PERLIOL(1) Perl Programmers Reference Guide PERLIOL(1)

PERLIOL(1) Perl Programmers Reference Guide PERLIOL(1) #

PERLIOL(1) Perl Programmers Reference Guide PERLIOL(1)

NNAAMMEE #

 perliol - C API for Perl's implementation of IO in Layers.

SSYYNNOOPPSSIISS #

     /* Defining a layer ... */
     #include <perliol.h>

DDEESSCCRRIIPPTTIIOONN #

 This document describes the behavior and implementation of the PerlIO
 abstraction described in perlapio when "USE_PERLIO" is defined.

HHiissttoorryy aanndd BBaacckkggrroouunndd The PerlIO abstraction was introduced in perl5.003_02 but languished as just an abstraction until perl5.7.0. However during that time a number of perl extensions switched to using it, so the API is mostly fixed to maintain (source) compatibility.

 The aim of the implementation is to provide the PerlIO API in a flexible
 and platform neutral manner. It is also a trial of an "Object Oriented C,
 with vtables" approach which may be applied to Raku.

BBaassiicc SSttrruuccttuurree PerlIO is a stack of layers.

 The low levels of the stack work with the low-level operating system
 calls (file descriptors in C) getting bytes in and out, the higher layers
 of the stack buffer, filter, and otherwise manipulate the I/O, and return
 characters (or bytes) to Perl.  Terms _a_b_o_v_e and _b_e_l_o_w are used to refer
 to the relative positioning of the stack layers.

 A layer contains a "vtable", the table of I/O operations (at C level a
 table of function pointers), and status flags.  The functions in the
 vtable implement operations like "open", "read", and "write".

 When I/O, for example "read", is requested, the request goes from Perl
 first down the stack using "read" functions of each layer, then at the
 bottom the input is requested from the operating system services, then
 the result is returned up the stack, finally being interpreted as Perl
 data.

 The requests do not necessarily go always all the way down to the
 operating system: that's where PerlIO buffering comes into play.

 When you do an ooppeenn(()) and specify extra PerlIO layers to be deployed, the
 layers you specify are "pushed" on top of the already existing default
 stack.  One way to see it is that "operating system is on the left" and
 "Perl is on the right".

 What exact layers are in this default stack depends on a lot of things:
 your operating system, Perl version, Perl compile time configuration, and
 Perl runtime configuration.  See PerlIO, "PERLIO" in perlrun, and open
 for more information.

 bbiinnmmooddee(()) operates similarly to ooppeenn(()): by default the specified layers
 are pushed on top of the existing stack.

 However, note that even as the specified layers are "pushed on top" for
 ooppeenn(()) and bbiinnmmooddee(()), this doesn't mean that the effects are limited to
 the "top": PerlIO layers can be very 'active' and inspect and affect
 layers also deeper in the stack.  As an example there is a layer called
 "raw" which repeatedly "pops" layers until it reaches the first layer
 that has declared itself capable of handling binary data.  The "pushed"
 layers are processed in left-to-right order.

 ssyyssooppeenn(()) operates (unsurprisingly) at a lower level in the stack than
 ooppeenn(()).  For example in Unix or Unix-like systems ssyyssooppeenn(()) operates
 directly at the level of file descriptors: in the terms of PerlIO layers,
 it uses only the "unix" layer, which is a rather thin wrapper on top of
 the Unix file descriptors.

LLaayyeerrss vvss DDiisscciipplliinneess Initial discussion of the ability to modify IO streams behaviour used the term “discipline” for the entities which were added. This came (I believe) from the use of the term in “sfio”, which in turn borrowed it from “line disciplines” on Unix terminals. However, this document (and the C code) uses the term “layer”.

 This is, I hope, a natural term given the implementation, and should
 avoid connotations that are inherent in earlier uses of "discipline" for
 things which are rather different.

DDaattaa SSttrruuccttuurreess The basic data structure is a PerlIOl:

         typedef struct _PerlIO PerlIOl;
         typedef struct _PerlIO_funcs PerlIO_funcs;
         typedef PerlIOl *PerlIO;

         struct _PerlIO
         {
          PerlIOl *      next;       /* Lower layer */
          PerlIO_funcs * tab;        /* Functions for this layer */
          U32            flags;      /* Various flags for state */
         };

 A "PerlIOl *" is a pointer to the struct, and the _a_p_p_l_i_c_a_t_i_o_n level
 "PerlIO *" is a pointer to a "PerlIOl *" - i.e. a pointer to a pointer to
 the struct. This allows the application level "PerlIO *" to remain
 constant while the actual "PerlIOl *" underneath changes. (Compare perl's
 "SV *" which remains constant while its "sv_any" field changes as the
 scalar's type changes.) An IO stream is then in general represented as a
 pointer to this linked-list of "layers".

