Daftar Isi

• NAME
• SYNOPSIS
• DESCRIPTION
  • History and Background
  • Basic Structure
  • Layers vs Disciplines
  • Data Structures
  • Functions and Attributes
  • Per-instance Data
  • Layers in action.
  • Per-instance flag bits
  • Methods in Detail
  • Utilities
  • Implementing PerlIO Layers
  • Core Layers
  • Extension Layers
• TODO

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 PerlIOabstraction described in perlapio when USE_PERLIO is defined (andUSE_SFIO is not).

History and Background

The PerlIO abstraction was introduced in perl5.003_02 but languished asjust an abstraction until perl5.7.0. However during that time a numberof perl extensions switched to using it, so the API is mostly fixed tomaintain (source) compatibility.

The aim of the implementation is to provide the PerlIO API in a flexibleand platform neutral manner. It is also a trial of an "Object OrientedC, 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 systemcalls (file descriptors in C) getting bytes in and out, the higherlayers of the stack buffer, filter, and otherwise manipulate the I/O,and return characters (or bytes) to Perl. Terms above and beloware used to refer to the relative positioning of the stack layers.

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

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

The requests do not necessarily go always all the way down to theoperating 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 existingdefault stack. One way to see it is that "operating system ison the left" and "Perl is on the right".

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

binmode() operates similarly to open(): by default the specifiedlayers 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 arelimited to the "top": PerlIO layers can be very 'active' and inspectand affect layers also deeper in the stack. As an example thereis a layer called "raw" which repeatedly "pops" layers untilit reaches the first layer that has declared itself capable ofhandling binary data. The "pushed" layers are processed in left-to-rightorder.

sysopen() operates (unsurprisingly) at a lower level in the stack thanopen(). For example in Unix or Unix-like systems sysopen() operatesdirectly at the level of file descriptors: in the terms of PerlIOlayers, it uses only the "unix" layer, which is a rather thin wrapperon top of the Unix file descriptors.

Layers vs Disciplines

Initial discussion of the ability to modify IO streams behaviour usedthe term "discipline" for the entities which were added. This came (Ibelieve) from the use of the term in "sfio", which in turn borrowed itfrom "line disciplines" on Unix terminals. However, this document (andthe C code) uses the term "layer".

This is, I hope, a natural term given the implementation, and shouldavoid 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 */
 
 IVflags;  /* Various flags for state */
 
};
 

A PerlIOl * is a pointer to the struct, and the applicationlevel PerlIO * is a pointer to a PerlIOl * - i.e. a pointerto a pointer to the struct. This allows the application level PerlIO *to remain constant while the actual PerlIOl * underneathchanges. (Compare perl's SV * which remains constant while itssv_any field changes as the scalar's type changes.) An IO stream isthen 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 degreeat 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") arefixed, and are defined by the PerlIO_funcs type. They are broadly thesame as the public PerlIO_xxxxx functions:

  struct _PerlIO_funcs
 
  {
 
   Size_tfsize;
 
   char *name;
 
   Size_tsize;
 
   IVkind;
 
   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 forcompatibility 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 bufferinglayer), 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 wholetable has to be present. Unimplemented slots can be NULL (which willresult in an error when called) or can be filled in with stubs to"inherit" behaviour from a "base class". This "inheritance" is fixedfor all instances of the layer, but as the layer chooses which stubsto populate the table, limited "multiple inheritance" is possible.

Per-instance Data

The per-instance data are held in memory beyond the basic PerlIOlstruct, by making a PerlIOl the first member of the layer's structthus:

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_tposn;   /* Offset of buf into the file */
 
 Size_tbufsiz; /* Real size of buffer */
 
 IVoneword; /* Emergency buffer */
 
} PerlIOBuf;
 

In this way (as for perl's scalars) a pointer to a PerlIOBuf can betreated 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 slotsin the table correspond to stdin,stdout and stderr. The tablein turn points to the current "top" layer for the handle - in this casean instance of the generic buffering layer "perlio". That layer in turnpoints to the next layer down - in this case the low-level "unix" layer.

The above is roughly equivalent to a "stdio" buffered stream, but withmuch 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 evendynamically) with a "socket" layer.

• Different handles can have different buffering schemes. The "top"layer could be the "mmap" layer if reading disk files was quickerusing mmap than read. An "unbuffered" stream can be implementedsimply 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+ - weneeded a mechanism to allow data to be translated between perl'sinternal 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 layercan be used on any platform, not just those that normally do suchthings.

