perlguts - Introduction to the Perl API
This document attempts to describe how to use the Perl API, as well as 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.
Perl has three typedefs that handle Perl's three main data types:
SV Scalar ValueAV Array ValueHV Hash ValueEach typedef has specific routines that manipulate the various data types.
Perl uses a special typedef IV 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 UV, which is simply an unsigned IV.
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.
An SV can be created and loaded with one command. There are five types of values that can be loaded: an integer value (IV), an unsigned integer value (UV), a double (NV), a string (PV), and another scalar (SV). ("PV" stands for "Pointer Value". You might think that it is misnamed because it is described as pointing only to strings. However, it is possible to have it point to other things. For example, it could point to an array of UVs. But, using it for non-strings requires care, as the underlying assumption of much of the internals is that PVs are just for strings. Often, for example, a trailingNUL is tacked on automatically. The non-string use is documented only in this paragraph.)
The seven routines are:
SV* newSViv(IV);SV* newSVuv(UV);SV* newSVnv(double);SV* newSVpv(const char*, STRLEN);SV* newSVpvn(const char*, STRLEN);SV* newSVpvf(const char*, ...);SV* newSVsv(SV*);STRLEN is an integer type (Size_t, usually defined assize_t inconfig.h) 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 initialization, you can create an empty SV with newSV(len). Iflen is 0 an empty SV of type NULL is returned, else an SV of type PV is returned with len + 1 (for theNUL) bytes of storage allocated, accessible via SvPVX. In both cases the SV has the undef value.
SV *sv = newSV(0); /* no storage allocated */SV *sv = newSV(10); /* 10 (+1) bytes of uninitialised storage * allocated */To change the value of analready-existing 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*, STRLEN)void sv_setpvf(SV*, const char*, ...);void sv_vsetpvfn(SV*, const char*, STRLEN, va_list *, SV **, Size_t, bool *);void sv_setsv(SV*, SV*);Notice that you can choose to specify the length of the string to be assigned by usingsv_setpvn,newSVpvn, ornewSVpv, or you may allow Perl to calculate the length by usingsv_setpv or by specifying 0 as the second argument tonewSVpv. Be warned, though, that Perl will determine the string's length by usingstrlen, which depends on the string terminating with aNUL character, and not otherwise containing NULs.
The arguments ofsv_setpvf are processed likesprintf, and the formatted output becomes the value.
sv_vsetpvfn is an analogue ofvsprintf, 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 (seeperlsec). This pointer may be NULL if that information is not important. Note that this function requires you to specify the length of the format.
Thesv_set*() functions are not generic enough to operate on values that have "magic". See"Magic Virtual Tables" later in this document.
All SVs that contain strings should be terminated with aNUL character. If it is notNUL-terminated there is a risk of core dumps and corruptions from code which passes the string to C functions or system calls which expect aNUL-terminated string. Perl's own functions typically add a trailingNUL for this reason. Nevertheless, you should be very careful when you pass a string stored in an SV to a C function or system call.
To access the actual value that an SV points to, you can use the macros:
SvIV(SV*)SvUV(SV*)SvNV(SV*)SvPV(SV*, STRLEN len)SvPV_nolen(SV*)which will automatically coerce the actual scalar type into an IV, UV, double, or string.
In theSvPV macro, the length of the string returned is placed into the variablelen (this is a macro, so you donot use&len). If you do not care what the length of the data is, use theSvPV_nolen macro. Historically theSvPV macro with the global variablePL_na has been used in this case. But that can be quite inefficient becausePL_na 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 aNUL.
Also remember that C doesn't allow you to safely sayfoo(SvPV(s, len), len);. It might work with your compiler, but it won't work for everyone. Break this sort of statement up into separate assignments:
SV *s;STRLEN len;char *ptr;ptr = SvPV(s, len);foo(ptr, len);If you want to know if the scalar value is TRUE, you can use:
SvTRUE(SV*)Although Perl will automatically grow strings for you, if you need to force Perl to allocate more memory for your SV, you can use the macro
SvGROW(SV*, STRLEN newlen)which will determine if more memory needs to be allocated. If so, it will call the functionsv_grow. Note thatSvGROW can only increase, not decrease, the allocated memory of an SV and that it does not automatically add space for the trailingNUL byte (perl's own string functions typically doSvGROW(sv, len + 1)).
If you want to write to an existing SV's buffer and set its value to a string, use SvPV_force() or one of its variants to force the SV to be a PV. This will remove any of various types of non-stringness from the SV while preserving the content of the SV in the PV. This can be used, for example, to append data from an API function to a buffer without extra copying:
(void)SvPVbyte_force(sv, len);s = SvGROW(sv, len + needlen + 1);/* something that modifies up to needlen bytes at s+len, but modifies newlen bytes eg. newlen = read(fd, s + len, needlen); ignoring errors for these examples */s[len + newlen] = '\0';SvCUR_set(sv, len + newlen);SvUTF8_off(sv);SvSETMAGIC(sv);If you already have the data in memory or if you want to keep your code simple, you can use one of the sv_cat*() variants, such as sv_catpvn(). If you want to insert anywhere in the string you can use sv_insert() or sv_insert_flags().
If you don't need the existing content of the SV, you can avoid some copying with:
SvPVCLEAR(sv);s = SvGROW(sv, needlen + 1);/* something that modifies up to needlen bytes at s, but modifies newlen bytes eg. newlen = read(fd, s. needlen); */s[newlen] = '\0';SvCUR_set(sv, newlen);SvPOK_only(sv); /* also clears SVf_UTF8 */SvSETMAGIC(sv);Again, if you already have the data in memory or want to avoid the complexity of the above, you can use sv_setpvn().
If you have a buffer allocated with Newx() and want to set that as the SV's value, you can use sv_usepvn_flags(). That has some requirements if you want to avoid perl re-allocating the buffer to fit the trailing NUL:
Newx(buf, somesize+1, char);/* ... fill in buf ... */buf[somesize] = '\0';sv_usepvn_flags(sv, buf, somesize, SV_SMAGIC | SV_HAS_TRAILING_NUL);/* buf now belongs to perl, don't release it */If you have an SV 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 SV you have.
SvIOK(SV*)SvNOK(SV*)SvPOK(SV*)You can get and set the current length of the string stored in an SV with the following macros:
SvCUR(SV*)SvCUR_set(SV*, I32 val)You can also get a pointer to the end of the string stored in the SV with the macro:
SvEND(SV*)But note that these last three macros are valid only ifSvPOK() is true.
If you want to append something to the end of string stored in anSV*, you can use the following functions:
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*);The first function calculates the length of the string to be appended by usingstrlen. In the second, you specify the length of the string yourself. The third function processes its arguments likesprintf and appends the formatted output. The fourth function works likevsprintf. 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 SV with the string stored in the second SV. It also forces the second SV to be interpreted as a string.
Thesv_cat*() functions are not generic enough to operate on values that have "magic". See"Magic Virtual Tables" later in this document.
If you know the name of a scalar variable, you can get a pointer to its SV by using the following:
SV* get_sv("package::varname", 0);This returns NULL if the variable does not exist.
If you want to know if this variable (or any other SV) is actuallydefined, you can call:
SvOK(SV*)The scalarundef value is stored in an SV instance calledPL_sv_undef.
Its address can be used whenever anSV* is needed. Make sure that you don't try to compare a random sv with&PL_sv_undef. For example when interfacing Perl code, it'll work correctly for:
foo(undef);But won't work when called as:
$x = undef;foo($x);So to repeat always use SvOK() to check whether an sv is defined.
Also you have to be careful when using&PL_sv_undef as a value in AVs or HVs (see"AVs, HVs and undefined values").
There are also the two valuesPL_sv_yes andPL_sv_no, which contain boolean TRUE and FALSE values, respectively. LikePL_sv_undef, their addresses can be used whenever anSV* is needed.
Do not be fooled into thinking that(SV *) 0 is the same as&PL_sv_undef. Take this code:
SV* sv = (SV*) 0;if (I-am-to-return-a-real-value) { sv = sv_2mortal(newSViv(42));}sv_setsv(ST(0), sv);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&PL_sv_undef in the first line and all will be well.
To free an SV that you've created, callSvREFCNT_dec(SV*). Normally this call is not necessary (see"Reference Counts and Mortality").
Perl provides the functionsv_chop to efficiently remove characters from the beginning of a string; you give it an SV and a pointer to somewhere inside the PV, and it discards everything before the pointer. The efficiency comes by means of a little hack: instead of actually removing the characters,sv_chop sets the flagOOK (offset OK) to signal to other functions that the offset hack is in effect, and it moves the PV pointer (calledSvPVX) forward by the number of bytes chopped off, and adjustsSvCUR andSvLEN accordingly. (A portion of the space between the old and new PV pointers is used to store the count of chopped bytes.)
Hence, at this point, the start of the buffer that we allocated lives atSvPVX(sv) - SvIV(sv) in memory and the PV pointer is pointing into the middle of this allocated storage.
This is best demonstrated by example. Normally copy-on-write will prevent the substitution from operator from using this hack, but if you can craft a string for which copy-on-write is not possible, you can see it in play. In the current implementation, the final byte of a string buffer is used as a copy-on-write reference count. If the buffer is not big enough, then copy-on-write is skipped. First have a look at an empty string:
% ./perl -Ilib -MDevel::Peek -le '$a=""; $a .= ""; Dump $a'SV = PV(0x7ffb7c008a70) at 0x7ffb7c030390 REFCNT = 1 FLAGS = (POK,pPOK) PV = 0x7ffb7bc05b50 ""\0 CUR = 0 LEN = 10Notice here the LEN is 10. (It may differ on your platform.) Extend the length of the string to one less than 10, and do a substitution:
% ./perl -Ilib -MDevel::Peek -le '$a=""; $a.="123456789"; $a=~s/.//; \ Dump($a)'SV = PV(0x7ffa04008a70) at 0x7ffa04030390 REFCNT = 1 FLAGS = (POK,OOK,pPOK) OFFSET = 1 PV = 0x7ffa03c05b61 ( "\1" . ) "23456789"\0 CUR = 8 LEN = 9Here the number of bytes chopped off (1) is shown next as the OFFSET. The portion of the string between the "real" and the "fake" beginnings is shown in parentheses, and the values ofSvCUR andSvLEN reflect the fake beginning, not the real one. (The first character of the string buffer happens to have changed to "\1" here, not "1", because the current implementation stores the offset count in the string buffer. This is subject to change.)
Something similar to the offset hack is performed on AVs to enable efficient shifting and splicing off the beginning of the array; whileAvARRAY points to the first element in the array that is visible from Perl,AvALLOC points to the real start of the C array. These are usually the same, but ashift operation can be carried out by increasingAvARRAY by one and decreasingAvFILL andAvMAX. Again, the location of the real start of the C array only comes into play when freeing the array. Seeav_shift inav.c.
Recall that the usual method of determining the type of scalar you have is to useSv*OK macros. Because a scalar can be both a number and a string, usually these macros will always return TRUE and calling theSv*V macros will do the appropriate conversion of string to integer/double or integer/double to string.
If youreally need to know if you have an integer, double, or string pointer in an SV, you can use the following three macros instead:
SvIOKp(SV*)SvNOKp(SV*)SvPOKp(SV*)These will tell you if you truly have an integer, double, or string pointer stored in your SV. The "p" stands for private.
There are various ways in which the private and public flags may differ. For example, in perl 5.16 and earlier a tied SV may have a valid underlying value in the IV slot (so SvIOKp is true), but the data should be accessed via the FETCH routine rather than directly, so SvIOK is false. (In perl 5.18 onwards, tied scalars use the flags the same way as untied scalars.) Another is when numeric conversion has occurred and precision has been lost: only the private flag is set on 'lossy' values. So when an NV is converted to an IV with loss, SvIOKp, SvNOKp and SvNOK will be set, while SvIOK wont be.
In general, though, it's best to use theSv*V macros.
There are two ways to create and load an AV. The first method creates an empty AV:
AV* newAV();The second method both creates the AV and initially populates it with SVs:
AV* av_make(SSize_t num, SV **ptr);The second argument points to an array containingnumSV*'s. Once the AV has been created, the SVs can be destroyed, if so desired.
Once the AV has been created, the following operations are possible on it:
void av_push(AV*, SV*);SV* av_pop(AV*);SV* av_shift(AV*);void av_unshift(AV*, SSize_t num);These should be familiar operations, with the exception ofav_unshift. This routine addsnum elements at the front of the array with theundef value. You must then useav_store (described below) to assign values to these new elements.
Here are some other functions:
SSize_t av_top_index(AV*);SV** av_fetch(AV*, SSize_t key, I32 lval);SV** av_store(AV*, SSize_t key, SV* val);Theav_top_index function returns the highest index value in an array (just like $#array in Perl). If the array is empty, -1 is returned. Theav_fetch function returns the value at indexkey, but iflval is non-zero, thenav_fetch will store an undef value at that index. Theav_store function stores the valueval at indexkey, and does not increment the reference count ofval. Thus the caller is responsible for taking care of that, and ifav_store returns NULL, the caller will have to decrement the reference count to avoid a memory leak. Note thatav_fetch andav_store both returnSV**'s, notSV*'s as their return value.