 It should be noted that because of the double indirection in a "PerlIO
 *", a "&(perlio->next)" "is" a "PerlIO *", and so to some degree at least
 one layer can use the "standard" API on the next layer down.

 A "layer" is composed of two parts:

 1.  The functions and attributes of the "layer class".

 2.  The per-instance data for a particular handle.

FFuunnccttiioonnss aanndd AAttttrriibbuutteess The functions and attributes are accessed via the “tab” (for table) member of “PerlIOl”. The functions (methods of the layer “class”) are fixed, and are defined by the “PerlIO_funcs” type. They are broadly the same as the public “PerlIO_xxxxx” functions:

  struct _PerlIO_funcs
  {
   Size_t     fsize;
   char *     name;
   Size_t     size;
   IV         kind;
   IV         (*Pushed)(pTHX_ PerlIO *f,
                              const char *mode,
                              SV *arg,
                              PerlIO_funcs *tab);
   IV         (*Popped)(pTHX_ PerlIO *f);
   PerlIO *   (*Open)(pTHX_ PerlIO_funcs *tab,
                            PerlIO_list_t *layers, IV n,
                            const char *mode,
                            int fd, int imode, int perm,
                            PerlIO *old,
                            int narg, SV **args);
   IV         (*Binmode)(pTHX_ PerlIO *f);
   SV *       (*Getarg)(pTHX_ PerlIO *f, CLONE_PARAMS *param, int flags)
   IV         (*Fileno)(pTHX_ PerlIO *f);
   PerlIO *   (*Dup)(pTHX_ PerlIO *f,
                           PerlIO *o,
                           CLONE_PARAMS *param,
                           int flags)
   /* Unix-like functions - cf sfio line disciplines */
   SSize_t    (*Read)(pTHX_ PerlIO *f, void *vbuf, Size_t count);
   SSize_t    (*Unread)(pTHX_ PerlIO *f, const void *vbuf, Size_t count);
   SSize_t    (*Write)(pTHX_ PerlIO *f, const void *vbuf, Size_t count);
   IV         (*Seek)(pTHX_ PerlIO *f, Off_t offset, int whence);
   Off_t      (*Tell)(pTHX_ PerlIO *f);
   IV         (*Close)(pTHX_ PerlIO *f);
   /* Stdio-like buffered IO functions */
   IV         (*Flush)(pTHX_ PerlIO *f);
   IV         (*Fill)(pTHX_ PerlIO *f);
   IV         (*Eof)(pTHX_ PerlIO *f);
   IV         (*Error)(pTHX_ PerlIO *f);
   void       (*Clearerr)(pTHX_ PerlIO *f);
   void       (*Setlinebuf)(pTHX_ PerlIO *f);
   /* Perl's snooping functions */
   STDCHAR *  (*Get_base)(pTHX_ PerlIO *f);
   Size_t     (*Get_bufsiz)(pTHX_ PerlIO *f);
   STDCHAR *  (*Get_ptr)(pTHX_ PerlIO *f);
   SSize_t    (*Get_cnt)(pTHX_ PerlIO *f);
   void       (*Set_ptrcnt)(pTHX_ PerlIO *f,STDCHAR *ptr,SSize_t cnt);
  };

 The first few members of the struct give a function table size for
 compatibility check "name" for the layer, the  size to "malloc" for the
 per-instance data, and some flags which are attributes of the class as
 whole (such as whether it is a buffering layer), then follow the
 functions which fall into four basic groups:

 1.  Opening and setup functions

 2.  Basic IO operations

 3.  Stdio class buffering options.

 4.  Functions to support Perl's traditional "fast" access to the buffer.

 A layer does not have to implement all the functions, but the whole table
 has to be present. Unimplemented slots can be NULL (which will result in
 an error when called) or can be filled in with stubs to "inherit"
 behaviour from a "base class". This "inheritance" is fixed for all
 instances of the layer, but as the layer chooses which stubs to populate
 the table, limited "multiple inheritance" is possible.

PPeerr--iinnssttaannccee DDaattaa The per-instance data are held in memory beyond the basic PerlIOl struct, by making a PerlIOl the first member of the layer’s struct thus:

         typedef struct
         {
          struct _PerlIO base;       /* Base "class" info */
          STDCHAR *      buf;        /* Start of buffer */
          STDCHAR *      end;        /* End of valid part of buffer */
          STDCHAR *      ptr;        /* Current position in buffer */
          Off_t          posn;       /* Offset of buf into the file */
          Size_t         bufsiz;     /* Real size of buffer */
          IV             oneword;    /* Emergency buffer */
         } PerlIOBuf;

 In this way (as for perl's scalars) a pointer to a PerlIOBuf can be
 treated as a pointer to a PerlIOl.

