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NAME ^

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

SYNOPSIS ^

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

DESCRIPTION ^

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 extentions 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 perl6.

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

The first few members of the struct give a "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 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 lowlevel "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.

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 occured (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 existance 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 an 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

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

The only absoultely 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).

IV (*Popped)(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.

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,
                        AV *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 AV 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 permited. The 'b' suffix means file should be binary, and 't' means it is text. (Binary/Text should be ignored by almost all layers and binary IO done, with PerlIO. The :crlf layer should be pushed to handle the distinction.)

If old is not NULL then this is a PerlIO_reopen. Perl iteself 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 an 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(*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 normaly call the Open() method of next layer down (if any) and then push itself on top if that succeeds.

SV * (*Getarg)(PerlIO *f);

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".

IV (*Fileno)(PerlIO *f);

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

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

Basic read operation. Returns actual bytes read, or -1 on an error. Typically will call Fill and manipulate pointers (possibly via the API). PerlIOBuf_read() may be suitable for derived classes which provide "fast gets" methods.

SSize_t (*Unread)(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.

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

Basic write operation. Returns bytes written or -1 on an error.

IV (*Seek)(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.

Off_t (*Tell)(PerlIO *f);

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

IV (*Close)(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.

IV (*Flush)(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 fron below but not actually consumed. (Should perhaps Unread() such data to the lower layer.)

IV (*Fill)(PerlIO *f);

The buffer for this layer should be filled (for read) from layer below.

IV (*Eof)(PerlIO *f);

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

IV (*Error)(PerlIO *f);

Return error indicator. PerlIOBase_error() is normally sufficient.

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

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

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

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

Size_t (*Get_bufsiz)(PerlIO *f);

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

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

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

SSize_t (*Get_cnt)(PerlIO *f);

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

void (*Set_ptrcnt)(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.)

Core Layers

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 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.

"pending"

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.)

"raw"

A dummy layer which never exists on the layer stack. Instead when "pushed" it actually pops the stack(!), removing itself, and any other layers until it reaches a layer with the class PERLIO_K_RAW bit set.

"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 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:

":encoding"
   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)
":Scalar"

Provides support for

   open($fh,"...",\$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.

":Object" or ":Perl"

May be provided to allow layers to be implemented as perl code - implementation is being investigated.

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