A few more:
void av_clear(AV*);void av_undef(AV*);void av_extend(AV*, SSize_t key);Theav_clear function deletes all the elements in the AV* array, but does not actually delete the array itself. Theav_undef function will delete all the elements in the array plus the array itself. Theav_extend function extends the array so that it contains at leastkey+1 elements. Ifkey+1 is less than the currently allocated length of the array, then nothing is done.
If you know the name of an array variable, you can get a pointer to its AV by using the following:
AV* get_av("package::varname", 0);This returns NULL if the variable does not exist.
See"Understanding the Magic of Tied Hashes and Arrays" for more information on how to use the array access functions on tied arrays.
To create an HV, you use the following routine:
HV* newHV();Once the HV has been created, the following operations are possible on it:
SV** hv_store(HV*, const char* key, U32 klen, SV* val, U32 hash);SV** hv_fetch(HV*, const char* key, U32 klen, I32 lval);Theklen parameter is the length of the key being passed in (Note that you cannot pass 0 in as a value ofklen to tell Perl to measure the length of the key). Theval argument contains the SV pointer to the scalar being stored, andhash is the precomputed hash value (zero if you wanthv_store to calculate it for you). Thelval 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 HV with the supplied key andhv_fetch will return as if the value had already existed.
Remember thathv_store andhv_fetch returnSV**'s and not justSV*. 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 NULL before dereferencing it.
The first of these two functions checks if a hash table entry exists, and the second deletes it.
bool hv_exists(HV*, const char* key, U32 klen);SV* hv_delete(HV*, const char* key, U32 klen, I32 flags);Ifflags does not include theG_DISCARD flag thenhv_delete will create and return a mortal copy of the deleted value.
And more miscellaneous functions:
void hv_clear(HV*);void hv_undef(HV*);Like their AV counterparts,hv_clear deletes all the entries in the hash table but does not actually delete the hash table. Thehv_undef deletes both the entries and the hash table itself.
Perl keeps the actual data in a linked list of structures with a typedef of HE. These contain the actual key and value pointers (plus extra administrative overhead). The key is a string pointer; the value is anSV*. However, once you have anHE*, to get the actual key and value, use the routines specified below.
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 */If you know the name of a hash variable, you can get a pointer to its HV by using the following:
HV* get_hv("package::varname", 0);This returns NULL if the variable does not exist.
The hash algorithm is defined in thePERL_HASH macro:
PERL_HASH(hash, key, klen)The exact implementation of this macro varies by architecture and version of perl, and the return value may change per invocation, so the value is only valid for the duration of a single perl process.
See"Understanding the Magic of Tied Hashes and Arrays" for more information on how to use the hash access functions on tied hashes.
Beginning with version 5.004, the following functions are also supported:
HE* hv_fetch_ent (HV* tb, SV* key, I32 lval, U32 hash);HE* hv_store_ent (HV* tb, SV* key, SV* val, U32 hash);bool hv_exists_ent (HV* tb, SV* key, U32 hash);SV* hv_delete_ent (HV* tb, SV* key, I32 flags, U32 hash);SV* hv_iterkeysv (HE* entry);Note that these functions takeSV* keys, which simplifies writing of extension code that deals with hash structures. These functions also allow passing ofSV* keys totie functions without forcing you to stringify the keys (unlike the previous set of functions).
They also return and accept whole hash entries (HE*), making their use more efficient (since the hash number for a particular string doesn't have to be recomputed every time). Seeperlapi for detailed descriptions.
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. Seeperlapi for detailed descriptions of these macros.
HePV(HE* he, STRLEN len)HeVAL(HE* he)HeHASH(HE* he)HeSVKEY(HE* he)HeSVKEY_force(HE* he)HeSVKEY_set(HE* he, SV* sv)These two lower level macros are defined, but must only be used when dealing with keys that are notSV*s:
HeKEY(HE* he)HeKLEN(HE* he)Note that bothhv_store andhv_store_ent do not increment the reference count of the storedval, which is the caller's responsibility. If these functions return a NULL value, the caller will usually have to decrement the reference count ofval to avoid a memory leak.
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&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. In perl 5.18 and earlier, AVs use&PL_sv_undef as a marker for indicating that an array element has not yet been initialized. Thus,exists $av[0] would be true for the above Perl code, but false for the array generated by the XS code. In perl 5.20, storing &PL_sv_undef will create a read-only element, because the scalar &PL_sv_undef itself is stored, not a copy.
Similar problems can occur when storing&PL_sv_undef in HVs:
hv_store( hv, "key", 3, &PL_sv_undef, 0 );This will indeed make the valueundef, but if you try to modify the value ofkey, you'll get the following error:
Modification of non-creatable hash value attemptedIn perl 5.8.0,&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 thehv_exists function.
You can run into similar problems when you store&PL_sv_yes or&PL_sv_no into AVs or HVs. Trying to modify such elements will give you the following error:
Modification of a read-only value attemptedTo make a long story short, you can use the special variables&PL_sv_undef,&PL_sv_yes and&PL_sv_no 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&PL_sv_undef, but rather create a new undefined value using thenewSV function, for example:
av_store( av, 42, newSV(0) );hv_store( hv, "foo", 3, newSV(0), 0 );References are a special type of scalar that point to other data types (including other references).
To create a reference, use either of the following functions:
SV* newRV_inc((SV*) thing);SV* newRV_noinc((SV*) thing);Thething argument can be any of anSV*,AV*, orHV*. The functions are identical except thatnewRV_inc increments the reference count of thething, whilenewRV_noinc does not. For historical reasons,newRV is a synonym fornewRV_inc.
Once you have a reference, you can use the following macro to dereference the reference:
SvRV(SV*)then call the appropriate routines, casting the returnedSV* to either anAV* orHV*, if required.
To determine if an SV is a reference, you can use the following macro:
SvROK(SV*)To discover what type of value the reference refers to, use the following macro and then check the return value.
SvTYPE(SvRV(SV*))The most useful types that will be returned are:
< SVt_PVAV ScalarSVt_PVAV ArraySVt_PVHV HashSVt_PVCV CodeSVt_PVGV Glob (possibly a file handle)See"svtype" in perlapi for more details.
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.
A reference can be blessed into a package with the following function:
SV* sv_bless(SV* sv, HV* stash);Thesv argument must be a reference value. Thestash argument specifies which class the reference will belong to. See"Stashes and Globs" for information on converting class names into stashes.
/* Still under construction */
The following function upgrades rv to reference if not already one. Creates a new SV for rv to point to. Ifclassname is non-null, the SV is blessed into the specified class. SV is returned.
SV* newSVrv(SV* rv, const char* classname);The following three functions copy integer, unsigned integer or double into an SV whose reference isrv. SV is blessed ifclassname is non-null.
SV* sv_setref_iv(SV* rv, const char* classname, IV iv);SV* sv_setref_uv(SV* rv, const char* classname, UV uv);SV* sv_setref_nv(SV* rv, const char* classname, NV iv);The following function copies the pointer value (the address, not the string!) into an SV whose reference is rv. SV is blessed ifclassname is non-null.
SV* sv_setref_pv(SV* rv, const char* classname, void* pv);The following function copies a string into an SV whose reference isrv. Set length to 0 to let Perl calculate the string length. SV is blessed ifclassname is non-null.
SV* sv_setref_pvn(SV* rv, const char* classname, char* pv, STRLEN length);The following function tests whether the SV is blessed into the specified class. It does not check inheritance relationships.
int sv_isa(SV* sv, const char* name);The following function tests whether the SV is a reference to a blessed object.
int sv_isobject(SV* sv);The following function tests whether the SV is derived from the specified class. SV can be either a reference to a blessed object or a string containing a class name. This is the function implementing theUNIVERSAL::isa functionality.
bool sv_derived_from(SV* sv, const char* name);To check if you've got an object derived from a specific class you have to write:
if (sv_isobject(sv) && sv_derived_from(sv, class)) { ... }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.
SV* get_sv("package::varname", GV_ADD);AV* get_av("package::varname", GV_ADD);HV* get_hv("package::varname", GV_ADD);Notice the use of GV_ADD as the second parameter. The new variable can now be set, using the routines appropriate to the data type.
There are additional macros whose values may be bitwise OR'ed with theGV_ADD argument to enable certain extra features. Those bits are:
Marks the variable as multiply defined, thus preventing the:
Name <varname> used only once: possible typowarning.
Issues the warning:
Had to create <varname> unexpectedlyif the variable did not exist before the function was called.
If you do not specify a package name, the variable is created in the current package.
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. At the most basic internal level, reference counts can be manipulated with the following macros:
int SvREFCNT(SV* sv);SV* SvREFCNT_inc(SV* sv);void SvREFCNT_dec(SV* sv);(There are also suffixed versions of the increment and decrement macros, for situations where the full generality of these basic macros can be exchanged for some performance.)
However, the way a programmer should think about references is not so much in terms of the bare reference count, but in terms ofownership of references. A reference to an xV can be owned by any of a variety of entities: another xV, the Perl interpreter, an XS data structure, a piece of running code, or a dynamic scope. An xV generally does not know what entities own the references to it; it only knows how many references there are, which is the reference count.
To correctly maintain reference counts, it is essential to keep track of what references the XS code is manipulating. The programmer should always know where a reference has come from and who owns it, and be aware of any creation or destruction of references, and any transfers of ownership. Because ownership isn't represented explicitly in the xV data structures, only the reference count need be actually maintained by the code, and that means that this understanding of ownership is not actually evident in the code. For example, transferring ownership of a reference from one owner to another doesn't change the reference count at all, so may be achieved with no actual code. (The transferring code doesn't touch the referenced object, but does need to ensure that the former owner knows that it no longer owns the reference, and that the new owner knows that it now does.)
An xV that is visible at the Perl level should not become unreferenced and thus be destroyed. Normally, an object will only become unreferenced when it is no longer visible, often by the same means that makes it invisible. For example, a Perl reference value (RV) owns a reference to its referent, so if the RV is overwritten that reference gets destroyed, and the no-longer-reachable referent may be destroyed as a result.
Many functions have some kind of reference manipulation as part of their purpose. Sometimes this is documented in terms of ownership of references, and sometimes it is (less helpfully) documented in terms of changes to reference counts. For example, thenewRV_inc() function is documented to create a new RV (with reference count 1) and increment the reference count of the referent that was supplied by the caller. This is best understood as creating a new reference to the referent, which is owned by the created RV, and returning to the caller ownership of the sole reference to the RV. ThenewRV_noinc() function instead does not increment the reference count of the referent, but the RV nevertheless ends up owning a reference to the referent. It is therefore implied that the caller ofnewRV_noinc() is relinquishing a reference to the referent, making this conceptually a more complicated operation even though it does less to the data structures.
For example, imagine you want to return a reference from an XSUB function. Inside the XSUB routine, you create an SV which initially has just a single reference, owned by the XSUB routine. This reference needs to be disposed of before the routine is complete, otherwise it will leak, preventing the SV from ever being destroyed. So to create an RV referencing the SV, it is most convenient to pass the SV tonewRV_noinc(), which consumes that reference. Now the XSUB routine no longer owns a reference to the SV, but does own a reference to the RV, which in turn owns a reference to the SV. The ownership of the reference to the RV is then transferred by the process of returning the RV from the XSUB.
There are some convenience functions available that can help with the destruction of xVs. These functions introduce the concept of "mortality". Much documentation speaks of an xV itself being mortal, but this is misleading. It is reallya reference to an xV that is mortal, and it is possible for there to be more than one mortal reference to a single xV. For a reference to be mortal means that it is owned by the temps stack, one of perl's many internal stacks, which will destroy that reference "a short time later". Usually the "short time later" is the end of the current Perl statement. However, it gets more complicated around dynamic scopes: there can be multiple sets of mortal references hanging around at the same time, with different death dates. Internally, the actual determinant for when mortal xV references are destroyed depends on two macros, SAVETMPS and FREETMPS. Seeperlcall andperlxs for more details on these macros.
Mortal references are mainly used for xVs that are placed on perl's main stack. The stack is problematic for reference tracking, because it contains a lot of xV references, but doesn't own those references: they are not counted. Currently, there are many bugs resulting from xVs being destroyed while referenced by the stack, because the stack's uncounted references aren't enough to keep the xVs alive. So when putting an (uncounted) reference on the stack, it is vitally important to ensure that there will be a counted reference to the same xV that will last at least as long as the uncounted reference. But it's also important that that counted reference be cleaned up at an appropriate time, and not unduly prolong the xV's life. For there to be a mortal reference is often the best way to satisfy this requirement, especially if the xV was created especially to be put on the stack and would otherwise be unreferenced.