LLaayyeerrss iinn aaccttiioonn.. table perlio unix | | +———–+ +———-+ +——–+ PerlIO ->| |—>| next |—>| NULL | +———–+ +———-+ +——–+ | | | buffer | | fd | +———–+ | | +——–+ | | +———-+

 The above attempts to show how the layer scheme works in a simple case.
 The application's "PerlIO *" points to an entry in the table(s)
 representing open (allocated) handles. For example the first three slots
 in the table correspond to "stdin","stdout" and "stderr". The table in
 turn points to the current "top" layer for the handle - in this case an
 instance of the generic buffering layer "perlio". That layer in turn
 points to the next layer down - in this case the low-level "unix" layer.

 The above is roughly equivalent to a "stdio" buffered stream, but with
 much more flexibility:

 •   If Unix level "read"/"write"/"lseek" is not appropriate for (say)
     sockets then the "unix" layer can be replaced (at open time or even
     dynamically) with a "socket" layer.

 •   Different handles can have different buffering schemes. The "top"
     layer could be the "mmap" layer if reading disk files was quicker
     using "mmap" than "read". An "unbuffered" stream can be implemented
     simply by not having a buffer layer.

 •   Extra layers can be inserted to process the data as it flows through.
     This was the driving need for including the scheme in perl 5.7.0+ -
     we needed a mechanism to allow data to be translated between perl's
     internal encoding (conceptually at least Unicode as UTF-8), and the
     "native" format used by the system. This is provided by the
     ":encoding(xxxx)" layer which typically sits above the buffering
     layer.

 •   A layer can be added that does "\n" to CRLF translation. This layer
     can be used on any platform, not just those that normally do such
     things.

PPeerr--iinnssttaannccee ffllaagg bbiittss The generic flag bits are a hybrid of “O_XXXXX” style flags deduced from the mode string passed to “PerlIO_open()”, and state bits for typical buffer layers.

PERLIO_F_EOF #

     End of file.

PERLIO_F_CANWRITE #

     Writes are permitted, i.e. opened as "w" or "r+" or "a", etc.

PERLIO_F_CANREAD #

     Reads are permitted i.e. opened "r" or "w+" (or even "a+" - ick).

PERLIO_F_ERROR #

     An error has occurred (for "PerlIO_error()").

PERLIO_F_TRUNCATE #

     Truncate file suggested by open mode.

PERLIO_F_APPEND #

     All writes should be appends.

PERLIO_F_CRLF #

     Layer is performing Win32-like "\n" mapped to CR,LF for output and
     CR,LF mapped to "\n" for input. Normally the provided "crlf" layer is
     the only layer that need bother about this. "PerlIO_binmode()" will
     mess with this flag rather than add/remove layers if the
     "PERLIO_K_CANCRLF" bit is set for the layers class.

PERLIO_F_UTF8 #

     Data written to this layer should be UTF-8 encoded; data provided by
     this layer should be considered UTF-8 encoded. Can be set on any
     layer by ":utf8" dummy layer. Also set on ":encoding" layer.

PERLIO_F_UNBUF #

     Layer is unbuffered - i.e. write to next layer down should occur for
     each write to this layer.

PERLIO_F_WRBUF #

     The buffer for this layer currently holds data written to it but not
     sent to next layer.

PERLIO_F_RDBUF #

     The buffer for this layer currently holds unconsumed data read from
     layer below.

PERLIO_F_LINEBUF #

     Layer is line buffered. Write data should be passed to next layer
     down whenever a "\n" is seen. Any data beyond the "\n" should then be
     processed.

PERLIO_F_TEMP #

     File has been "unlink()"ed, or should be deleted on "close()".

PERLIO_F_OPEN #

     Handle is open.

PERLIO_F_FASTGETS #

     This instance of this layer supports the "fast "gets"" interface.
     Normally set based on "PERLIO_K_FASTGETS" for the class and by the
     existence of the function(s) in the table. However a class that
     normally provides that interface may need to avoid it on a particular
     instance. The "pending" layer needs to do this when it is pushed
     above a layer which does not support the interface.  (Perl's
     "sv_gets()" does not expect the streams fast "gets" behaviour to
     change during one "get".)

