Skunkware 5/Skunkware 5.iso/man/cat.1/perlguts.1

Text File | 1995-07-25 | 28KB | 727 lines

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     NNNNAAAAMMMMEEEE
          perlguts - Perl's Internal Functions

     DDDDEEEESSSSCCCCRRRRIIIIPPPPTTTTIIIIOOOONNNN
          This document attempts to describe some of the internal
          functions of the Perl executable.  It is far from complete
          and probably contains many errors.  Please refer any
          questions or comments to the author below.

     DDDDaaaattttaaaattttyyyyppppeeeessss
          Perl has three typedefs that handle Perl's three main data
          types:

              SV  Scalar Value
              AV  Array Value
              HV  Hash Value

          Each typedef has specific routines that manipulate the
          various data type.

          WWWWhhhhaaaatttt iiiissss aaaannnn

          Perl uses a special typedef IV which is large enough to hold
          either an integer or a pointer.

          Perl also uses a special typedef I32 which will always be a
          32-bit integer.

          WWWWoooorrrrkkkkiiiinnnngggg wwwwiiiitttthhhh SSSSVVVV''''ssss

          An SV can be created and loaded with one command.  There are
          four types of values that can be loaded: an integer value
          (IV), a double (NV), a string, (PV), and another scalar
          (SV).

          The four routines are:

              SV*  newSViv(IV);
              SV*  newSVnv(double);
              SV*  newSVpv(char*, int);
              SV*  newSVsv(SV*);

          To change the value of an *already-existing* scalar, there
          are five routines:

              void  sv_setiv(SV*, IV);
              void  sv_setnv(SV*, double);
              void  sv_setpvn(SV*, char*, int)
              void  sv_setpv(SV*, char*);
              void  sv_setsv(SV*, SV*);

          Notice that you can choose to specify the length of the



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          string to be assigned by using sv_setpvn, or allow Perl to
          calculate the length by using sv_setpv.  Be warned, though,
          that sv_setpv determines the string's length by using
          strlen, which depends on the string terminating with a NUL
          character.

          To access the actual value that an SV points to, you can use
          the macros:

              SvIV(SV*)
              SvNV(SV*)
              SvPV(SV*, STRLEN len)

          which will automatically coerce the actual scalar type into
          an IV, double, or string.

          In the SvPV macro, the length of the string returned is
          placed into the variable len (this is a macro, so you do _n_o_t
          use &len).  If you do not care what the length of the data
          is, use the global variable na.  Remember, however, that
          Perl allows arbitrary strings of data that may both contain
          NUL's and not be terminated by a NUL.

          If you simply 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 function sv_grow.  Note that SvGROW
          can only increase, not decrease, the allocated memory of an
          SV.

          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)



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          But note that these are valid only if SvPOK() is true.

          If you know the name of a scalar variable, you can get a
          pointer to its SV by using the following:

              SV*  perl_get_sv("varname", FALSE);

          This returns NULL if the variable does not exist.

          If you want to know if this variable (or any other SV) is
          actually defined, you can call:

              SvOK(SV*)

          The scalar undef value is stored in an SV instance called
          sv_undef.  Its address can be used whenever an SV* is
          needed.

          There are also the two values sv_yes and sv_no, which
          contain Boolean TRUE and FALSE values, respectively.  Like
          sv_undef, their addresses can be used whenever an SV* is
          needed.

          Do not be fooled into thinking that (SV *) 0 is the same as
          &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, or just weird
          results.  Change the zero to &sv_undef in the first line and
          all will be well.

          To free an SV that you've created, call SvREFCNT_dec(SV*).
          Normally this call is not necessary.  See the section on
          MMMMOOOORRRRTTTTAAAALLLLIIIITTTTYYYY.

          PPPPrrrriiiivvvvaaaatttteeee aaaannnndddd PPPPuuuubbbblllliiiicccc VVVVaaaalllluuuueeeessss

          Recall that the usual method of determining the type of
          scalar you have is to use Sv[INP]OK macros.  Since a scalar
          can be both a number and a string, usually these macros will
          always return TRUE and calling the Sv[INP]V macros will do
          the appropriate conversion of string to integer/double or
          integer/double to string.




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          If you _r_e_a_l_l_y 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.

          In general, though, it's best to just use the Sv[INP]V
          macros.

          WWWWoooorrrrkkkkiiiinnnngggg wwwwiiiitttthhhh AAAAVVVV''''ssss

          There are two ways to create and load an AV.  The first
          method just creates an empty AV:

              AV*  newAV();

          The second method both creates the AV and initially
          populates it with SV's:

              AV*  av_make(I32 num, SV **ptr);

          The second argument points to an array containing num SV*'s.

          Once the AV has been created, the following operations are
          possible on AV's:

              void  av_push(AV*, SV*);
              SV*   av_pop(AV*);
              SV*   av_shift(AV*);
              void  av_unshift(AV*, I32 num);

          These should be familiar operations, with the exception of
          av_unshift.  This routine adds num elements at the front of
          the array with the undef value.  You must then use av_store
          (described below) to assign values to these new elements.