To create a mortal reference, use the functions:
SV* sv_newmortal()SV* sv_mortalcopy(SV*)SV* sv_2mortal(SV*)sv_newmortal() creates an SV (with the undefined value) whose sole reference is mortal.sv_mortalcopy() creates an xV whose value is a copy of a supplied xV and whose sole reference is mortal.sv_2mortal() mortalises an existing xV reference: it transfers ownership of a reference from the caller to the temps stack. Becausesv_newmortal gives the new SV no value, it must normally be given one viasv_setpv,sv_setiv, etc. :
SV *tmp = sv_newmortal();sv_setiv(tmp, an_integer);As that is multiple C statements it is quite common so see this idiom instead:
SV *tmp = sv_2mortal(newSViv(an_integer));The mortal routines are not just for SVs; AVs and HVs can be made mortal by passing their address (type-casted toSV*) to thesv_2mortal orsv_mortalcopy routines.
Astash 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, including (but not limited to) the following:
Scalar ValueArray ValueHash ValueI/O HandleFormatSubroutineThere is a single stash calledPL_defstash that holds the items that exist in themain package. To get at the items in other packages, append the string "::" to the package name. The items in theFoo package are in the stashFoo:: in PL_defstash. The items in theBar::Baz package are in the stashBaz:: inBar::'s stash.
To get the stash pointer for a particular package, use the function:
HV* gv_stashpv(const char* name, I32 flags)HV* gv_stashsv(SV*, I32 flags)The first function takes a literal string, the second uses the string stored in the SV. Remember that a stash is just a hash table, so you get back anHV*. Theflags flag will create a new package if it is set to GV_ADD.
The name thatgv_stash*v wants is the name of the package whose symbol table you want. The default package is calledmain. If you have multiply nested packages, pass their names togv_stash*v, separated by:: as in the Perl language itself.
Alternately, if you have an SV that is a blessed reference, you can find out the stash pointer by using:
HV* SvSTASH(SvRV(SV*));then use the following to get the package name itself:
char* HvNAME(HV* stash);If you need to bless or re-bless an object you can use the following function:
SV* sv_bless(SV*, HV* stash)where the first argument, anSV*, must be a reference, and the second argument is a stash. The returnedSV* can now be used in the same way as any other SV.
For more information on references and blessings, consultperlref.
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.
Some scalar variables contain more than one type of scalar data. For example, the variable$! contains either the numeric value oferrno or its string equivalent from eitherstrerror orsys_errlist[].
To force multiple data values into an SV, you must do two things: use thesv_set*v 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:
SvIOK_onSvNOK_onSvPOK_onSvROK_onThe particular macro you must use depends on whichsv_set*v routine you called first. This is because everysv_set*v routine turns on only the bit for the particular type of data being set, and turns off all the rest.
For example, to create a new Perl variable called "dberror" that contains both the numeric and descriptive string error values, you could use the following code:
extern int dberror;extern char *dberror_list;SV* sv = get_sv("dberror", GV_ADD);sv_setiv(sv, (IV) dberror);sv_setpv(sv, dberror_list[dberror]);SvIOK_on(sv);If the order ofsv_setiv andsv_setpv had been reversed, then the macroSvPOK_on would need to be called instead ofSvIOK_on.
In Perl 5.16 and earlier, copy-on-write (see the next section) shared a flag bit with read-only scalars. So the only way to test whethersv_setsv, etc., will raise a "Modification of a read-only value" error in those versions is:
SvREADONLY(sv) && !SvIsCOW(sv)Under Perl 5.18 and later, SvREADONLY only applies to read-only variables, and, under 5.20, copy-on-write scalars can also be read-only, so the above check is incorrect. You just want:
SvREADONLY(sv)If you need to do this check often, define your own macro like this:
#if PERL_VERSION >= 18# define SvTRULYREADONLY(sv) SvREADONLY(sv)#else# define SvTRULYREADONLY(sv) (SvREADONLY(sv) && !SvIsCOW(sv))#endifPerl implements a copy-on-write (COW) mechanism for scalars, in which string copies are not immediately made when requested, but are deferred until made necessary by one or the other scalar changing. This is mostly transparent, but one must take care not to modify string buffers that are shared by multiple SVs.
You can test whether an SV is using copy-on-write withSvIsCOW(sv).
You can force an SV to make its own copy of its string buffer by callingsv_force_normal(sv) or SvPV_force_nolen(sv).
If you want to make the SV drop its string buffer, usesv_force_normal_flags(sv, SV_COW_DROP_PV) or simplysv_setsv(sv, NULL).
All of these functions will croak on read-only scalars (see the previous section for more on those).
To test that your code is behaving correctly and not modifying COW buffers, on systems that supportmmap(2) (i.e., Unix) you can configure perl with-Accflags=-DPERL_DEBUG_READONLY_COW and it will turn buffer violations into crashes. You will find it to be marvellously slow, so you may want to skip perl's own tests.
[This section still under construction. Ignore everything here. Post no bills. Everything not permitted is forbidden.]
Any SV may be magical, that is, it has special features that a normal SV does not have. These features are stored in the SV structure in a linked list ofstruct magic's, typedef'ed toMAGIC.
struct magic { MAGIC* mg_moremagic; MGVTBL* mg_virtual; U16 mg_private; char mg_type; U8 mg_flags; I32 mg_len; SV* mg_obj; char* mg_ptr;};Note this is current as of patchlevel 0, and could change at any time.
Perl adds magic to an SV using the sv_magic function:
void sv_magic(SV* sv, SV* obj, int how, const char* name, I32 namlen);Thesv argument is a pointer to the SV that is to acquire a new magical feature.
Ifsv is not already magical, Perl uses theSvUPGRADE macro to convertsv to typeSVt_PVMG. 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 SV.
Thename andnamlen arguments are used to associate a string with the magic, typically the name of a variable.namlen is stored in themg_len field and ifname is non-null then either asavepvn copy ofname orname itself is stored in themg_ptr field, depending on whethernamlen is greater than zero or equal to zero respectively. As a special case, if(name && namlen == HEf_SVKEY) thenname is assumed to contain anSV* and is stored as-is with its REFCNT incremented.
The sv_magic function useshow to determine which, if any, predefined "Magic Virtual Table" should be assigned to themg_virtual field. See the"Magic Virtual Tables" section below. Thehow argument is also stored in themg_type field. The value ofhow should be chosen from the set of macrosPERL_MAGIC_foo found inperl.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 thanPERL_MAGIC_uvar for example.
Theobj argument is stored in themg_obj field of theMAGIC structure. If it is not the same as thesv argument, the reference count of theobj object is incremented. If it is the same, or if thehow argument isPERL_MAGIC_arylen,PERL_MAGIC_regdatum,PERL_MAGIC_regdata, or if it is a NULL pointer, thenobj is merely stored, without the reference count being incremented.
See alsosv_magicext inperlapi for a more flexible way to add magic to an SV.
There is also a function to add magic to anHV:
void hv_magic(HV *hv, GV *gv, int how);This simply callssv_magic and coerces thegv argument into anSV.
To remove the magic from an SV, call the function sv_unmagic:
int sv_unmagic(SV *sv, int type);Thetype argument should be equal to thehow value when theSV was initially made magical.
However, note thatsv_unmagic removes all magic of a certaintype from theSV. If you want to remove only certain magic of atype based on the magic virtual table, usesv_unmagicext instead:
int sv_unmagicext(SV *sv, int type, MGVTBL *vtbl);Themg_virtual field in theMAGIC structure is a pointer to anMGVTBL, which is a structure of function pointers and stands for "Magic Virtual Table" to handle the various operations that might be applied to that variable.
TheMGVTBL has five (or sometimes eight) pointers to the following routine types:
int (*svt_get) (pTHX_ SV* sv, MAGIC* mg);int (*svt_set) (pTHX_ SV* sv, MAGIC* mg);U32 (*svt_len) (pTHX_ SV* sv, MAGIC* mg);int (*svt_clear)(pTHX_ SV* sv, MAGIC* mg);int (*svt_free) (pTHX_ SV* sv, MAGIC* mg);int (*svt_copy) (pTHX_ SV *sv, MAGIC* mg, SV *nsv, const char *name, I32 namlen);int (*svt_dup) (pTHX_ MAGIC *mg, CLONE_PARAMS *param);int (*svt_local)(pTHX_ SV *nsv, MAGIC *mg);This MGVTBL structure is set at compile-time inperl.h and there are currently 32 types. These different structures contain pointers to various routines that perform additional actions depending on which function is being called.
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.svt_copy copy tied variable magic to a tied elementsvt_dup duplicate a magic structure during thread cloningsvt_local copy magic to local value during 'local'For instance, the MGVTBL structure calledvtbl_sv (which corresponds to anmg_type ofPERL_MAGIC_sv) contains:
{ magic_get, magic_set, magic_len, 0, 0 }Thus, when an SV is determined to be magical and of typePERL_MAGIC_sv, if a get operation is being performed, the routinemagic_get is called. All the various routines for the various magical types begin withmagic_. NOTE: the magic routines are not considered part of the Perl API, and may not be exported by the Perl library.
The last three slots are a recent addition, and for source code compatibility they are only checked for if one of the three flags MGf_COPY, MGf_DUP or MGf_LOCAL is set in mg_flags. This means that most code can continue declaring a vtable as a 5-element value. These three are currently used exclusively by the threading code, and are highly subject to change.
The current kinds of Magic Virtual Tables are:
mg_type(old-style char and macro) MGVTBL Type of magic-------------------------- ------ -------------\0 PERL_MAGIC_sv vtbl_sv Special scalar variable# PERL_MAGIC_arylen vtbl_arylen Array length ($#ary)% PERL_MAGIC_rhash (none) Extra data for restricted hashes* PERL_MAGIC_debugvar vtbl_debugvar $DB::single, signal, trace vars. PERL_MAGIC_pos vtbl_pos pos() lvalue: PERL_MAGIC_symtab (none) Extra data for symbol tables< PERL_MAGIC_backref vtbl_backref For weak ref data@ PERL_MAGIC_arylen_p (none) To move arylen out of XPVAVB PERL_MAGIC_bm vtbl_regexp Boyer-Moore (fast string search)c PERL_MAGIC_overload_table vtbl_ovrld Holds overload table (AMT) on stashD PERL_MAGIC_regdata vtbl_regdata Regex match position data (@+ and @- vars)d PERL_MAGIC_regdatum vtbl_regdatum Regex match position data elementE PERL_MAGIC_env vtbl_env %ENV hashe PERL_MAGIC_envelem vtbl_envelem %ENV hash elementf PERL_MAGIC_fm vtbl_regexp Formline ('compiled' format)g PERL_MAGIC_regex_global vtbl_mglob m//g targetH PERL_MAGIC_hints vtbl_hints %^H hashh PERL_MAGIC_hintselem vtbl_hintselem %^H hash elementI PERL_MAGIC_isa vtbl_isa @ISA arrayi PERL_MAGIC_isaelem vtbl_isaelem @ISA array elementk PERL_MAGIC_nkeys vtbl_nkeys scalar(keys()) lvalueL PERL_MAGIC_dbfile (none) Debugger %_<filenamel PERL_MAGIC_dbline vtbl_dbline Debugger %_<filename elementN PERL_MAGIC_shared (none) Shared between threadsn PERL_MAGIC_shared_scalar (none) Shared between threadso PERL_MAGIC_collxfrm vtbl_collxfrm Locale transformationP PERL_MAGIC_tied vtbl_pack Tied array or hashp PERL_MAGIC_tiedelem vtbl_packelem Tied array or hash elementq PERL_MAGIC_tiedscalar vtbl_packelem Tied scalar or handler PERL_MAGIC_qr vtbl_regexp Precompiled qr// regexS PERL_MAGIC_sig (none) %SIG hashs PERL_MAGIC_sigelem vtbl_sigelem %SIG hash elementt PERL_MAGIC_taint vtbl_taint TaintednessU PERL_MAGIC_uvar vtbl_uvar Available for use by extensionsu PERL_MAGIC_uvar_elem (none) Reserved for use by extensionsV PERL_MAGIC_vstring (none) SV was vstring literalv PERL_MAGIC_vec vtbl_vec vec() lvaluew PERL_MAGIC_utf8 vtbl_utf8 Cached UTF-8 informationx PERL_MAGIC_substr vtbl_substr substr() lvalueY PERL_MAGIC_nonelem vtbl_nonelem Array element that does not existy PERL_MAGIC_defelem vtbl_defelem Shadow "foreach" iterator variable / smart parameter vivification\ PERL_MAGIC_lvref vtbl_lvref Lvalue reference constructor] PERL_MAGIC_checkcall vtbl_checkcall Inlining/mutation of call to this CV~ PERL_MAGIC_ext (none) Available for use by extensionsWhen an uppercase and lowercase letter both exist in the table, then the uppercase letter is typically used to represent some kind of composite type (a list or a hash), and the lowercase letter is used to represent an element of that composite type. Some internals code makes use of this case relationship. However, 'v' and 'V' (vec and v-string) are in no way related.