MMeetthhooddss iinn DDeettaaiill fsize Size_t fsize;

     Size of the function table. This is compared against the value PerlIO
     code "knows" as a compatibility check. Future versions _m_a_y be able to
     tolerate layers compiled against an old version of the headers.

 name
             char * name;

     The name of the layer whose ooppeenn(()) method Perl should invoke on
     ooppeenn(()).  For example if the layer is called APR, you will call:

       open $fh, ">:APR", ...

     and Perl knows that it has to invoke the PPeerrllIIOOAAPPRR__ooppeenn(()) method
     implemented by the APR layer.

 size
             Size_t size;

     The size of the per-instance data structure, e.g.:

       sizeof(PerlIOAPR)

     If this field is zero then "PerlIO_pushed" does not malloc anything
     and assumes layer's Pushed function will do any required layer stack
     manipulation - used to avoid malloc/free overhead for dummy layers.
     If the field is non-zero it must be at least the size of "PerlIOl",
     "PerlIO_pushed" will allocate memory for the layer's data structures
     and link new layer onto the stream's stack. (If the layer's Pushed
     method returns an error indication the layer is popped again.)

 kind
             IV kind;

• PERLIO_K_BUFFERED #

         The layer is buffered.

• PERLIO_K_RAW #

         The layer is acceptable to have in a binmode(FH) stack - i.e. it
         does not (or will configure itself not to) transform bytes
         passing through it.

• PERLIO_K_CANCRLF #

         Layer can translate between "\n" and CRLF line ends.

• PERLIO_K_FASTGETS #

         Layer allows buffer snooping.

• PERLIO_K_MULTIARG #

         Used when the layer's ooppeenn(()) accepts more arguments than usual.
         The extra arguments should come not before the "MODE" argument.
         When this flag is used it's up to the layer to validate the args.

 Pushed
      IV     (*Pushed)(pTHX_ PerlIO *f,const char *mode, SV *arg);

     The only absolutely mandatory method. Called when the layer is pushed
     onto the stack.  The "mode" argument may be NULL if this occurs post-
     open. The "arg" will be non-"NULL" if an argument string was passed.
     In most cases this should call "PerlIOBase_pushed()" to convert
     "mode" into the appropriate "PERLIO_F_XXXXX" flags in addition to any
     actions the layer itself takes.  If a layer is not expecting an
     argument it need neither save the one passed to it, nor provide
     "Getarg()" (it could perhaps "Perl_warn" that the argument was un-
     expected).

     Returns 0 on success. On failure returns -1 and should set errno.

 Popped
             IV      (*Popped)(pTHX_ PerlIO *f);

     Called when the layer is popped from the stack. A layer will normally
     be popped after "Close()" is called. But a layer can be popped
     without being closed if the program is dynamically managing layers on
     the stream. In such cases "Popped()" should free any resources
     (buffers, translation tables, ...) not held directly in the layer's
     struct.  It should also "Unread()" any unconsumed data that has been
     read and buffered from the layer below back to that layer, so that it
     can be re-provided to what ever is now above.

     Returns 0 on success and failure.  If "Popped()" returns _t_r_u_e then
     _p_e_r_l_i_o_._c assumes that either the layer has popped itself, or the
     layer is super special and needs to be retained for other reasons.
     In most cases it should return _f_a_l_s_e.

 Open
             PerlIO *        (*Open)(...);

     The "Open()" method has lots of arguments because it combines the
     functions of perl's "open", "PerlIO_open", perl's "sysopen",
     "PerlIO_fdopen" and "PerlIO_reopen".  The full prototype is as
     follows:

      PerlIO *       (*Open)(pTHX_ PerlIO_funcs *tab,
                             PerlIO_list_t *layers, IV n,
                             const char *mode,
                             int fd, int imode, int perm,
                             PerlIO *old,
                             int narg, SV **args);

     Open should (perhaps indirectly) call "PerlIO_allocate()" to allocate
     a slot in the table and associate it with the layers information for
     the opened file, by calling "PerlIO_push".  The _l_a_y_e_r_s is an array of
     all the layers destined for the "PerlIO *", and any arguments passed
     to them, _n is the index into that array of the layer being called.
     The macro "PerlIOArg" will return a (possibly "NULL") SV * for the
     argument passed to the layer.

     Where a layer opens or takes ownership of a file descriptor, that
     layer is responsible for getting the file descriptor's close-on-exec
     flag into the correct state.  The flag should be clear for a file
     descriptor numbered less than or equal to "PL_maxsysfd", and set for
     any file descriptor numbered higher.  For thread safety, when a layer
     opens a new file descriptor it should if possible open it with the
     close-on-exec flag initially set.

     The _m_o_d_e string is an ""fopen()"-like" string which would match the
     regular expression "/^[I#]?[rwa]\+?[bt]?$/".