Per-instance flag bits

The generic flag bits are a hybrid of O_XXXXX style flags deducedfrom the mode string passed to PerlIO_open(), and state bits fortypical 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,LFmapped to "\n" for input. Normally the provided "crlf" layer is the onlylayer that need bother about this. PerlIO_binmode() will mess with thisflag rather than add/remove layers if the PERLIO_K_CANCRLF bit is setfor the layers class.

• PERLIO_F_UTF8

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

• PERLIO_F_UNBUF

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

• PERLIO_F_WRBUF

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

• PERLIO_F_RDBUF

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

• PERLIO_F_LINEBUF

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

• 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 theexistence of the function(s) in the table. However a class thatnormally provides that interface may need to avoid it on aparticular instance. The "pending" layer needs to do this whenit is pushed above a layer which does not support the interface.(Perl's sv_gets() does not expect the streams fast gets behaviourto change during one "get".)

Methods in Detail

• fsize

  Size_t fsize;
   

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

• name

  char * name;
   

  The name of the layer whose open() method Perl should invoke onopen(). 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() methodimplemented 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 anythingand assumes layer's Pushed function will do any required layer stackmanipulation - 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 structuresand link new layer onto the stream's stack. (If the layer's Pushedmethod 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 open() accepts more arguments than usual. Theextra arguments should come not before the MODE argument. When thisflag 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 pushedonto the stack. The mode argument may be NULL if this occurspost-open. The arg will be non-NULL if an argument string waspassed. In most cases this should call PerlIOBase_pushed() toconvert mode into the appropriate PERLIO_F_XXXXX flags inaddition to any actions the layer itself takes. If a layer is notexpecting an argument it need neither save the one passed to it, norprovide Getarg() (it could perhaps Perl_warn that the argumentwas 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 normallybe popped after Close() is called. But a layer can be poppedwithout being closed if the program is dynamically managing layers onthe stream. In such cases Popped() should free any resources(buffers, translation tables, ...) not held directly in the layer'sstruct. It should also Unread() any unconsumed data that has beenread and buffered from the layer below back to that layer, so that itcan be re-provided to what ever is now above.

  Returns 0 on success and failure. If Popped() returns true thenperlio.c assumes that either the layer has popped itself, or thelayer is super special and needs to be retained for other reasons.In most cases it should return false.

• Open

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

  The Open() method has lots of arguments because it combines thefunctions of perl's open, PerlIO_open, perl's sysopen,PerlIO_fdopen and PerlIO_reopen. The full prototype is asfollows:

   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 allocatea slot in the table and associate it with the layers information forthe opened file, by calling PerlIO_push. The layers is anarray of all the layers destined for the PerlIO *, and anyarguments passed to them, n is the index into that array of thelayer being called. The macro PerlIOArg will return a (possiblyNULL) SV * for the argument passed to the layer.

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

  The 'I' prefix is used during creation of stdin..stderr viaspecial PerlIO_fdopen calls; the '#' prefix means that this issysopen and that imode and perm should be passed toPerlLIO_open3; 'r' means read, 'w' means write and'a' means append. The '+' suffix means that both reading andwriting/appending are permitted. The 'b' suffix means file shouldbe binary, and 't' means it is text. (Almost all layers should dothe IO in binary mode, and ignore the b/t bits. The :crlf layershould be pushed to handle the distinction.)

  If old is not NULL then this is a PerlIO_reopen. Perl itselfdoes 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 modestring, the call is thus equivalent to PerlIO_fdopen. In this casenargs will be zero.

  If nargs is greater than zero then it gives the number of argumentspassed to open, otherwise it will be 1 if for examplePerlIO_open was called. In simple cases SvPV_nolen(*args) is thepathname 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 thatsucceeds. PerlIOBase_open is provided to do exactly that, so inmost cases you don't have to write your own Open() method. If thismethod is not defined, other layers may have difficulty pushingthemselves on top of it during open.

  If PerlIO_push was performed and open has failed, it mustPerlIO_pop itself, since if it's not, the layer won't be removedand 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 resultof binmode(FH)). If not present layer will be popped. If presentshould configure layer as binary (or pop itself) and return 0.If it returns -1 for error binmode will fail with layerstill on the stack.