ThePERL_MAGIC_ext andPERL_MAGIC_uvar magic types are defined specifically for use by extensions and will not be used by perl itself. Extensions can usePERL_MAGIC_ext magic to 'attach' private information to variables (typically objects). This is especially useful because there is no way for normal perl code to corrupt this private information (unlike using extra elements of a hash object).
Similarly,PERL_MAGIC_uvar magic can be used much like tie() to call a C function any time a scalar's value is used or changed. TheMAGIC'smg_ptr field points to aufuncs structure:
struct ufuncs { I32 (*uf_val)(pTHX_ IV, SV*); I32 (*uf_set)(pTHX_ IV, SV*); IV uf_index;};When the SV is read from or written to, theuf_val oruf_set function will be called withuf_index as the first arg and a pointer to the SV as the second. A simple example of how to addPERL_MAGIC_uvar magic is shown below. Note that the ufuncs structure is copied by sv_magic, so you can safely allocate it on the stack.
voidUmagic(sv) SV *sv;PREINIT: struct ufuncs uf;CODE: uf.uf_val = &my_get_fn; uf.uf_set = &my_set_fn; uf.uf_index = 0; sv_magic(sv, 0, PERL_MAGIC_uvar, (char*)&uf, sizeof(uf));AttachingPERL_MAGIC_uvar to arrays is permissible but has no effect.
For hashes there is a specialized hook that gives control over hash keys (but not values). This hook callsPERL_MAGIC_uvar 'get' magic if the "set" function in theufuncs structure is NULL. The hook is activated whenever the hash is accessed with a key specified as anSV through the functionshv_store_ent,hv_fetch_ent,hv_delete_ent, andhv_exists_ent. Accessing the key as a string through the functions without the..._ent suffix circumvents the hook. See"GUTS" in Hash::Util::FieldHash for a detailed description.
Note that because multiple extensions may be usingPERL_MAGIC_ext orPERL_MAGIC_uvar magic, it is important for extensions to take extra care to avoid conflict. Typically only using the magic on objects blessed into the same class as the extension is sufficient. ForPERL_MAGIC_ext magic, it is usually a good idea to define anMGVTBL, even if all its fields will be0, so that individualMAGIC pointers can be identified as a particular kind of magic using their magic virtual table.mg_findext provides an easy way to do that:
STATIC MGVTBL my_vtbl = { 0, 0, 0, 0, 0, 0, 0, 0 };MAGIC *mg;if ((mg = mg_findext(sv, PERL_MAGIC_ext, &my_vtbl))) { /* this is really ours, not another module's PERL_MAGIC_ext */ my_priv_data_t *priv = (my_priv_data_t *)mg->mg_ptr; ...}Also note that thesv_set*() andsv_cat*() functions described earlier donot invoke 'set' magic on their targets. This must be done by the user either by calling theSvSETMAGIC() macro after calling these functions, or by using one of thesv_set*_mg() orsv_cat*_mg() functions. Similarly, generic C code must call theSvGETMAGIC() macro to invoke any 'get' magic if they use an SV obtained from external sources in functions that don't handle magic. Seeperlapi for a description of these functions. For example, calls to thesv_cat*() functions typically need to be followed bySvSETMAGIC(), but they don't need a priorSvGETMAGIC() since their implementation handles 'get' magic.
MAGIC *mg_find(SV *sv, int type); /* Finds the magic pointer of that * type */This routine returns a pointer to aMAGIC structure stored in the SV. If the SV does not have that magical feature,NULL is returned. If the SV has multiple instances of that magical feature, the first one will be returned.mg_findext can be used to find aMAGIC structure of an SV based on both its magic type and its magic virtual table:
MAGIC *mg_findext(SV *sv, int type, MGVTBL *vtbl);Also, if the SV passed tomg_find ormg_findext is not of type SVt_PVMG, Perl may core dump.
int mg_copy(SV* sv, SV* nsv, const char* key, STRLEN klen);This routine checks to see what types of magicsv has. If the mg_type field is an uppercase letter, then the mg_obj is copied tonsv, but the mg_type field is changed to be the lowercase letter.
Tied hashes and arrays are magical beasts of thePERL_MAGIC_tied magic type.
WARNING: As of the 5.004 release, proper usage of the array and hash access functions requires understanding a few caveats. Some of these caveats are actually considered bugs in the API, to be fixed in later releases, and are bracketed with [MAYCHANGE] below. If you find yourself actually applying such information in this section, be aware that the behavior may change in the future, umm, without warning.
The perl tie function associates a variable with an object that implements the various GET, SET, etc methods. To perform the equivalent of the perl tie function from an XSUB, you must mimic this behaviour. The code below carries out the necessary steps -- firstly it creates a new hash, and then creates a second hash which it blesses into the class which will implement the tie methods. Lastly it ties the two hashes together, and returns a reference to the new tied hash. Note that the code below does NOT call the TIEHASH method in the MyTie class - see"Calling Perl Routines from within C Programs" for details on how to do this.
SV*mytie()PREINIT: HV *hash; HV *stash; SV *tie;CODE: hash = newHV(); tie = newRV_noinc((SV*)newHV()); stash = gv_stashpv("MyTie", GV_ADD); sv_bless(tie, stash); hv_magic(hash, (GV*)tie, PERL_MAGIC_tied); RETVAL = newRV_noinc(hash);OUTPUT: RETVALTheav_store function, when given a tied array argument, merely copies the magic of the array onto the value to be "stored", usingmg_copy. It may also return NULL, indicating that the value did not actually need to be stored in the array. [MAYCHANGE] After a call toav_store on a tied array, the caller will usually need to callmg_set(val) to actually invoke the perl level "STORE" method on the TIEARRAY object. Ifav_store did return NULL, a call toSvREFCNT_dec(val) will also be usually necessary to avoid a memory leak. [/MAYCHANGE]
The previous paragraph is applicable verbatim to tied hash access using thehv_store andhv_store_ent functions as well.
av_fetch and the corresponding hash functionshv_fetch andhv_fetch_ent actually return an undefined mortal value whose magic has been initialized usingmg_copy. Note the value so returned does not need to be deallocated, as it is already mortal. [MAYCHANGE] But you will need to callmg_get() on the returned value in order to actually invoke the perl level "FETCH" method on the underlying TIE object. Similarly, you may also callmg_set() on the return value after possibly assigning a suitable value to it usingsv_setsv, which will invoke the "STORE" method on the TIE object. [/MAYCHANGE]
[MAYCHANGE] In other words, the array or hash fetch/store functions don't really fetch and store actual values in the case of tied arrays and hashes. They merely callmg_copy to attach magic to the values that were meant to be "stored" or "fetched". Later calls tomg_get andmg_set actually do the job of invoking the TIE methods on the underlying objects. Thus the magic mechanism currently implements a kind of lazy access to arrays and hashes.
Currently (as of perl version 5.004), use of the hash and array access functions requires the user to be aware of whether they are operating on "normal" hashes and arrays, or on their tied variants. The API may be changed to provide more transparent access to both tied and normal data types in future versions. [/MAYCHANGE]
You would do well to understand that the TIEARRAY and TIEHASH interfaces are mere sugar to invoke some perl method calls while using the uniform hash and array syntax. The use of this sugar imposes some overhead (typically about two to four extra opcodes per FETCH/STORE operation, in addition to the creation of all the mortal variables required to invoke the methods). This overhead will be comparatively small if the TIE methods are themselves substantial, but if they are only a few statements long, the overhead will not be insignificant.
Perl has a very handy construction
{ local $var = 2; ...}This construction isapproximately equivalent to
{ my $oldvar = $var; $var = 2; ... $var = $oldvar;}The biggest difference is that the first construction would reinstate the initial value of $var, irrespective of how control exits the block:goto,return,die/eval, etc. It is a little bit more efficient as well.
There is a way to achieve a similar task from C via Perl API: create apseudo-block, and arrange for some changes to be automatically undone at the end of it, either explicit, or via a non-local exit (via die()). Ablock-like construct is created by a pair ofENTER/LEAVE macros (see"Returning a Scalar" in perlcall). Such a construct may be created specially for some important localized task, or an existing one (like boundaries of enclosing Perl subroutine/block, or an existing pair for freeing TMPs) may be used. (In the second case the overhead of additional localization must be almost negligible.) Note that any XSUB is automatically enclosed in anENTER/LEAVE pair.
Inside such apseudo-block the following service is available:
SAVEINT(int i)SAVEIV(IV i)SAVEI32(I32 i)SAVELONG(long i)These macros arrange things to restore the value of integer variablei at the end of enclosingpseudo-block.
SAVESPTR(s)SAVEPPTR(p)These macros arrange things to restore the value of pointerss andp.s must be a pointer of a type which survives conversion toSV* and back,p should be able to survive conversion tochar* and back.
SAVEFREESV(SV *sv)The refcount ofsv will be decremented at the end ofpseudo-block. This is similar tosv_2mortal in that it is also a mechanism for doing a delayedSvREFCNT_dec. However, whilesv_2mortal extends the lifetime ofsv until the beginning of the next statement,SAVEFREESV extends it until the end of the enclosing scope. These lifetimes can be wildly different.
Also compareSAVEMORTALIZESV.
SAVEMORTALIZESV(SV *sv)Just likeSAVEFREESV, but mortalizessv at the end of the current scope instead of decrementing its reference count. This usually has the effect of keepingsv alive until the statement that called the currently live scope has finished executing.
SAVEFREEOP(OP *op)TheOP * is op_free()ed at the end ofpseudo-block.
SAVEFREEPV(p)The chunk of memory which is pointed to byp is Safefree()ed at the end ofpseudo-block.
SAVECLEARSV(SV *sv)Clears a slot in the current scratchpad which corresponds tosv at the end ofpseudo-block.
SAVEDELETE(HV *hv, char *key, I32 length)The keykey ofhv is deleted at the end ofpseudo-block. The string pointed to bykey is Safefree()ed. If one has akey in short-lived storage, the corresponding string may be reallocated like this:
SAVEDELETE(PL_defstash, savepv(tmpbuf), strlen(tmpbuf));SAVEDESTRUCTOR(DESTRUCTORFUNC_NOCONTEXT_t f, void *p)At the end ofpseudo-block the functionf is called with the only argumentp.
SAVEDESTRUCTOR_X(DESTRUCTORFUNC_t f, void *p)At the end ofpseudo-block the functionf is called with the implicit context argument (if any), andp.
SAVESTACK_POS()The current offset on the Perl internal stack (cf.SP) is restored at the end ofpseudo-block.
The following API list contains functions, thus one needs to provide pointers to the modifiable data explicitly (either C pointers, or PerlishGV *s). Where the above macros takeint, a similar function takesint *.
SV* save_scalar(GV *gv)Equivalent to Perl codelocal $gv.
AV* save_ary(GV *gv)HV* save_hash(GV *gv)Similar tosave_scalar, but localize@gv and%gv.
void save_item(SV *item)Duplicates the current value ofSV, on the exit from the currentENTER/LEAVEpseudo-block will restore the value ofSV using the stored value. It doesn't handle magic. Usesave_scalar if magic is affected.
void save_list(SV **sarg, I32 maxsarg)A variant ofsave_item which takes multiple arguments via an arraysarg ofSV* of lengthmaxsarg.
SV* save_svref(SV **sptr)Similar tosave_scalar, but will reinstate anSV *.
void save_aptr(AV **aptr)void save_hptr(HV **hptr)Similar tosave_svref, but localizeAV * andHV *.
TheAlias module implements localization of the basic types within thecaller's scope. People who are interested in how to localize things in the containing scope should take a look there too.
The XSUB mechanism is a simple way for Perl programs to access C subroutines. An XSUB routine will have a stack that contains the arguments from the Perl program, and a way to map from the Perl data structures to a C equivalent.
The stack arguments are accessible through theST(n) macro, which returns then'th stack argument. Argument 0 is the first argument passed in the Perl subroutine call. These arguments areSV*, and can be used anywhere anSV* is used.
Most of the time, output from the C routine can be handled through use of the RETVAL and OUTPUT directives. However, there are some cases where the argument stack is not already long enough to handle all the return values. An example is the POSIX tzname() call, which takes no arguments, but returns two, the local time zone's standard and summer time abbreviations.
To handle this situation, the PPCODE directive is used and the stack is extended using the macro:
EXTEND(SP, num);whereSP is the macro that represents the local copy of the stack pointer, andnum 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 usingPUSHs macro. The pushed values will often need to be "mortal" (See"Reference Counts and Mortality"):
PUSHs(sv_2mortal(newSViv(an_integer)))PUSHs(sv_2mortal(newSVuv(an_unsigned_integer)))PUSHs(sv_2mortal(newSVnv(a_double)))PUSHs(sv_2mortal(newSVpv("Some String",0)))/* Although the last example is better written as the more * efficient: */PUSHs(newSVpvs_flags("Some String", SVs_TEMP))And now the Perl program callingtzname, the two values will be assigned as in:
($standard_abbrev, $summer_abbrev) = POSIX::tzname;An alternate (and possibly simpler) method to pushing values on the stack is to use the macro:
XPUSHs(SV*)This macro automatically adjusts the stack for you, if needed. Thus, you do not need to callEXTEND to extend the stack.