     The 'I' prefix is used during creation of "stdin".."stderr" via
     special "PerlIO_fdopen" calls; the '#' prefix means that this is
     "sysopen" and that _i_m_o_d_e and _p_e_r_m should be passed to
     "PerlLIO_open3"; 'r' means rread, 'w' means wwrite and 'a' means
     aappend. The '+' suffix means that both reading and writing/appending
     are permitted.  The 'b' suffix means file should be binary, and 't'
     means it is text. (Almost all layers should do the IO in binary mode,
     and ignore the b/t bits. The ":crlf" layer should be pushed to handle
     the distinction.)

     If _o_l_d is not "NULL" then this is a "PerlIO_reopen". Perl itself does
     not use this (yet?) and semantics are a little vague.

     If _f_d not negative then it is the numeric file descriptor _f_d, which
     will be open in a manner compatible with the supplied mode string,
     the call is thus equivalent to "PerlIO_fdopen". In this case _n_a_r_g_s
     will be zero.  The file descriptor may have the close-on-exec flag
     either set or clear; it is the responsibility of the layer that takes
     ownership of it to get the flag into the correct state.

     If _n_a_r_g_s is greater than zero then it gives the number of arguments
     passed to "open", otherwise it will be 1 if for example "PerlIO_open"
     was called.  In simple cases SvPV_nolen(*args) is the pathname to
     open.

     If a layer provides "Open()" it should normally call the "Open()"
     method of next layer down (if any) and then push itself on top if
     that succeeds.  "PerlIOBase_open" is provided to do exactly that, so
     in most cases you don't have to write your own "Open()" method.  If
     this method is not defined, other layers may have difficulty pushing
     themselves on top of it during open.

     If "PerlIO_push" was performed and open has failed, it must
     "PerlIO_pop" itself, since if it's not, the layer won't be removed
     and may cause bad problems.

     Returns "NULL" on failure.

 Binmode
             IV        (*Binmode)(pTHX_ PerlIO *f);

     Optional. Used when ":raw" layer is pushed (explicitly or as a result
     of binmode(FH)). If not present layer will be popped. If present
     should configure layer as binary (or pop itself) and return 0.  If it
     returns -1 for error "binmode" will fail with layer still on the
     stack.

 Getarg
             SV *      (*Getarg)(pTHX_ PerlIO *f,
                                 CLONE_PARAMS *param, int flags);

     Optional. If present should return an SV * representing the string
     argument passed to the layer when it was pushed. e.g.
     ":encoding(ascii)" would return an SvPV with value "ascii". (_p_a_r_a_m
     and _f_l_a_g_s arguments can be ignored in most cases)

     "Dup" uses "Getarg" to retrieve the argument originally passed to
     "Pushed", so you must implement this function if your layer has an
     extra argument to "Pushed" and will ever be "Dup"ed.

 Fileno
             IV        (*Fileno)(pTHX_ PerlIO *f);

     Returns the Unix/Posix numeric file descriptor for the handle.
     Normally "PerlIOBase_fileno()" (which just asks next layer down) will
     suffice for this.

     Returns -1 on error, which is considered to include the case where
     the layer cannot provide such a file descriptor.

 Dup
             PerlIO * (*Dup)(pTHX_ PerlIO *f, PerlIO *o,
                             CLONE_PARAMS *param, int flags);

     XXX: Needs more docs.

     Used as part of the "clone" process when a thread is spawned (in
     which case param will be non-NULL) and when a stream is being
     duplicated via '&' in the "open".

     Similar to "Open", returns PerlIO* on success, "NULL" on failure.

 Read
             SSize_t (*Read)(pTHX_ PerlIO *f, void *vbuf, Size_t count);

     Basic read operation.

     Typically will call "Fill" and manipulate pointers (possibly via the
     API).  "PerlIOBuf_read()" may be suitable for derived classes which
     provide "fast gets" methods.

     Returns actual bytes read, or -1 on an error.

 Unread
             SSize_t (*Unread)(pTHX_ PerlIO *f,
                               const void *vbuf, Size_t count);

     A superset of stdio's "ungetc()". Should arrange for future reads to
     see the bytes in "vbuf". If there is no obviously better
     implementation then "PerlIOBase_unread()" provides the function by
     pushing a "fake" "pending" layer above the calling layer.

     Returns the number of unread chars.

 Write
             SSize_t (*Write)(PerlIO *f, const void *vbuf, Size_t count);

     Basic write operation.

     Returns bytes written or -1 on an error.

 Seek
             IV      (*Seek)(pTHX_ PerlIO *f, Off_t offset, int whence);

     Position the file pointer. Should normally call its own "Flush"
     method and then the "Seek" method of next layer down.

