perliol - Online in the Cloud

PROGRAM:

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.

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 */
U32 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_" 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:

· 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.

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 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".)

Methods in Detail
fsize
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.

name
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
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 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.

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 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.

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 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.

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". (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 "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 _read 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.)

Utilities
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:

· 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
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

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, 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 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 be 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 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 );

":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. 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 SETERRNO() 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).

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