          Here are some other functions:

              I32   av_len(AV*); /* Returns length of array */

              SV**  av_fetch(AV*, I32 key, I32 lval);
                      /* Fetches value at key offset, but it seems to
                         set the value to lval if lval is non-zero */
              SV**  av_store(AV*, I32 key, SV* val);
                      /* Stores val at offset key */





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              void  av_clear(AV*);
                      /* Clear out all elements, but leave the array */
              void  av_undef(AV*);
                      /* Undefines the array, removing all elements */

          If you know the name of an array variable, you can get a
          pointer to its AV by using the following:

              AV*  perl_get_av("varname", FALSE);

          This returns NULL if the variable does not exist.

          WWWWoooorrrrkkkkiiiinnnngggg wwwwiiiitttthhhh HHHHVVVV''''ssss

          To create an HV, you use the following routine:

              HV*  newHV();

          Once the HV has been created, the following operations are
          possible on HV's:

              SV**  hv_store(HV*, char* key, U32 klen, SV* val, U32 hash);
              SV**  hv_fetch(HV*, char* key, U32 klen, I32 lval);

          The klen parameter is the length of the key being passed in.
          The val argument contains the SV pointer to the scalar being
          stored, and hash is the pre-computed hash value (zero if you
          want hv_store to calculate it for you).  The lval parameter
          indicates whether this fetch is actually a part of a store
          operation.

          Remember that hv_store and hv_fetch return SV**'s and not
          just SV*.  In order 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.

          These two functions check if a hash table entry exists, and
          deletes it.

              bool  hv_exists(HV*, char* key, U32 klen);
              SV*   hv_delete(HV*, char* key, U32 klen);

          And more miscellaneous functions:

              void   hv_clear(HV*);
                      /* Clears all entries in hash table */
              void   hv_undef(HV*);
                      /* Undefines the hash table */






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              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 a SV pointer to the value of the HE
                         structure */

          If you know the name of a hash variable, you can get a
          pointer to its HV by using the following:

              HV*  perl_get_hv("varname", FALSE);

          This returns NULL if the variable does not exist.

          The hash algorithm, for those who are interested, is:

              i = klen;
              hash = 0;
              s = key;
              while (i--)
                  hash = hash * 33 + *s++;


          RRRReeeeffffeeeerrrreeeennnncccceeeessss

          References are a special type of scalar that point to other
          scalar types (including references).  To treat an AV or HV
          as a scalar, it is simply a matter of casting an AV or HV to
          an SV.

          To create a reference, use the following command:

              SV*  newRV((SV*) pointer);

          Once you have a reference, you can use the following macro
          with a cast to the appropriate typedef (SV, AV, HV):

              SvRV(SV*)

          then call the appropriate routines, casting the returned SV*
          to either an AV* or HV*.

          To determine, after dereferencing a reference, if you still
          have a reference, you can use the following macro:

              SvROK(SV*)




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     XXXXSSSSUUUUBBBB''''SSSS aaaannnndddd tttthhhheeee AAAArrrrgggguuuummmmeeeennnntttt SSSSttttaaaacccckkkk
          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 the ST(n) macro,
          which returns the n'th stack argument.  Argument 0 is the
          first argument passed in the Perl subroutine call.  These
          arguments are SV*, and can be used anywhere an SV* 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 _t_z_n_a_m_e() call, which takes no arguments, but
          returns two, the local timezone'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);

          where sp is the stack pointer, and num is the number of
          elements the stack should be extended by.

          Now that there is room on the stack, values can be pushed on
          it using the macros to push IV's, doubles, strings, and SV
          pointers respectively:

              PUSHi(IV)
              PUSHn(double)
              PUSHp(char*, I32)
              PUSHs(SV*)

          And now the Perl program calling tzname, 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 macros:

              XPUSHi(IV)
              XPUSHn(double)
              XPUSHp(char*, I32)
              XPUSHs(SV*)

          These macros automatically adjust the stack for you, if
          needed.




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     MMMMoooorrrrttttaaaalllliiiittttyyyy
          In Perl, values are normally "immortal" -- that is, they are
          not freed unless explicitly done so (via the Perl undef call
          or other routines in Perl itself).

          In the above example with tzname, we needed to create two
          new SV's to push onto the argument stack, that being the two
          strings.  However, we don't want these new SV's to stick
          around forever because they will eventually be copied into
          the SV's that hold the two scalar variables.

          An SV (or AV or HV) that is "mortal" acts in all ways as a
          normal "immortal" SV, AV, or HV, but is only valid in the
          "current context".  When the Perl interpreter leaves the
          current context, the mortal SV, AV, or HV is automatically
          freed.  Generally the "current context" means a single Perl
          statement.