     Returns 0 on success, -1 on failure.

 Tell
             Off_t   (*Tell)(pTHX_ PerlIO *f);

     Return the file pointer. May be based on layers cached concept of
     position to avoid overhead.

     Returns -1 on failure to get the file pointer.

 Close
             IV      (*Close)(pTHX_ PerlIO *f);

     Close the stream. Should normally call "PerlIOBase_close()" to flush
     itself and close layers below, and then deallocate any data
     structures (buffers, translation tables, ...) not  held directly in
     the data structure.

     Returns 0 on success, -1 on failure.

 Flush
             IV      (*Flush)(pTHX_ PerlIO *f);

     Should make stream's state consistent with layers below. That is, any
     buffered write data should be written, and file position of lower
     layers adjusted for data read from below but not actually consumed.
     (Should perhaps "Unread()" such data to the lower layer.)

     Returns 0 on success, -1 on failure.

 Fill
             IV      (*Fill)(pTHX_ PerlIO *f);

     The buffer for this layer should be filled (for read) from layer
     below.  When you "subclass" PerlIOBuf layer, you want to use its
     ___r_e_a_d method and to supply your own fill method, which fills the
     PerlIOBuf's buffer.

     Returns 0 on success, -1 on failure.

 Eof
             IV      (*Eof)(pTHX_ PerlIO *f);

     Return end-of-file indicator. "PerlIOBase_eof()" is normally
     sufficient.

     Returns 0 on end-of-file, 1 if not end-of-file, -1 on error.

 Error
             IV      (*Error)(pTHX_ PerlIO *f);

     Return error indicator. "PerlIOBase_error()" is normally sufficient.

     Returns 1 if there is an error (usually when "PERLIO_F_ERROR" is
     set), 0 otherwise.

 Clearerr
             void    (*Clearerr)(pTHX_ PerlIO *f);

     Clear end-of-file and error indicators. Should call
     "PerlIOBase_clearerr()" to set the "PERLIO_F_XXXXX" flags, which may
     suffice.

 Setlinebuf
             void    (*Setlinebuf)(pTHX_ PerlIO *f);

     Mark the stream as line buffered. "PerlIOBase_setlinebuf()" sets the
     PERLIO_F_LINEBUF flag and is normally sufficient.

 Get_base
             STDCHAR *       (*Get_base)(pTHX_ PerlIO *f);

     Allocate (if not already done so) the read buffer for this layer and
     return pointer to it. Return NULL on failure.

 Get_bufsiz
             Size_t  (*Get_bufsiz)(pTHX_ PerlIO *f);

     Return the number of bytes that last "Fill()" put in the buffer.

 Get_ptr
             STDCHAR *       (*Get_ptr)(pTHX_ PerlIO *f);

     Return the current read pointer relative to this layer's buffer.

 Get_cnt
             SSize_t (*Get_cnt)(pTHX_ PerlIO *f);

     Return the number of bytes left to be read in the current buffer.

 Set_ptrcnt
             void    (*Set_ptrcnt)(pTHX_ PerlIO *f,
                                   STDCHAR *ptr, SSize_t cnt);

     Adjust the read pointer and count of bytes to match "ptr" and/or
     "cnt".  The application (or layer above) must ensure they are
     consistent.  (Checking is allowed by the paranoid.)

UUttiilliittiieess To ask for the next layer down use PerlIONext(PerlIO *f).

 To check that a PerlIO* is valid use PerlIOValid(PerlIO *f).  (All this
 does is really just to check that the pointer is non-NULL and that the
 pointer behind that is non-NULL.)

 PerlIOBase(PerlIO *f) returns the "Base" pointer, or in other words, the
 "PerlIOl*" pointer.

 PerlIOSelf(PerlIO* f, type) return the PerlIOBase cast to a type.

 Perl_PerlIO_or_Base(PerlIO* f, callback, base, failure, args) either
 calls the _c_a_l_l_b_a_c_k from the functions of the layer _f (just by the name of
 the IO function, like "Read") with the _a_r_g_s, or if there is no such
 callback, calls the _b_a_s_e version of the callback with the same args, or
 if the f is invalid, set errno to EBADF and return _f_a_i_l_u_r_e.

 Perl_PerlIO_or_fail(PerlIO* f, callback, failure, args) either calls the
 _c_a_l_l_b_a_c_k of the functions of the layer _f with the _a_r_g_s, or if there is no
 such callback, set errno to EINVAL.  Or if the f is invalid, set errno to
 EBADF and return _f_a_i_l_u_r_e.