Despite their suggestions in earlier versions of this document the macros(X)PUSH[iunp] arenot suited to XSUBs which return multiple results. For that, either stick to the(X)PUSHs macros shown above, or use the newm(X)PUSH[iunp] macros instead; see"Putting a C value on Perl stack".
For more information, consultperlxs andperlxstut.
If an AUTOLOAD routine is an XSUB, as with Perl subroutines, Perl puts the fully-qualified name of the autoloaded subroutine in the $AUTOLOAD variable of the XSUB's package.
But it also puts the same information in certain fields of the XSUB itself:
HV *stash = CvSTASH(cv);const char *subname = SvPVX(cv);STRLEN name_length = SvCUR(cv); /* in bytes */U32 is_utf8 = SvUTF8(cv);SvPVX(cv) contains just the sub name itself, not including the package. For an AUTOLOAD routine in UNIVERSAL or one of its superclasses,CvSTASH(cv) returns NULL during a method call on a nonexistent package.
Note: Setting $AUTOLOAD stopped working in 5.6.1, which did not support XS AUTOLOAD subs at all. Perl 5.8.0 introduced the use of fields in the XSUB itself. Perl 5.16.0 restored the setting of $AUTOLOAD. If you need to support 5.8-5.14, use the XSUB's fields.
There are four routines that can be used to call a Perl subroutine from within a C program. These four are:
I32 call_sv(SV*, I32);I32 call_pv(const char*, I32);I32 call_method(const char*, I32);I32 call_argv(const char*, I32, char**);The routine most often used iscall_sv. TheSV* argument contains either the name of the Perl subroutine to be called, or a reference to the subroutine. The second argument consists of flags that control the context in which the subroutine is called, whether or not the subroutine is being passed arguments, how errors should be trapped, and how to treat return values.
All four routines return the number of arguments that the subroutine returned on the Perl stack.
These routines used to be calledperl_call_sv, etc., before Perl v5.6.0, but those names are now deprecated; macros of the same name are provided for compatibility.
When using any of these routines (exceptcall_argv), the programmer must manipulate the Perl stack. These include the following macros and functions:
dSPSPPUSHMARK()PUTBACKSPAGAINENTERSAVETMPSFREETMPSLEAVEXPUSH*()POP*()For a detailed description of calling conventions from C to Perl, consultperlcall.
A lot of opcodes (this is an elementary operation in the internal perl stack machine) put an SV* on the stack. However, as an optimization the corresponding SV is (usually) not recreated each time. The opcodes reuse specially assigned SVs (targets) which are (as a corollary) not constantly freed/created.
Each of the targets is created only once (but see"Scratchpads and recursion" below), and when an opcode needs to put an integer, a double, or a string on stack, it just sets the corresponding parts of itstarget and puts thetarget on stack.
The macro to put this target on stack isPUSHTARG, and it is directly used in some opcodes, as well as indirectly in zillions of others, which use it via(X)PUSH[iunp].
Because the target is reused, you must be careful when pushing multiple values on the stack. The following code will not do what you think:
XPUSHi(10);XPUSHi(20);This translates as "setTARG to 10, push a pointer toTARG onto the stack; setTARG to 20, push a pointer toTARG onto the stack". At the end of the operation, the stack does not contain the values 10 and 20, but actually contains two pointers toTARG, which we have set to 20.
If you need to push multiple different values then you should either use the(X)PUSHs macros, or else use the newm(X)PUSH[iunp] macros, none of which make use ofTARG. The(X)PUSHs macros simply push an SV* on the stack, which, as noted under"XSUBs and the Argument Stack", will often need to be "mortal". The newm(X)PUSH[iunp] macros make this a little easier to achieve by creating a new mortal for you (via(X)PUSHmortal), pushing that onto the stack (extending it if necessary in the case of themXPUSH[iunp] macros), and then setting its value. Thus, instead of writing this to "fix" the example above:
XPUSHs(sv_2mortal(newSViv(10)))XPUSHs(sv_2mortal(newSViv(20)))you can simply write:
mXPUSHi(10)mXPUSHi(20)On a related note, if you do use(X)PUSH[iunp], then you're going to need adTARG in your variable declarations so that the*PUSH* macros can make use of the local variableTARG. See alsodTARGET anddXSTARG.
The question remains on when the SVs which aretargets for opcodes are created. The answer is that they are created when the current unit--a subroutine or a file (for opcodes for statements outside of subroutines)--is compiled. During this time a special anonymous Perl array is created, which is called a scratchpad for the current unit.
A scratchpad keeps SVs which are lexicals for the current unit and are targets for opcodes. A previous version of this document stated that one can deduce that an SV lives on a scratchpad by looking on its flags: lexicals haveSVs_PADMY set, andtargets haveSVs_PADTMP set. But this has never been fully true.SVs_PADMY could be set on a variable that no longer resides in any pad. Whiletargets do haveSVs_PADTMP set, it can also be set on variables that have never resided in a pad, but nonetheless act liketargets. As of perl 5.21.5, theSVs_PADMY flag is no longer used and is defined as 0.SvPADMY() now returns true for anything withoutSVs_PADTMP.
The correspondence between OPs andtargets is not 1-to-1. Different OPs in the compile tree of the unit can use the same target, if this would not conflict with the expected life of the temporary.
In fact it is not 100% true that a compiled unit contains a pointer to the scratchpad AV. In fact it contains a pointer to an AV of (initially) one element, and this element is the scratchpad AV. Why do we need an extra level of indirection?
The answer isrecursion, and maybethreads. Both these can create several execution pointers going into the same subroutine. For the subroutine-child not write over the temporaries for the subroutine-parent (lifespan of which covers the call to the child), the parent and the child should have different scratchpads. (And the lexicals should be separate anyway!)
So each subroutine is born with an array of scratchpads (of length 1). On each entry to the subroutine it is checked that the current depth of the recursion is not more than the length of this array, and if it is, new scratchpad is created and pushed into the array.
Thetargets on this scratchpad areundefs, but they are already marked with correct flags.
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 used within perl.
It is suggested that you enable the version of malloc that is distributed 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 :
Newx(pointer, number, type);Newxc(pointer, number, type, cast);Newxz(pointer, number, type);The first argumentpointer should be the name of a variable that will point to the newly allocated memory.
The second and third argumentsnumber andtype specify how many of the specified type of data structure should be allocated. The argumenttype is passed tosizeof. The final argument toNewxc,cast, should be used if thepointer argument is different from thetype argument.
Unlike theNewx andNewxc macros, theNewxz macro callsmemzero to zero out all the newly allocated memory.
Renew(pointer, number, type);Renewc(pointer, number, type, cast);Safefree(pointer)These three macros are used to change a memory buffer size or to free a piece of memory no longer needed. The arguments toRenew andRenewc match those ofNew andNewc with the exception of not needing the "magic cookie" argument.
Move(source, dest, number, type);Copy(source, dest, number, type);Zero(dest, number, type);These three macros are used to move, copy, or zero out previously allocated memory. Thesource anddest arguments point to the source and destination starting points. Perl will move, copy, or zero outnumber instances of the size of thetype data structure (using thesizeof function).
The most recent development releases of Perl have been experimenting with removing Perl's dependency on the "normal" standard I/O suite and allowing other stdio implementations to be used. This involves creating a new abstraction layer that then calls whichever implementation of stdio Perl was compiled with. All XSUBs should now use the functions in the PerlIO abstraction layer and not make any assumptions about what kind of stdio is being used.
For a complete description of the PerlIO abstraction, consultperlapio.
Here we describe the internal form your code is converted to by Perl. Start with a simple example:
$a = $b + $c;This is converted to a tree similar to this one:
assign-to / \ + $a / \$b $c(but slightly more complicated). This tree reflects the way Perl parsed your code, but has nothing to do with the execution order. There is an additional "thread" going through the nodes of the tree which shows the order of execution of the nodes. In our simplified example above it looks like:
$b ---> $c ---> + ---> $a ---> assign-toBut with the actual compile tree for$a = $b + $c it is different: some nodesoptimized away. As a corollary, though the actual tree contains more nodes than our simplified example, the execution order is the same as in our example.
If you have your perl compiled for debugging (usually done with-DDEBUGGING on theConfigure command line), you may examine the compiled tree by specifying-Dx on the Perl command line. The output takes several lines per node, and for$b+$c it looks like this:
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 } }This tree has 5 nodes (one perTYPE 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{} pairs on the same level of indentation, thus this listing corresponds to the tree:
add / \null null | |gvsv gvsvThe execution order is indicated by===> marks, thus it is3 4 5 6 (node6 is not included into above listing), i.e.,gvsv gvsv add whatever.
Each of these nodes represents an op, a fundamental operation inside the Perl core. The code which implements each operation can be found in thepp*.c files; the function which implements the op with typegvsv ispp_gvsv, and so on. As the tree above shows, different ops have different numbers of children:add is a binary operator, as one would expect, and so has two children. To accommodate the various different numbers of children, there are various types of op data structure, and they link together in different ways.
The simplest type of op structure isOP: this has no children. Unary operators,UNOPs, have one child, and this is pointed to by theop_first field. Binary operators (BINOPs) have not only anop_first field but also anop_last field. The most complex type of op is aLISTOP, which has any number of children. In this case, the first child is pointed to byop_first and the last child byop_last. The children in between can be found by iteratively following theOpSIBLING pointer from the first child to the last (but see below).
There are also some other op types: aPMOP holds a regular expression, and has no children, and aLOOP may or may not have children. If theop_children field is non-zero, it behaves like aLISTOP. To complicate matters, if aUNOP is actually anull op after optimization (see"Compile pass 2: context propagation") it will still have children in accordance with its former type.
Finally, there is aLOGOP, or logic op. Like aLISTOP, this has one or more children, but it doesn't have anop_last field: so you have to followop_first and then theOpSIBLING chain itself to find the last child. Instead it has anop_other field, which is comparable to theop_next field described below, and represents an alternate execution path. Operators likeand,or and? areLOGOPs. Note that in general,op_other may not point to any of the direct children of theLOGOP.
Starting in version 5.21.2, perls built with the experimental define-DPERL_OP_PARENT add an extra boolean flag for each op,op_moresib. When not set, this indicates that this is the last op in anOpSIBLING chain. This frees up theop_sibling field on the last sibling to point back to the parent op. Under this build, that field is also renamedop_sibparent to reflect its joint role. The macroOpSIBLING(o) wraps this special behaviour, and always returns NULL on the last sibling. With this build theop_parent(o) function can be used to find the parent of any op. Thus for forward compatibility, you should always use theOpSIBLING(o) macro rather than accessingop_sibling directly.
Another way to examine the tree is to use a compiler back-end module, such asB::Concise.
The tree is created by the compiler whileyacc code feeds it the constructions it recognizes. Sinceyacc works bottom-up, so does the first pass of perl compilation.
What makes this pass interesting for perl developers is that some optimization may be performed on this pass. This is optimization by so-called "check routines". The correspondence between node names and corresponding check routines is described inopcode.pl (do not forget to runmake regen_headers if you modify this file).
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.
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).
By convention, check routines have namesck_*. They are usually called fromnew*OP subroutines (orconvert) (which in turn are called fromperly.y).
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 aconstant node with the "return value" of the corresponding subtree is substituted instead. The subtree is deleted.
If constant folding was not performed, the execution-order thread is created.
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.
Additional context-dependent optimizations are performed at this time. Since at this moment the compile tree contains back-references (via "thread" pointers), nodes cannot be free()d now. To allow optimized-away nodes at this stage, such nodes are null()ified instead of free()ing (i.e. their type is changed to OP_NULL).
After the compile tree for a subroutine (or for aneval 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). Optimizations performed at this stage are subject to the same restrictions as in the pass 2.
Peephole optimizations are done by calling the function pointed to by the global variablePL_peepp. By default,PL_peepp just calls the function pointed to by the global variablePL_rpeepp. By default, that performs some basic op fixups and optimisations along the execution-order op chain, and recursively callsPL_rpeepp for each side chain of ops (resulting from conditionals). Extensions may provide additional optimisations or fixups, hooking into either the per-subroutine or recursive stage, like this:
static peep_t prev_peepp;static void my_peep(pTHX_ OP *o){ /* custom per-subroutine optimisation goes here */ prev_peepp(aTHX_ o); /* custom per-subroutine optimisation may also go here */}BOOT: prev_peepp = PL_peepp; PL_peepp = my_peep;static peep_t prev_rpeepp;static void my_rpeep(pTHX_ OP *o){ OP *orig_o = o; for(; o; o = o->op_next) { /* custom per-op optimisation goes here */ } prev_rpeepp(aTHX_ orig_o);}BOOT: prev_rpeepp = PL_rpeepp; PL_rpeepp = my_rpeep;The compile tree is executed in a runops function. There are two runops functions, inrun.c and indump.c.Perl_runops_debug is used with DEBUGGING andPerl_runops_standard is used otherwise. For fine control over the execution of the compile tree it is possible to provide your own runops function.