          To create a mortal variable, use the functions:

              SV*  sv_newmortal()
              SV*  sv_2mortal(SV*)
              SV*  sv_mortalcopy(SV*)

          The first call creates a mortal SV, the second converts an
          existing SV to a mortal SV, the third creates a mortal copy
          of an existing SV.

          The mortal routines are not just for SV's -- AV's and HV's
          can be made mortal by passing their address (and casting
          them to SV*) to the sv_2mortal or sv_mortalcopy routines.

     CCCCrrrreeeeaaaattttiiiinnnngggg NNNNeeeewwww VVVVaaaarrrriiiiaaaabbbblllleeeessss
          To create a new Perl variable, which can be accessed from
          your Perl script, use the following routines, depending on
          the variable type.

              SV*  perl_get_sv("varname", TRUE);
              AV*  perl_get_av("varname", TRUE);
              HV*  perl_get_hv("varname", TRUE);

          Notice the use of TRUE as the second parameter.  The new
          variable can now be set, using the routines appropriate to
          the data type.

     SSSSttttaaaasssshhhheeeessss aaaannnndddd OOOObbbbjjjjeeeeccccttttssss
          A stash is a hash table (associative array) that contains
          all of the different objects that are contained within a
          package.  Each key of the hash table 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 called a GV
          (for Glob Value).  The GV in turn contains references to the



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          various objects of that name, including (but not limited to)
          the following:

              Scalar Value
              Array Value
              Hash Value
              File Handle
              Directory Handle
              Format
              Subroutine

          Perl stores various stashes in a GV structure (for global
          variable) but represents them with an HV structure.

          To get the HV pointer for a particular package, use the
          function:

              HV*  gv_stashpv(char* name, I32 create)
              HV*  gv_stashsv(SV*, I32 create)

          The first function takes a literal string, the second uses
          the string stored in the SV.

          The name that gv_stash*v wants is the name of the package
          whose symbol table you want.  The default package is called
          main.  If you have multiply nested packages, it is legal to
          pass their names to gv_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 return a blessed value to your Perl script,
          you can use the following function:

              SV*  sv_bless(SV*, HV* stash)

          where the first argument, an SV*, must be a reference, and
          the second argument is a stash.  The returned SV* can now be
          used in the same way as any other SV.

     MMMMaaaaggggiiiicccc
          [This section under construction]

     DDDDoooouuuubbbblllleeee----TTTTyyyyppppeeeedddd SSSSVVVV''''ssss
          Scalar variables normally contain only one type of value, an



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          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 of errno or its string equivalent from
          sys_errlist[].

          To force multiple data values into an SV, you must do two
          things: use the sv_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_on
                  SvNOK_on
                  SvPOK_on
                  SvROK_on

          The particular macro you must use depends on which sv_set*v
          routine you called first.  This is because every sv_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 = perl_get_sv("dberror", TRUE);
              sv_setiv(sv, (IV) dberror);
              sv_setpv(sv, dberror_list[dberror]);
              SvIOK_on(sv);

          If the order of sv_setiv and sv_setpv had been reversed,
          then the macro SvPOK_on would need to be called instead of
          SvIOK_on.

     CCCCaaaalllllllliiiinnnngggg PPPPeeeerrrrllll RRRRoooouuuuttttiiiinnnneeeessss ffffrrrroooommmm wwwwiiiitttthhhhiiiinnnn CCCC PPPPrrrrooooggggrrrraaaammmmssss
          There are four routines that can be used to call a Perl
          subroutine from within a C program.  These four are:

              I32  perl_call_sv(SV*, I32);
              I32  perl_call_pv(char*, I32);
              I32  perl_call_method(char*, I32);
              I32  perl_call_argv(char*, I32, register char**);

          The routine most often used should be perl_call_sv.  The SV*
          argument contains either the name of the Perl subroutine to



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          be called, or a reference to the subroutine.  The second
          argument tells the appropriate routine what, if any,
          variables are being returned by the Perl subroutine.

          All four routines return the number of arguments that the
          subroutine returned on the Perl stack.

          When using these four routines, the programmer must
          manipulate the Perl stack.  These include the following
          macros and functions:

              dSP
              PUSHMARK()
              PUTBACK
              SPAGAIN
              ENTER
              SAVETMPS
              FREETMPS
              LEAVE
              XPUSH*()

          For more information, consult the _p_e_r_l_c_a_l_l manpage.

     MMMMeeeemmmmoooorrrryyyy AAAAllllllllooooccccaaaattttiiiioooonnnn
          [This section under construction]

     AAAAUUUUTTTTHHHHOOOORRRR
          Jeff Okamoto <[e-mail]>

          With lots of help and suggestions from Dean Roehrich,
          Malcolm Beattie, Andreas Koenig, Paul Hudson, Ilya
          Zakharevich, Paul Marquess, and Neil Bowers.

     DDDDAAAATTTTEEEE
          Version 12: 1994/10/16




















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