 Perl_PerlIO_or_Base_void(PerlIO* f, callback, base, args) either calls
 the _c_a_l_l_b_a_c_k of the functions of the layer _f with the _a_r_g_s, or if there
 is no such callback, calls the _b_a_s_e version of the callback with the same
 args, or if the f is invalid, set errno to EBADF.

 Perl_PerlIO_or_fail_void(PerlIO* f, callback, args) either calls the
 _c_a_l_l_b_a_c_k of the functions of the layer _f with the _a_r_g_s, or if there is no
 such callback, set errno to EINVAL.  Or if the f is invalid, set errno to

EBADF. #

IImmpplleemmeennttiinngg PPeerrllIIOO LLaayyeerrss If you find the implementation document unclear or not sufficient, look at the existing PerlIO layer implementations, which include:

 •   C implementations

     The _p_e_r_l_i_o_._c and _p_e_r_l_i_o_l_._h in the Perl core implement the "unix",
     "perlio", "stdio", "crlf", "utf8", "byte", "raw", "pending" layers,
     and also the "mmap" and "win32" layers if applicable.  (The "win32"
     is currently unfinished and unused, to see what is used instead in
     Win32, see "Querying the layers of filehandles" in PerlIO .)

     PerlIO::encoding, PerlIO::scalar, PerlIO::via in the Perl core.

     PerlIO::gzip and APR::PerlIO (mod_perl 2.0) on CPAN.

 •   Perl implementations

     PerlIO::via::QuotedPrint in the Perl core and PerlIO::via::* on CPAN.

 If you are creating a PerlIO layer, you may want to be lazy, in other
 words, implement only the methods that interest you.  The other methods
 you can either replace with the "blank" methods

     PerlIOBase_noop_ok
     PerlIOBase_noop_fail

 (which do nothing, and return zero and -1, respectively) or for certain
 methods you may assume a default behaviour by using a NULL method.  The
 Open method looks for help in the 'parent' layer.  The following table
 summarizes the behaviour:

     method      behaviour with NULL

     Clearerr    PerlIOBase_clearerr
     Close       PerlIOBase_close
     Dup         PerlIOBase_dup
     Eof         PerlIOBase_eof
     Error       PerlIOBase_error
     Fileno      PerlIOBase_fileno
     Fill        FAILURE
     Flush       SUCCESS
     Getarg      SUCCESS
     Get_base    FAILURE
     Get_bufsiz  FAILURE
     Get_cnt     FAILURE
     Get_ptr     FAILURE
     Open        INHERITED
     Popped      SUCCESS
     Pushed      SUCCESS
     Read        PerlIOBase_read
     Seek        FAILURE
     Set_cnt     FAILURE
     Set_ptrcnt  FAILURE
     Setlinebuf  PerlIOBase_setlinebuf
     Tell        FAILURE
     Unread      PerlIOBase_unread
     Write       FAILURE

  FAILURE        Set errno (to EINVAL in Unixish, to LIB$_INVARG in VMS)
                 and return -1 (for numeric return values) or NULL (for
                 pointers)
  INHERITED      Inherited from the layer below
  SUCCESS        Return 0 (for numeric return values) or a pointer

CCoorree LLaayyeerrss The file “perlio.c” provides the following layers:

 "unix"
     A basic non-buffered layer which calls Unix/POSIX "read()",
     "write()", "lseek()", "close()". No buffering. Even on platforms that
     distinguish between O_TEXT and O_BINARY this layer is always

O_BINARY. #

 "perlio"
     A very complete generic buffering layer which provides the whole of
     PerlIO API. It is also intended to be used as a "base class" for
     other layers. (For example its "Read()" method is implemented in
     terms of the "Get_cnt()"/"Get_ptr()"/"Set_ptrcnt()" methods).

     "perlio" over "unix" provides a complete replacement for stdio as
     seen via PerlIO API. This is the default for USE_PERLIO when system's
     stdio does not permit perl's "fast gets" access, and which do not
     distinguish between "O_TEXT" and "O_BINARY".

 "stdio"
     A layer which provides the PerlIO API via the layer scheme, but
     implements it by calling system's stdio. This is (currently) the
     default if system's stdio provides sufficient access to allow perl's
     "fast gets" access and which do not distinguish between "O_TEXT" and

“O_BINARY”. #

 "crlf"
     A layer derived using "perlio" as a base class. It provides
     Win32-like "\n" to CR,LF translation. Can either be applied above
     "perlio" or serve as the buffer layer itself. "crlf" over "unix" is
     the default if system distinguishes between "O_TEXT" and "O_BINARY"
     opens. (At some point "unix" will be replaced by a "native" Win32 IO
     layer on that platform, as Win32's read/write layer has various
     drawbacks.) The "crlf" layer is a reasonable model for a layer which
     transforms data in some way.

