string. So don't do that! .Sh "How does Perl store \s-1UTF8\s0 strings?" .IX Subsection "How does Perl store UTF8 strings?" Currently, Perl deals with Unicode strings and non-Unicode strings slightly differently. If a string has been identified as being \s-1UTF\-8\s0 encoded, Perl will set a flag in the \s-1SV\s0, \f(CW\*(C`SVf_UTF8\*(C'\fR. You can check and manipulate this flag with the following macros: .PP .Vb 3 \& SvUTF8(sv) \& SvUTF8_on(sv) \& SvUTF8_off(sv) .Ve .PP This flag has an important effect on Perl's treatment of the string: if Unicode data is not properly distinguished, regular expressions, \&\f(CW\*(C`length\*(C'\fR, \f(CW\*(C`substr\*(C'\fR and other string handling operations will have undesirable results. .PP The problem comes when you have, for instance, a string that isn't flagged is \s-1UTF8\s0, and contains a byte sequence that could be \s-1UTF8\s0 \- especially when combining non\-UTF8 and \s-1UTF8\s0 strings. .PP Never forget that the \f(CW\*(C`SVf_UTF8\*(C'\fR flag is separate to the \s-1PV\s0 value; you need be sure you don't accidentally knock it off while you're manipulating SVs. More specifically, you cannot expect to do this: .PP .Vb 4 \& SV *sv; \& SV *nsv; \& STRLEN len; \& char *p; .Ve .PP .Vb 3 \& p = SvPV(sv, len); \& frobnicate(p); \& nsv = newSVpvn(p, len); .Ve .PP The \f(CW\*(C`char*\*(C'\fR string does not tell you the whole story, and you can't copy or reconstruct an \s-1SV\s0 just by copying the string value. Check if the old \s-1SV\s0 has the \s-1UTF8\s0 flag set, and act accordingly: .PP .Vb 5 \& p = SvPV(sv, len); \& frobnicate(p); \& nsv = newSVpvn(p, len); \& if (SvUTF8(sv)) \& SvUTF8_on(nsv); .Ve .PP In fact, your \f(CW\*(C`frobnicate\*(C'\fR function should be made aware of whether or not it's dealing with \s-1UTF8\s0 data, so that it can handle the string appropriately. .Sh "How do I convert a string to \s-1UTF8\s0?" .IX Subsection "How do I convert a string to UTF8?" If you're mixing \s-1UTF8\s0 and non\-UTF8 strings, you might find it necessary to upgrade one of the strings to \s-1UTF8\s0. If you've got an \s-1SV\s0, the easiest way to do this is: .PP .Vb 1 \& sv_utf8_upgrade(sv); .Ve .PP However, you must not do this, for example: .PP .Vb 2 \& if (!SvUTF8(left)) \& sv_utf8_upgrade(left); .Ve .PP 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. .PP Instead, \f(CW\*(C`bytes_to_utf8\*(C'\fR will give you a UTF8\-encoded \fBcopy\fR of its string argument. This is useful for having the data available for comparisons and so on, without harming the original \s-1SV\s0. There's also \&\f(CW\*(C`utf8_to_bytes\*(C'\fR to go the other way, but naturally, this will fail if the string contains any characters above 255 that can't be represented in a single byte. .Sh "Is there anything else I need to know?" .IX Subsection "Is there anything else I need to know?" Not really. Just remember these things: .IP "\(bu" 3 There's no way to tell if a string is \s-1UTF8\s0 or not. You can tell if an \s-1SV\s0 is \s-1UTF8\s0 by looking at is \f(CW\*(C`SvUTF8\*(C'\fR flag. Don't forget to set the flag if something should be \s-1UTF8\s0. Treat the flag as part of the \s-1PV\s0, even though it's not \- if you pass on the \s-1PV\s0 to somewhere, pass on the flag too. .IP "\(bu" 3 If a string is \s-1UTF8\s0, \fBalways\fR use \f(CW\*(C`utf8_to_uv\*(C'\fR to get at the value, unless \f(CW\*(C`!