• Getarg

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

  Optional. If present should return an SV * representing the stringargument passed to the layer when it waspushed. e.g. ":encoding(ascii)" would return an SvPV with value"ascii". (param and flags arguments can be ignored in mostcases)

  Dup uses Getarg to retrieve the argument originally passed toPushed, so you must implement this function if your layer has anextra argument to Pushed and will ever be Duped.

• Fileno

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

  Returns the Unix/Posix numeric file descriptor for the handle. NormallyPerlIOBase_fileno() (which just asks next layer down) will sufficefor this.

  Returns -1 on error, which is considered to include the case where thelayer 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 whichcase 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 theAPI). PerlIOBuf_read() may be suitable for derived classes whichprovide "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 tosee the bytes in vbuf. If there is no obviously better implementationthen 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 Flushmethod 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 ofposition 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 flushitself and close layers below, and then deallocate any data structures(buffers, translation tables, ...) not held directly in the datastructure.

  Returns 0 on success, -1 on failure.

• Flush

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

  Should make stream's state consistent with layers below. That is, anybuffered write data should be written, and file position of lower layersadjusted 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 layerbelow. When you "subclass" PerlIOBuf layer, you want to use its_read method and to supply your own fill method, which fills thePerlIOBuf'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 thePERLIO_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 andreturn 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.)

Utilities

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

To check that a PerlIO* is valid use PerlIOValid(PerlIO *f). (Allthis does is really just to check that the pointer is non-NULL andthat 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) eithercalls the callback from the functions of the layer f (just bythe name of the IO function, like "Read") with the args, or ifthere is no such callback, calls the base version of the callbackwith the same args, or if the f is invalid, set errno to EBADF andreturn failure.

Perl_PerlIO_or_fail(PerlIO* f, callback, failure, args) either callsthe 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 isinvalid, set errno to EBADF and return failure.

Perl_PerlIO_or_Base_void(PerlIO* f, callback, base, args) either callsthe callback of the functions of the layer f with the args,or if there is no such callback, calls the base version of thecallback with the same args, or if the f is invalid, set errno toEBADF.

Perl_PerlIO_or_fail_void(PerlIO* f, callback, args) either calls thecallback of the functions of the layer f with the args, or ifthere is no such callback, set errno to EINVAL. Or if the f isinvalid, 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:

• C implementations

  The perlio.c and perliol.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 usedinstead 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 otherwords, implement only the methods that interest you. The other methodsyou can either replace with the "blank" methods

 PerlIOBase_noop_ok
 
 PerlIOBase_noop_fail
 

(which do nothing, and return zero and -1, respectively) or forcertain methods you may assume a default behaviour by using a NULLmethod. 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:

• "unix"

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

• "perlio"

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

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

• "stdio"

  A layer which provides the PerlIO API via the layer scheme, butimplements it by calling system's stdio. This is (currently) the defaultif 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 serveas the buffer layer itself. "crlf" over "unix" is the default if systemdistinguishes 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 isa 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 thefile. Performance improvement is marginal on modern systems, so it ismainly there as a proof of concept. It is likely to be unbundled fromthe core at some point. The "mmap" layer is a reasonable model for aminimalist "derived" layer.

• "pending"

  An "internal" derivative of "perlio" which can be used to provideUnread() function for layers which have no buffer or cannot bebothered. (Basically this layer's Fill() pops itself off the stackand 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 callsBinmode function table entry on all the layers in the stack - normallythis (via PerlIOBase_binmode) removes any layers which do not havePERLIO_K_RAW bit set. Layers can modify that behaviour by definingtheir own Binmode entry.

• "utf8"

  Another dummy layer. When pushed it pops itself and sets thePERLIO_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 classeswhich do not need to do anything special for a particular method.

Extension Layers

Layers can be made available by extension modules. When an unknown layeris 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 openwill fail.

The following extension layers are bundled with perl:

• ":encoding"

     use Encoding;
   

  makes this layer available, although PerlIO.pm "knows" where tofind it. It is an example of a layer which takes an argument as it iscalled 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 valueof $scalar, and writes change the value. In both cases the positionin $scalar starts as zero but can be altered via seek, anddetermined via tell.

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

TODO

Things that need to be done to improve this document.

• Explain how to make a valid fh without going through open()(i.e. applya layer). For example if the file is not opened through perl, but wewant 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 justdelegated to the top layer.

  Probably give some hints on using SETERRNO() or pointers to where theycan be found.

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