 "mmap"
     If Configure detects "mmap()" functions this layer is provided (with
     "perlio" as a "base") which does "read" operations by mmmmaapp(())ing the
     file. Performance improvement is marginal on modern systems, so it is
     mainly there as a proof of concept. It is likely to be unbundled from
     the core at some point. The "mmap" layer is a reasonable model for a
     minimalist "derived" layer.

 "pending"
     An "internal" derivative of "perlio" which can be used to provide
     UUnnrreeaadd(()) function for layers which have no buffer or cannot be
     bothered.  (Basically this layer's "Fill()" pops itself off the stack
     and so resumes reading from layer below.)

 "raw"
     A dummy layer which never exists on the layer stack. Instead when
     "pushed" it actually pops the stack removing itself, it then calls
     Binmode function table entry on all the layers in the stack -
     normally this (via PerlIOBase_binmode) removes any layers which do
     not have "PERLIO_K_RAW" bit set. Layers can modify that behaviour by
     defining their own Binmode entry.

 "utf8"
     Another dummy layer. When pushed it pops itself and sets the
     "PERLIO_F_UTF8" flag on the layer which was (and now is once more)
     the top of the stack.

 In addition _p_e_r_l_i_o_._c also provides a number of "PerlIOBase_xxxx()"
 functions which are intended to be used in the table slots of classes
 which do not need to do anything special for a particular method.

EExxtteennssiioonn LLaayyeerrss Layers can be made available by extension modules. When an unknown layer is encountered the PerlIO code will perform the equivalent of :

    use PerlIO 'layer';

 Where _l_a_y_e_r is the unknown layer. _P_e_r_l_I_O_._p_m will then attempt to:

    require PerlIO::layer;

 If after that process the layer is still not defined then the "open" will
 fail.

 The following extension layers are bundled with perl:

 ":encoding"
        use Encoding;

     makes this layer available, although _P_e_r_l_I_O_._p_m "knows" where to find
     it.  It is an example of a layer which takes an argument as it is
     called thus:

        open( $fh, "<:encoding(iso-8859-7)", $pathname );

 ":scalar"
     Provides support for reading data from and writing data to a scalar.

        open( $fh, "+<:scalar", \$scalar );

     When a handle is so opened, then reads get bytes from the string
     value of _$_s_c_a_l_a_r, and writes change the value. In both cases the
     position in _$_s_c_a_l_a_r starts as zero but can be altered via "seek", and
     determined via "tell".

     Please note that this layer is implied when calling ooppeenn(()) thus:

        open( $fh, "+<", \$scalar );

 ":via"
     Provided to allow layers to be implemented as Perl code.  For
     instance:

        use PerlIO::via::StripHTML;
        open( my $fh, "<:via(StripHTML)", "index.html" );

     See PerlIO::via for details.

TTOODDOO #

 Things that need to be done to improve this document.

 •   Explain how to make a valid fh without going through ooppeenn(())(i.e.
     apply a layer). For example if the file is not opened through perl,
     but we want to get back a fh, like it was opened by Perl.

     How PerlIO_apply_layera fits in, where its docs, was it made public?

     Currently the example could be something like this:

       PerlIO *foo_to_PerlIO(pTHX_ char *mode, ...)
       {
           char *mode; /* "w", "r", etc */
           const char *layers = ":APR"; /* the layer name */
           PerlIO *f = PerlIO_allocate(aTHX);
           if (!f) {
               return NULL;
           }

           PerlIO_apply_layers(aTHX_ f, mode, layers);

           if (f) {
               PerlIOAPR *st = PerlIOSelf(f, PerlIOAPR);
               /* fill in the st struct, as in _open() */
               st->file = file;
               PerlIOBase(f)->flags |= PERLIO_F_OPEN;

               return f;
           }
           return NULL;
       }

 •   fix/add the documentation in places marked as XXX.

 •   The handling of errors by the layer is not specified. e.g. when $!
     should be set explicitly, when the error handling should be just
     delegated to the top layer.

     Probably give some hints on using SSEETTEERRRRNNOO(()) or pointers to where
     they can be found.

 •   I think it would help to give some concrete examples to make it
     easier to understand the API. Of course I agree that the API has to
     be concise, but since there is no second document that is more of a
     guide, I think that it'd make it easier to start with the doc which
     is an API, but has examples in it in places where things are unclear,
     to a person who is not a PerlIO guru (yet).

perl v5.36.3 2023-02-15 PERLIOL(1)