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


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


This document describes the behavior and implementation of the PerlIO abstraction described in perlapio when USE_PERLIO is defined (and USE_SFIO is not).

History and Background

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 Perl 6.

Basic Structure

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 above and below 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 open() 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.

binmode() operates similarly to open(): 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 open() and binmode(), 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.

sysopen() operates (unsurprisingly) at a lower level in the stack than open(). For example in Unix or Unix-like systems sysopen() 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.

Layers vs Disciplines

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.

Data Structures

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 */
         IV             flags;      /* Various flags for state */

A PerlIOl * is a pointer to the struct, and the application 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.

Functions and Attributes

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.

Per-instance Data

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.

Layers in action.

                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:

Per-instance flag bits

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.


End of file.


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


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


An error has occurred (for PerlIO_error()).


Truncate file suggested by open mode.


All writes should be appends.


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.


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.


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


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


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


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.


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


Handle is open.


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".)

Methods in Detail

        Size_t fsize;

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

        char * name;

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

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

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

        Size_t size;

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


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.)

        IV kind;

    The layer is buffered.


    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.


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


    Layer allows buffer snooping.


    Used when the layer's open() 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.

        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.

        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 true then perlio.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 false.

        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 layers 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.

The mode 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 imode and perm should be passed to PerlLIO_open3; 'r' means read, 'w' means write and 'a' means append. 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 old is not NULL then this is a PerlIO_reopen. Perl itself does not use this (yet?) and semantics are a little vague.

If fd not negative then it is the numeric file descriptor fd, which will be open in a manner compatible with the supplied mode string, the call is thus equivalent to PerlIO_fdopen. In this case nargs will be zero.

If nargs 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.

Having said all that translation-only layers do not need to provide Open() at all, but rather leave the opening to a lower level layer and wait to be "pushed". If a layer does provide Open() it should normally call the Open() method of next layer down (if any) and then push itself on top if that succeeds.

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.

        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.

        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". (param and flags 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 Duped.

        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.

                        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.


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.

        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.

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

Basic write operation.

Returns bytes written or -1 on an error.


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.

        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.

        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.

        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.

        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 _read method and to supply your own fill method, which fills the PerlIOBuf's buffer.

Returns 0 on success, -1 on failure.

        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.

        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.

        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.

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

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


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

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

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


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

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

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

        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.)


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

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

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

Implementing PerlIO Layers

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

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


(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 

Core Layers

The file perlio.c provides the following layers:


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.


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.


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.


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.


If Configure detects mmap() functions this layer is provided (with "perlio" as a "base") which does "read" operations by mmap()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.


An "internal" derivative of "perlio" which can be used to provide Unread() 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.)


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.


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 perlio.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.

Extension Layers

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

   use PerlIO 'layer';

Where layer is the unknown layer. PerlIO.pm 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:

   use Encoding;

makes this layer available, although PerlIO.pm "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 );

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 $scalar, and writes change the value. In both cases the position in $scalar starts as zero but can be altered via seek, and determined via tell.

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

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

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.


Things that need to be done to improve this document.

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