It's probably best to copy one of the existing runops functions and change it to suit your needs. Then, in the BOOT section of your XS file, add the line:
PL_runops = my_runops;This function should be as efficient as possible to keep your programs running as fast as possible.
As of perl 5.14 it is possible to hook into the compile-time lexical scope mechanism usingPerl_blockhook_register. This is used like this:
STATIC void my_start_hook(pTHX_ int full);STATIC BHK my_hooks;BOOT: BhkENTRY_set(&my_hooks, bhk_start, my_start_hook); Perl_blockhook_register(aTHX_ &my_hooks);This will arrange to havemy_start_hook called at the start of compiling every lexical scope. The available hooks are:
void bhk_start(pTHX_ int full)This is called just after starting a new lexical scope. Note that Perl code like
if ($x) { ... }creates two scopes: the first starts at the( and hasfull == 1, the second starts at the{ and hasfull == 0. Both end at the}, so calls tostart andpre/post_end will match. Anything pushed onto the save stack by this hook will be popped just before the scope ends (between thepre_ andpost_end hooks, in fact).
void bhk_pre_end(pTHX_ OP **o)This is called at the end of a lexical scope, just before unwinding the stack.o is the root of the optree representing the scope; it is a double pointer so you can replace the OP if you need to.
void bhk_post_end(pTHX_ OP **o)This is called at the end of a lexical scope, just after unwinding the stack.o is as above. Note that it is possible for calls topre_ andpost_end to nest, if there is something on the save stack that calls string eval.
void bhk_eval(pTHX_ OP *const o)This is called just before starting to compile aneval STRING,do FILE,require oruse, after the eval has been set up.o is the OP that requested the eval, and will normally be anOP_ENTEREVAL,OP_DOFILE orOP_REQUIRE.
Once you have your hook functions, you need aBHK structure to put them in. It's best to allocate it statically, since there is no way to free it once it's registered. The function pointers should be inserted into this structure using theBhkENTRY_set macro, which will also set flags indicating which entries are valid. If you do need to allocate yourBHK dynamically for some reason, be sure to zero it before you start.
Once registered, there is no mechanism to switch these hooks off, so if that is necessary you will need to do this yourself. An entry in%^H is probably the best way, so the effect is lexically scoped; however it is also possible to use theBhkDISABLE andBhkENABLE macros to temporarily switch entries on and off. You should also be aware that generally speaking at least one scope will have opened before your extension is loaded, so you will see somepre/post_end pairs that didn't have a matchingstart.
dump functionsTo aid debugging, the source filedump.c contains a number of functions which produce formatted output of internal data structures.
The most commonly used of these functions isPerl_sv_dump; it's used for dumping SVs, AVs, HVs, and CVs. TheDevel::Peek module callssv_dump to produce debugging output from Perl-space, so users of that module should already be familiar with its format.
Perl_op_dump can be used to dump anOP structure or any of its derivatives, and produces output similar toperl -Dx; in fact,Perl_dump_eval will dump the main root of the code being evaluated, exactly like-Dx.
Other useful functions arePerl_dump_sub, which turns aGV into an op tree,Perl_dump_packsubs which callsPerl_dump_sub on all the subroutines in a package like so: (Thankfully, these are all xsubs, so there is no op tree)
(gdb) print Perl_dump_packsubs(PL_defstash)SUB attributes::bootstrap = (xsub 0x811fedc 0)SUB UNIVERSAL::can = (xsub 0x811f50c 0)SUB UNIVERSAL::isa = (xsub 0x811f304 0)SUB UNIVERSAL::VERSION = (xsub 0x811f7ac 0)SUB DynaLoader::boot_DynaLoader = (xsub 0x805b188 0)andPerl_dump_all, which dumps all the subroutines in the stash and the op tree of the main root.
The Perl interpreter can be regarded as a closed box: it has an API 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.
One macro controls the major Perl build flavor: MULTIPLICITY. The MULTIPLICITY build has a C structure that packages all the interpreter state. With multiplicity-enabled perls, PERL_IMPLICIT_CONTEXT is also normally defined, and enables the support for passing in a "hidden" first argument that represents all three data structures. MULTIPLICITY makes multi-threaded perls possible (with the ithreads threading model, related to the macro USE_ITHREADS.)
Two other "encapsulation" macros are the PERL_GLOBAL_STRUCT and PERL_GLOBAL_STRUCT_PRIVATE (the latter turns on the former, and the former turns on MULTIPLICITY.) The PERL_GLOBAL_STRUCT causes all the internal variables of Perl to be wrapped inside a single global struct, struct perl_vars, accessible as (globals) &PL_Vars or PL_VarsPtr or the function Perl_GetVars(). The PERL_GLOBAL_STRUCT_PRIVATE goes one step further, there is still a single struct (allocated in main() either from heap or from stack) but there are no global data symbols pointing to it. In either case the global struct should be initialized as the very first thing in main() using Perl_init_global_struct() and correspondingly tear it down after perl_free() using Perl_free_global_struct(), please seeminiperlmain.c for usage details. You may also need to usedVAR in your coding to "declare the global variables" when you are using them. dTHX does this for you automatically.
To see whether you have non-const data you can use a BSD (or GNU) compatiblenm:
nm libperl.a | grep -v ' [TURtr] 'If this displays anyD ord symbols (or possiblyC orc), you have non-const data. The symbols thegrep removed are as follows:Tt aretext, or code, theRr areread-only (const) data, and theU is <undefined>, external symbols referred to.
The testt/porting/libperl.t does this kind of symbol sanity checking onlibperl.a.
For backward compatibility reasons defining just PERL_GLOBAL_STRUCT doesn't actually hide all symbols inside a big global struct: some PerlIO_xxx vtables are left visible. The PERL_GLOBAL_STRUCT_PRIVATE then hides everything (see how the PERLIO_FUNCS_DECL is used).
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.
First problem: deciding which functions will be public API functions and which will be private. All functions whose names beginS_ are private (think "S" for "secret" or "static"). All other functions begin with "Perl_", but just because a function begins with "Perl_" does not mean it is part of the API. (See"Internal Functions".) The easiest way to besure a function is part of the API is to find its entry inperlapi. If it exists inperlapi, it's part of the API. If it doesn't, and you think it should be (i.e., you need it for your extension), send mail viaperlbug explaining why you think it should be.
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:
STATIC voidS_incline(pTHX_ char *s)STATIC becomes "static" in C, and may be #define'd to nothing in some configurations in the future.
A public function (i.e. part of the internal API, but not necessarily sanctioned for use in extensions) begins like this:
voidPerl_sv_setiv(pTHX_ SV* dsv, IV num)pTHX_ is one of a number of macros (inperl.h) that hide the details of the interpreter's context. THX stands for "thread", "this", or "thingy", as the case may be. (And no, George Lucas is not involved. :-) The first character could be 'p' for aprototype, 'a' forargument, or 'd' fordeclaration, so we havepTHX,aTHX anddTHX, and their variants.
When Perl is built without options that set PERL_IMPLICIT_CONTEXT, there is no first argument containing the interpreter's context. The trailing underscore in the pTHX_ macro indicates that the macro expansion needs a comma after the context argument because other arguments follow it. If PERL_IMPLICIT_CONTEXT is not defined, pTHX_ will be ignored, and the 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.
When a core function calls another, it must pass the context. This is normally hidden via macros. Considersv_setiv. It expands into something like this:
#ifdef PERL_IMPLICIT_CONTEXT #define sv_setiv(a,b) Perl_sv_setiv(aTHX_ a, b) /* can't do this for vararg functions, see below */#else #define sv_setiv Perl_sv_setiv#endifThis works well, and means that XS authors can gleefully write:
sv_setiv(foo, bar);and still have it work under all the modes Perl could have been compiled with.
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, passingaTHX_ as the first argument (the Perl core tends to do this with functions like Perl_warner), or use a context-free version.
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#define warner Perl_warner_nocontext so that extensions get source compatibility at the expense of performance. (Passing an arg is cheaper than grabbing it from thread-local storage.)
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.
dTHR was introduced in perl 5.005 to support the older thread model. The older thread model now uses theTHX mechanism to pass context pointers around, sodTHR 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.
When Perl is built with PERL_IMPLICIT_CONTEXT, extensions that call any functions in the Perl API 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 PERL_IMPLICIT_CONTEXT enabled.
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: wheneverXSUB.h is #included, it redefines the aTHX and aTHX_ macros to call a function that will return the context. Thus, something like:
sv_setiv(sv, num);in your extension will translate to this when PERL_IMPLICIT_CONTEXT is in effect:
Perl_sv_setiv(Perl_get_context(), sv, num);or to this otherwise:
Perl_sv_setiv(sv, num);You don't have to do anything new in your extension to get this; since the Perl library provides Perl_get_context(), it will all just work.
The second, more efficient way is to use the following template for your Foo.xs:
#define PERL_NO_GET_CONTEXT /* we want efficiency */#include "EXTERN.h"#include "perl.h"#include "XSUB.h"STATIC void my_private_function(int arg1, int arg2);STATIC voidmy_private_function(int arg1, int arg2){ dTHX; /* fetch context */ ... call many Perl API functions ...}[... etc ...]MODULE = Foo PACKAGE = Foo/* typical XSUB */voidmy_xsub(arg) int arg CODE: my_private_function(arg, 10);Note that the only two changes from the normal way of writing an extension is the addition of a#define PERL_NO_GET_CONTEXT before including the Perl headers, followed by adTHX; declaration at the start of every function that will call the Perl API. (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 XS() macro is correctly defined to pass in the implicit context if needed.
The third, even more efficient way is to ape how it is done within the Perl guts:
#define PERL_NO_GET_CONTEXT /* we want efficiency */#include "EXTERN.h"#include "perl.h"#include "XSUB.h"/* pTHX_ only needed for functions that call Perl API */STATIC void my_private_function(pTHX_ int arg1, int arg2);STATIC voidmy_private_function(pTHX_ int arg1, int arg2){ /* dTHX; not needed here, because THX is an argument */ ... call Perl API functions ...}[... etc ...]MODULE = Foo PACKAGE = Foo/* typical XSUB */voidmy_xsub(arg) int arg CODE: my_private_function(aTHX_ arg, 10);This implementation never has to fetch the context using a function call, since it is always passed as an extra argument. Depending on your needs for simplicity or efficiency, you may mix the previous two approaches freely.
Never add a comma afterpTHX 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.
If one is compiling Perl with the-DPERL_GLOBAL_STRUCT thedVAR definition is needed if the Perl global variables (seeperlvars.h orglobvar.sym) are accessed in the function anddTHX is not used (thedTHX includes thedVAR if necessary). One notices the need fordVAR only with the said compile-time define, because otherwise the Perl global variables are visible as-is.
If you create interpreters in one thread and then proceed to call them in another, you need to make sure perl's own Thread Local Storage (TLS) slot is initialized correctly in each of those threads.
Theperl_alloc andperl_clone API functions will automatically set the TLS slot to the interpreter they created, so that there is no need to do anything special if the interpreter is always accessed in the same thread that created it, and that thread did not create or call any other interpreters afterwards. If that is not the case, you have to set the TLS slot of the thread before calling any functions in the Perl API on that particular interpreter. This is done by calling thePERL_SET_CONTEXT macro in that thread as the first thing you do:
/* do this before doing anything else with some_perl */PERL_SET_CONTEXT(some_perl);... other Perl API calls on some_perl go here ...Just as PERL_IMPLICIT_CONTEXT provides a way to bundle up everything that the interpreter knows about itself and pass it around, so too are there plans to allow the interpreter to bundle up everything it knows about the environment it's running on. This is enabled with the PERL_IMPLICIT_SYS macro. Currently it only works with USE_ITHREADS on Windows.
This allows the ability to provide an extra pointer (called the "host" 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 (seewin32/perllib.c) for the default perl executable, but for a more ambitious host (like the one that would do fork() emulation) all the extra work needed to pretend that different interpreters are actually different "processes", would be done here.
The Perl engine/interpreter and the host are orthogonal entities. There could be one or more interpreters in a process, and one or more "hosts", with free association between them.
All of Perl's internal functions which will be exposed to the outside world are prefixed byPerl_ 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 withPL_. (By convention, static functions start withS_.)
Inside the Perl core (PERL_CORE defined), you can get at the functions either with or without thePerl_ prefix, thanks to a bunch of defines that live inembed.h. Note that extension code shouldnot setPERL_CORE; this exposes the full perl internals, and is likely to cause breakage of the XS in each new perl release.
The fileembed.h is generated automatically fromembed.pl andembed.fnc.embed.pl also creates the prototyping header files for the internal functions, generates the documentation and a lot of other bits and pieces. It's important that when you add a new function to the core or change an existing one, you change the data in the table inembed.fnc as well. Here's a sample entry from that table:
Apd |SV** |av_fetch |AV* ar|I32 key|I32 lvalThe 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:
This function is a part of the public API. All such functions should also have 'd', very few do not.