(*s & 0x80)\*(C'\fR in which case you can use \f(CW*s\fR. .IP "\(bu" 3 When writing to a \s-1UTF8\s0 string, \fBalways\fR use \f(CW\*(C`uv_to_utf8\*(C'\fR, unless \&\f(CW\*(C`uv < 0x80\*(C'\fR in which case you can use \f(CW\*(C`*s = uv\*(C'\fR. .IP "\(bu" 3 Mixing \s-1UTF8\s0 and non\-UTF8 strings is tricky. Use \f(CW\*(C`bytes_to_utf8\*(C'\fR to get a new string which is \s-1UTF8\s0 encoded. There are tricks you can use to delay deciding whether you need to use a \s-1UTF8\s0 string until you get to a high character \- \f(CW\*(C`HALF_UPGRADE\*(C'\fR is one of those. .SH "Custom Operators" .IX Header "Custom Operators" Custom operator support is a new experimental feature that allows you to define your own ops. This is primarily to allow the building of interpreters for other languages in the Perl core, but it also allows optimizations through the creation of \*(L"macro\-ops\*(R" (ops which perform the functions of multiple ops which are usually executed together, such as \&\f(CW\*(C`gvsv, gvsv, add\*(C'\fR.) .PP This feature is implemented as a new op type, \f(CW\*(C`OP_CUSTOM\*(C'\fR. The Perl core does not \*(L"know\*(R" anything special about this op type, and so it will not be involved in any optimizations. This also means that you can define your custom ops to be any op structure \- unary, binary, list and so on \- you like. .PP It's important to know what custom operators won't do for you. They won't let you add new syntax to Perl, directly. They won't even let you add new keywords, directly. In fact, they won't change the way Perl compiles a program at all. You have to do those changes yourself, after Perl has compiled the program. You do this either by manipulating the op tree using a \f(CW\*(C`CHECK\*(C'\fR block and the \f(CW\*(C`B::Generate\*(C'\fR module, or by adding a custom peephole optimizer with the \f(CW\*(C`optimize\*(C'\fR module. .PP When you do this, you replace ordinary Perl ops with custom ops by creating ops with the type \f(CW\*(C`OP_CUSTOM\*(C'\fR and the \f(CW\*(C`pp_addr\*(C'\fR of your own \&\s-1PP\s0 function. This should be defined in \s-1XS\s0 code, and should look like the \s-1PP\s0 ops in \f(CW\*(C`pp_*.c\*(C'\fR. You are responsible for ensuring that your op takes the appropriate number of values from the stack, and you are responsible for adding stack marks if necessary. .PP You should also \*(L"register\*(R" your op with the Perl interpreter so that it can produce sensible error and warning messages. Since it is possible to have multiple custom ops within the one \*(L"logical\*(R" op type \f(CW\*(C`OP_CUSTOM\*(C'\fR, Perl uses the value of \f(CW\*(C`o\->op_ppaddr\*(C'\fR as a key into the \&\f(CW\*(C`PL_custom_op_descs\*(C'\fR and \f(CW\*(C`PL_custom_op_names\*(C'\fR hashes. This means you need to enter a name and description for your op at the appropriate place in the \f(CW\*(C`PL_custom_op_names\*(C'\fR and \f(CW\*(C`PL_custom_op_descs\*(C'\fR hashes. .PP Forthcoming versions of \f(CW\*(C`B::Generate\*(C'\fR (version 1.0 and above) should directly support the creation of custom ops by name; \f(CW\*(C`Opcodes::Custom\*(C'\fR will provide functions which make it trivial to \*(L"register\*(R" custom ops to the Perl interpreter. .SH "AUTHORS" .IX Header "AUTHORS" Until May 1997, this document was maintained by Jeff Okamoto . It is now maintained as part of Perl itself by the Perl 5 Porters . .PP With lots of help and suggestions from Dean Roehrich, Malcolm Beattie, Andreas Koenig, Paul Hudson, Ilya Zakharevich, Paul Marquess, Neil Bowers, Matthew Green, Tim Bunce, Spider Boardman, Ulrich Pfeifer, Stephen McCamant, and Gurusamy Sarathy. .PP \&\s-1API\s0 Listing originally by Dean Roehrich . .PP Modifications to autogenerate the \s-1API\s0 listing (perlapi) by Benjamin Stuhl. .SH "SEE ALSO" .IX Header "SEE ALSO" \&\fIperlapi\fR\|(1), \fIperlintern\fR\|(1), \fIperlxs\fR\|(1), \fIperlembed\fR\|(1)