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 [Toc] [Back]
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 perl6.
Basic Structure [Toc] [Back]
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 [Toc] [Back]
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 [Toc] [Back]
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 applica-
tion 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 [Toc] [Back]
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,
AV *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 [Toc] [Back]
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. [Toc] [Back]
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:
o 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.
o 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.
o 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.
o A layer can be added that does "0 to CRLF translation.
This layer can be used on any platform, not just
those that normally do such things.
Per-instance flag bits [Toc] [Back]
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 "0 mapped to CR,LF
for output and CR,LF mapped to "0 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 "0 is seen. Any data
beyond the "0 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 [Toc] [Back]
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 Per-
lIOAPR_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 "0 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,
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 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.
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 [Toc] [Back]
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 [Toc] [Back]
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 [Toc] [Back]
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 "0 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 [Toc] [Back]
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 reading data from and writing
data to a scalar.
open( $fh, "+<:scalar", calar );
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, "+<", calar );
":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.
Things that need to be done to improve this document.
o 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;
}
o fix/add the documentation in places marked as XXX.
o 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.
o I think it would help to give some concrete examples
to make it easier to understand the API. Of course I
agree that the API has to be concise, but since there
is no second document that is more of a guide, I think
that it'd make it easier to start with the doc which
is an API, but has examples in it in places where
things are unclear, to a person who is not a PerlIO
guru (yet).
perl v5.8.5 2002-11-06 20 [ Back ] |