This function has aPerl_ prefix; i.e. it is defined asPerl_av_fetch.
This function has documentation using theapidoc feature which we'll look at in a second. Some functions have 'd' but not 'A'; docs are good.
Other available flags are:
This is a static function and is defined asSTATIC S_whatever, and usually called within the sources aswhatever(...).
This does not need an interpreter context, so the definition has nopTHX, and it follows that callers don't useaTHX. (See"Background and PERL_IMPLICIT_CONTEXT".)
This function never returns;croak,exit and friends.
This function takes a variable number of arguments,printf style. The argument list should end with..., like this:
Afprd |void |croak |const char* pat|...This function is part of the experimental development API, and may change or disappear without notice.
This function should not have a compatibility macro to define, say,Perl_parse toparse. It must be called asPerl_parse.
This function isn't exported out of the Perl core.
This is implemented as a macro.
This function is explicitly exported.
This function is visible to extensions included in the Perl core.
Binary backward compatibility; this function is a macro but also has aPerl_ implementation (which is exported).
See the comments at the top ofembed.fnc for others.
If you editembed.pl orembed.fnc, you will need to runmake regen_headers to force a rebuild ofembed.h and other auto-generated files.
If you are printing IVs, UVs, or NVS instead of the stdio(3) style formatting codes like%d,%ld,%f, you should use the following macros for portability
IVdf IV in decimalUVuf UV in decimalUVof UV in octalUVxf UV in hexadecimalNVef NV %e-likeNVff NV %f-likeNVgf NV %g-likeThese will take care of 64-bit integers and long doubles. For example:
printf("IV is %"IVdf"\n", iv);The IVdf will expand to whatever is the correct format for the IVs.
Note that there are different "long doubles": Perl will use whatever the compiler has.
If you are printing addresses of pointers, use UVxf combined with PTR2UV(), do not use %lx or %p.
Size_t andSSize_tThe most general way to do this is to cast them to a UV or IV, and print as in theprevious section.
But if you're usingPerlIO_printf(), it's less typing and visual clutter to use the"%z" length modifier (forsiZe):
PerlIO_printf("STRLEN is %zu\n", len);This modifier is not portable, so its use should be restricted toPerlIO_printf().
Because pointer size does not necessarily equal integer size, use the follow macros to do it right.
PTR2UV(pointer)PTR2IV(pointer)PTR2NV(pointer)INT2PTR(pointertotype, integer)For example:
IV iv = ...;SV *sv = INT2PTR(SV*, iv);and
AV *av = ...;UV uv = PTR2UV(av);There are a couple of macros to do very basic exception handling in XS modules. You have to defineNO_XSLOCKS before includingXSUB.h to be able to use these macros:
#define NO_XSLOCKS#include "XSUB.h"You can use these macros if you call code that may croak, but you need to do some cleanup before giving control back to Perl. For example:
dXCPT; /* set up necessary variables */XCPT_TRY_START { code_that_may_croak();} XCPT_TRY_ENDXCPT_CATCH{ /* do cleanup here */ XCPT_RETHROW;}Note that you always have to rethrow an exception that has been caught. Using these macros, it is not possible to just catch the exception and ignore it. If you have to ignore the exception, you have to use thecall_* function.
The advantage of using the above macros is that you don't have to setup an extra function forcall_*, and that using these macros is faster than usingcall_*.
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 XS writers.perlintern is the autogenerated manual for the functions which are not part of the API and are supposedly for internal use only.
Source documentation is created by putting POD comments into the C source, like this:
/*=for apidoc sv_setivCopies an integer into the given SV. Does not handle 'set' magic. SeeL<perlapi/sv_setiv_mg>.=cut*/Please try and supply some documentation if you add functions to the Perl core.
The Perl API changes over time. New functions are added or the interfaces of existing functions are changed. TheDevel::PPPort module tries to provide compatibility code for some of these changes, so XS writers don't have to code it themselves when supporting multiple versions of Perl.
Devel::PPPort generates a C header fileppport.h that can also be run as a Perl script. To generateppport.h, run:
perl -MDevel::PPPort -eDevel::PPPort::WriteFileBesides checking existing XS code, the script can also be used to retrieve compatibility information for various API calls using the--api-info command line switch. For example:
% perl ppport.h --api-info=sv_magicextFor details, seeperldoc ppport.h.
Perl 5.6.0 introduced Unicode support. It's important for porters and XS writers to understand this support and make sure that the code they write does not corrupt Unicode data.
In the olden, less enlightened times, we all used to use ASCII. Most of us did, anyway. The big problem with ASCII 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 ASCII, and the whole point of it being a standard was lost.
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 ASCII altogether, and build their own systems using pairs of numbers to refer 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 UTF-8. UTF-8 uses a variable number of bytes to represent a character. You can learn more about Unicode and Perl's Unicode model inperlunicode.
(On EBCDIC platforms, Perl uses instead UTF-EBCDIC, which is a form of UTF-8 adapted for EBCDIC platforms. Below, we just talk about UTF-8. UTF-EBCDIC is like UTF-8, but the details are different. The macros hide the differences from you, just remember that the particular numbers and bit patterns presented below will differ in UTF-EBCDIC.)
You can't. This is because UTF-8 data is stored in bytes just like non-UTF-8 data. The Unicode character 200, (0xC8 for you hex types) capital E with a grave accent, is represented by the two bytesv196.172. Unfortunately, the non-Unicode stringchr(196).chr(172) has that byte sequence as well. So you can't tell just by looking -- this is what makes Unicode input an interesting problem.
In general, you either have to know what you're dealing with, or you have to guess. The API functionis_utf8_string can help; it'll tell you if a string contains only valid UTF-8 characters, and the chances of a non-UTF-8 string looking like valid UTF-8 become very small very quickly with increasing string length. On a character-by-character basis,isUTF8_CHAR will tell you whether the current character in a string is valid UTF-8.
As mentioned above, UTF-8 uses a variable number of bytes to store a character. Characters with values 0...127 are stored in one byte, just like good ol' ASCII. Character 128 is stored asv194.128; this continues up to character 191, which isv194.191. Now we've run out of bits (191 is binary10111111) so we move on; character 192 isv195.128. And so it goes on, moving to three bytes at character 2048."Unicode Encodings" in perlunicode has pictures of how this works.
Assuming you know you're dealing with a UTF-8 string, you can find out how long the first character in it is with theUTF8SKIP macro:
char *utf = "\305\233\340\240\201";I32 len;len = UTF8SKIP(utf); /* len is 2 here */utf += len;len = UTF8SKIP(utf); /* len is 3 here */Another way to skip over characters in a UTF-8 string is to useutf8_hop, which takes a string and a number of characters to skip over. You're on your own about bounds checking, though, so don't use it lightly.
All bytes in a multi-byte 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 (theUTF8_IS_INVARIANT() is a macro that tests whether the byte is encoded as a single byte even in UTF-8):
U8 *utf; /* Initialize this to point to the beginning of the sequence to convert */U8 *utf_end; /* Initialize this to 1 beyond the end of the sequence pointed to by 'utf' */UV uv; /* Returned code point; note: a UV, not a U8, not a char */STRLEN len; /* Returned length of character in bytes */if (!UTF8_IS_INVARIANT(*utf)) /* Must treat this as UTF-8 */ uv = utf8_to_uvchr_buf(utf, utf_end, &len);else /* OK to treat this character as a byte */ uv = *utf;You can also see in that example that we useutf8_to_uvchr_buf to get the value of the character; the inverse functionuvchr_to_utf8 is available for putting a UV into UTF-8:
if (!UVCHR_IS_INVARIANT(uv)) /* Must treat this as UTF8 */ utf8 = uvchr_to_utf8(utf8, uv);else /* OK to treat this character as a byte */ *utf8++ = uv;Youmust convert characters to UVs using the above functions if 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 containsv196.172, and you skip that character, you can never match achr(200) in a non-UTF-8 string. So don't do that!
(Note that we don't have to test for invariant characters in the examples above. The functions work on any well-formed UTF-8 input. It's just that its faster to avoid the function overhead when it's not needed.)
Currently, Perl deals with UTF-8 strings and non-UTF-8 strings slightly differently. A flag in the SV,SVf_UTF8, indicates that the string is internally encoded as UTF-8. Without it, the byte value is the codepoint number and vice versa. This flag is only meaningful if the SV isSvPOK or immediately after stringification viaSvPV or a similar macro. You can check and manipulate this flag with the following macros:
SvUTF8(sv)SvUTF8_on(sv)SvUTF8_off(sv)This flag has an important effect on Perl's treatment of the string: if UTF-8 data is not properly distinguished, regular expressions,length,substr and other string handling operations will have undesirable (wrong) results.
The problem comes when you have, for instance, a string that isn't flagged as 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 theSVf_UTF8 flag is separate from the PV value; you need to be sure you don't accidentally knock it off while you're manipulating SVs. More specifically, you cannot expect to do this:
SV *sv;SV *nsv;STRLEN len;char *p;p = SvPV(sv, len);frobnicate(p);nsv = newSVpvn(p, len);Thechar* 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 (after theSvPV call), and act accordingly:
p = SvPV(sv, len);is_utf8 = SvUTF8(sv);frobnicate(p, is_utf8);nsv = newSVpvn(p, len);if (is_utf8) SvUTF8_on(nsv);In the above, yourfrobnicate function has been changed to be made aware of whether or 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 passing achar * to an XS function.
For full generality, use theDO_UTF8 macro to see if the string in an SV is to betreated as UTF-8. This takes into account if the call to the XS function is being made from within the scope ofuse bytes. If so, the underlying bytes that comprise the UTF-8 string are to be exposed, rather than the character they represent. But this pragma should only really be used for debugging and perhaps low-level testing at the byte level. Hence most XS code need not concern itself with this, but various areas of the perl core do need to support it.
And this isn't the whole story. Starting in Perl v5.12, strings that aren't encoded in UTF-8 may also be treated as Unicode under various conditions (see"ASCII Rules versus Unicode Rules" in perlunicode). This is only really a problem for characters whose ordinals are between 128 and 255, and their behavior varies under ASCII versus Unicode rules in ways that your code cares about (see"The "Unicode Bug"" in perlunicode). There is no published API for dealing with this, as it is subject to change, but you can look at the code forpp_lc inpp.c for an example as to how it's currently done.
If you're mixing UTF-8 and non-UTF-8 strings, it is necessary to upgrade the non-UTF-8 strings to UTF-8. If you've got an SV, the easiest way to do this is:
sv_utf8_upgrade(sv);However, you must not do this, for example:
if (!SvUTF8(left)) sv_utf8_upgrade(left);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 in deficient code.
Instead,bytes_to_utf8 will give you a UTF-8-encodedcopy of its string argument. This is useful for having the data available for comparisons and so on, without harming the original SV. There's alsoutf8_to_bytes 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.
"sv_cmp" in perlapi and"sv_cmp_flags" in perlapi do a lexigraphic comparison of two SV's, and handle UTF-8ness properly. Note, however, that Unicode specifies a much fancier mechanism for collation, available via theUnicode::Collate module.
To just compare two strings for equality/non-equality, you can just usememEQ() andmemNE() as usual, except the strings must be both UTF-8 or not UTF-8 encoded.
To compare two strings case-insensitively, usefoldEQ_utf8() (the strings don't have to have the same UTF-8ness).
Not really. Just remember these things:
There's no way to tell if achar * orU8 * string is UTF-8 or not. But you can tell if an SV is to be treated as UTF-8 by callingDO_UTF8 on it, after stringifying it withSvPV or a similar macro. And, you can tell if SV is actually UTF-8 (even if it is not to be treated as such) by looking at itsSvUTF8 flag (again after stringifying it). 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.
If a string is UTF-8,always useutf8_to_uvchr_buf to get at the value, unlessUTF8_IS_INVARIANT(*s) in which case you can use*s.
When writing a character UV to a UTF-8 string,always useuvchr_to_utf8, unlessUVCHR_IS_INVARIANT(uv)) in which case you can use*s = uv.
Mixing UTF-8 and non-UTF-8 strings is tricky. Usebytes_to_utf8 to get a new string which is UTF-8 encoded, and then combine them.
Custom operator support is an 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 "macro-ops" (ops which perform the functions of multiple ops which are usually executed together, such asgvsv, gvsv, add.)
This feature is implemented as a new op type,OP_CUSTOM. The Perl core does not "know" 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.
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 aCHECK block and theB::Generate module, or by adding a custom peephole optimizer with theoptimize module.
When you do this, you replace ordinary Perl ops with custom ops by creating ops with the typeOP_CUSTOM and theop_ppaddr of your own PP function. This should be defined in XS code, and should look like the PP ops inpp_*.c. 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.
You should also "register" 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 "logical" op typeOP_CUSTOM, Perl uses the value ofo->op_ppaddr to determine which custom op it is dealing with. You should create anXOP structure for each ppaddr you use, set the properties of the custom op withXopENTRY_set, and register the structure against the ppaddr usingPerl_custom_op_register. A trivial example might look like:
static XOP my_xop;static OP *my_pp(pTHX);BOOT: XopENTRY_set(&my_xop, xop_name, "myxop"); XopENTRY_set(&my_xop, xop_desc, "Useless custom op"); Perl_custom_op_register(aTHX_ my_pp, &my_xop);The available fields in the structure are:
A short name for your op. This will be included in some error messages, and will also be returned as$op->name by theB module, so it will appear in the output of module likeB::Concise.
A short description of the function of the op.
Which of the various*OP structures this op uses. This should be one of theOA_* constants fromop.h, namely
This should be interpreted as 'PVOP' only. The_OR_SVOP is because the only corePVOP,OP_TRANS, can sometimes be aSVOP instead.
The otherOA_* constants should not be used.
This member is of typePerl_cpeep_t, which expands tovoid (*Perl_cpeep_t)(aTHX_ OP *o, OP *oldop). If it is set, this function will be called fromPerl_rpeep when ops of this type are encountered by the peephole optimizer.o is the OP that needs optimizing;oldop is the previous OP optimized, whoseop_next points too.
B::Generate directly supports the creation of custom ops by name.
Note: this section describes a non-public internal API that is subject to change without notice.
In Perl, dynamic scoping refers to the runtime nesting of things like subroutine calls, evals etc, as well as the entering and exiting of block scopes. For example, the restoring of alocalised variable is determined by the dynamic scope.
Perl tracks the dynamic scope by a data structure called the context stack, which is an array ofPERL_CONTEXT structures, and which is itself a big union for all the types of context. Whenever a new scope is entered (such as a block, afor loop, or a subroutine call), a new context entry is pushed onto the stack. Similarly when leaving a block or returning from a subroutine call etc. a context is popped. Since the context stack represents the current dynamic scope, it can be searched. For example,next LABEL searches back through the stack looking for a loop context that matches the label;return pops contexts until it finds a sub or eval context or similar;caller examines sub contexts on the stack.
Each context entry is labelled with a context type,cx_type. Typical context types areCXt_SUB,CXt_EVAL etc., as well asCXt_BLOCK andCXt_NULL which represent a basic scope (as pushed bypp_enter) and a sort block. The type determines which part of the context union are valid.
The main division in the context struct is between a substitution scope (CXt_SUBST) and block scopes, which are everything else. The former is just used while executings///e, and won't be discussed further here.
All the block scope types share a common base, which corresponds toCXt_BLOCK. This stores the old values of various scope-related variables likePL_curpm, as well as information about the current scope, such asgimme. On scope exit, the old variables are restored.
Particular block scope types store extra per-type information. For example,CXt_SUB stores the currently executing CV, while the various for loop types might hold the original loop variable SV. On scope exit, the per-type data is processed; for example the CV has its reference count decremented, and the original loop variable is restored.
The macrocxstack returns the base of the current context stack, whilecxstack_ix is the index of the current frame within that stack.
In fact, the context stack is actually part of a stack-of-stacks system; whenever something unusual is done such as calling aDESTROY or tie handler, a new stack is pushed, then popped at the end.
Note that the API described here changed considerably in perl 5.24; prior to that, big macros likePUSHBLOCK andPOPSUB were used; in 5.24 they were replaced by the inline static functions described below. In addition, the ordering and detail of how these macros/function work changed in many ways, often subtly. In particular they didn't handle saving the savestack and temps stack positions, and required additionalENTER,SAVETMPS andLEAVE compared to the new functions. The old-style macros will not be described further.
For pushing a new context, the two basic functions arecx = cx_pushblock(), which pushes a new basic context block and returns its address, and a family of similar functions with names likecx_pushsub(cx) which populate the additional type-dependent fields in thecx struct. Note thatCXt_NULL andCXt_BLOCK don't have their own push functions, as they don't store any data beyond that pushed bycx_pushblock.
The fields of the context struct and the arguments to thecx_* functions are subject to change between perl releases, representing whatever is convenient or efficient for that release.
A typical context stack pushing can be found inpp_entersub; the following shows a simplified and stripped-down example of a non-XS call, along with comments showing roughly what each function does.
dMARK;U8 gimme = GIMME_V;bool hasargs = cBOOL(PL_op->op_flags & OPf_STACKED);OP *retop = PL_op->op_next;I32 old_ss_ix = PL_savestack_ix;CV *cv = ....;/* ... make mortal copies of stack args which are PADTMPs here ... *//* ... do any additional savestack pushes here ... *//* Now push a new context entry of type 'CXt_SUB'; initially just * doing the actions common to all block types: */cx = cx_pushblock(CXt_SUB, gimme, MARK, old_ss_ix); /* this does (approximately): CXINC; /* cxstack_ix++ (grow if necessary) */ cx = CX_CUR(); /* and get the address of new frame */ cx->cx_type = CXt_SUB; cx->blk_gimme = gimme; cx->blk_oldsp = MARK - PL_stack_base; cx->blk_oldsaveix = old_ss_ix; cx->blk_oldcop = PL_curcop; cx->blk_oldmarksp = PL_markstack_ptr - PL_markstack; cx->blk_oldscopesp = PL_scopestack_ix; cx->blk_oldpm = PL_curpm; cx->blk_old_tmpsfloor = PL_tmps_floor; PL_tmps_floor = PL_tmps_ix; *//* then update the new context frame with subroutine-specific info, * such as the CV about to be executed: */cx_pushsub(cx, cv, retop, hasargs); /* this does (approximately): cx->blk_sub.cv = cv; cx->blk_sub.olddepth = CvDEPTH(cv); cx->blk_sub.prevcomppad = PL_comppad; cx->cx_type |= (hasargs) ? CXp_HASARGS : 0; cx->blk_sub.retop = retop; SvREFCNT_inc_simple_void_NN(cv); */Note thatcx_pushblock() sets two new floors: for the args stack (toMARK) and the temps stack (toPL_tmps_ix). While executing at this scope level, everynextstate (amongst others) will reset the args and tmps stack levels to these floors. Note that sincecx_pushblock uses the current value ofPL_tmps_ix rather than it being passed as an arg, this dictates at what pointcx_pushblock should be called. In particular, any new mortals which should be freed only on scope exit (rather than at the nextnextstate) should be created first.
Most callers ofcx_pushblock simply set the new args stack floor to the top of the previous stack frame, but forCXt_LOOP_LIST it stores the items being iterated over on the stack, and so setsblk_oldsp to the top of these items instead. Note that, contrary to its name,blk_oldsp doesn't always represent the value to restorePL_stack_sp to on scope exit.
Note the early capture ofPL_savestack_ix toold_ss_ix, which is later passed as an arg tocx_pushblock. In the case ofpp_entersub, this is because, although most values needing saving are stored in fields of the context struct, an extra value needs saving only when the debugger is running, and it doesn't make sense to bloat the struct for this rare case. So instead it is saved on the savestack. Since this value gets calculated and saved before the context is pushed, it is necessary to pass the old value ofPL_savestack_ix tocx_pushblock, to ensure that the saved value gets freed during scope exit. For most users ofcx_pushblock, where nothing needs pushing on the save stack,PL_savestack_ix is just passed directly as an arg tocx_pushblock.
Note that where possible, values should be saved in the context struct rather than on the save stack; it's much faster that way.
Normallycx_pushblock should be immediately followed by the appropriatecx_pushfoo, with nothing between them; this is because if code in-between could die (e.g. a warning upgraded to fatal), then the context stack unwinding code indounwind would see (in the example above) aCXt_SUB context frame, but without all the subroutine-specific fields set, and crashes would soon ensue.
Where the two must be separate, initially set the type toCXt_NULL orCXt_BLOCK, and later change it toCXt_foo when doing thecx_pushfoo. This is exactly whatpp_enteriter does, once it's determined which type of loop it's pushing.
Contexts are popped usingcx_popsub() etc. andcx_popblock(). Note however, that unlikecx_pushblock, neither of these functions actually decrement the current context stack index; this is done separately usingCX_POP().
There are two main ways that contexts are popped. During normal execution as scopes are exited, functions likepp_leave,pp_leaveloop andpp_leavesub process and pop just one context usingcx_popfoo andcx_popblock. On the other hand, things likepp_return andnext may have to pop back several scopes until a sub or loop context is found, and exceptions (such asdie) need to pop back contexts until an eval context is found. Both of these are accomplished bydounwind(), which is capable of processing and popping all contexts above the target one.
Here is a typical example of context popping, as found inpp_leavesub (simplified slightly):
U8 gimme;PERL_CONTEXT *cx;SV **oldsp;OP *retop;cx = CX_CUR();gimme = cx->blk_gimme;oldsp = PL_stack_base + cx->blk_oldsp; /* last arg of previous frame */if (gimme == G_VOID) PL_stack_sp = oldsp;else leave_adjust_stacks(oldsp, oldsp, gimme, 0);CX_LEAVE_SCOPE(cx);cx_popsub(cx);cx_popblock(cx);retop = cx->blk_sub.retop;CX_POP(cx);return retop;The steps above are in a very specific order, designed to be the reverse order of when the context was pushed. The first thing to do is to copy and/or protect any any return arguments and free any temps in the current scope. Scope exits like an rvalue sub normally return a mortal copy of their return args (as opposed to lvalue subs). It is important to make this copy before the save stack is popped or variables are restored, or bad things like the following can happen:
sub f { my $x =...; $x } # $x freed before we get to copy itsub f { /(...)/; $1 } # PL_curpm restored before $1 copiedAlthough we wish to free any temps at the same time, we have to be careful not to free any temps which are keeping return args alive; nor to free the temps we have just created while mortal copying return args. Fortunately,leave_adjust_stacks() is capable of making mortal copies of return args, shifting args down the stack, and only processing those entries on the temps stack that are safe to do so.
In void context no args are returned, so it's more efficient to skip callingleave_adjust_stacks(). Also in void context, anextstate op is likely to be imminently called which will do aFREETMPS, so there's no need to do that either.
The next step is to pop savestack entries:CX_LEAVE_SCOPE(cx) is just defined as<LEAVE_SCOPE(cx-blk_oldsaveix)>>. Note that during the popping, it's possible for perl to call destructors, callSTORE to undo localisations of tied vars, and so on. Any of these can die or callexit(). In this case,dounwind() will be called, and the current context stack frame will be re-processed. Thus it is vital that all steps in popping a context are done in such a way to support reentrancy. The other alternative, of decrementingcxstack_ixbefore processing the frame, would lead to leaks and the like if something died halfway through, or overwriting of the current frame.
CX_LEAVE_SCOPE itself is safely re-entrant: if only half the savestack items have been popped before dying and getting trapped by eval, then theCX_LEAVE_SCOPEs indounwind orpp_leaveeval will continue where the first one left off.
The next step is the type-specific context processing; in this casecx_popsub. In part, this looks like:
cv = cx->blk_sub.cv;CvDEPTH(cv) = cx->blk_sub.olddepth;cx->blk_sub.cv = NULL;SvREFCNT_dec(cv);where its processing the just-executed CV. Note that before it decrements the CV's reference count, it nulls theblk_sub.cv. This means that if it re-enters, the CV won't be freed twice. It also means that you can't rely on such type-specific fields having useful values after the return fromcx_popfoo.
Next,cx_popblock restores all the various interpreter vars to their previous values or previous high water marks; it expands to:
PL_markstack_ptr = PL_markstack + cx->blk_oldmarksp;PL_scopestack_ix = cx->blk_oldscopesp;PL_curpm = cx->blk_oldpm;PL_curcop = cx->blk_oldcop;PL_tmps_floor = cx->blk_old_tmpsfloor;Note that itdoesn't restorePL_stack_sp; as mentioned earlier, which value to restore it to depends on the context type (specificallyfor (list) {}), and what args (if any) it returns; and that will already have been sorted out earlier byleave_adjust_stacks().
Finally, the context stack pointer is actually decremented byCX_POP(cx). After this point, it's possible that that the current context frame could be overwritten by other contexts being pushed. Although things like ties andDESTROY are supposed to work within a new context stack, it's best not to assume this. Indeed on debugging builds,CX_POP(cx) deliberately setscx to null to detect code that is still relying on the field values in that context frame. Note in thepp_leavesub() example above, we grabblk_sub.retopbefore callingCX_POP.
Finally, there iscx_topblock(cx), which acts like a super-nextstate as regards to resetting various vars to their base values. It is used in places likepp_next,pp_redo andpp_goto where rather than exiting a scope, we want to re-initialise the scope. As well as resettingPL_stack_sp likenextstate, it also resetsPL_markstack_ptr,PL_scopestack_ix andPL_curpm. Note that it doesn't do aFREETMPS.
Until May 1997, this document was maintained by Jeff Okamoto <okamoto@corp.hp.com>. It is now maintained as part of Perl itself by the Perl 5 Porters <perl5-porters@perl.org>.
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.
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