NAME
perliol - C API for Perls 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_PERLIOis defined (and
USE_SFIOis not).
History and Background
The PerlIO abstraction was introduced in perl5.003_02 but languished as just an abstraction until perl5.7.0. However during that time a number of perl extensions switched to using it, so the API is mostly fixed to maintain (source) compatibility.
The aim of the implementation is to provide the PerlIO API in a flexible and platform neutral manner. It is also a trial of an Object Oriented C, with vtables approach which may be applied to Perl 6.
Basic Structure
PerlIO is a stack of layers.
The low levels of the stack work with the low-level operating system calls (file descriptors in C) getting bytes in and out, the higher layers of the stack buffer, filter, and otherwise manipulate the I/O, and return characters (or bytes) to Perl. Terms above and below are used to refer to the relative positioning of the stack layers.
A layer contains a vtable, the table of I/O operations (at C level a table of function pointers), and status flags. The functions in the vtable implement operations like open, read, and write.
When I/O, for example read, is requested, the request goes from Perl first down the stack using read functions of each layer, then at the bottom the input is requested from the operating system services, then the result is returned up the stack, finally being interpreted as Perl data.
The requests do not necessarily go always all the way down to the operating system: thats 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 doesnt mean that the effects are limited to the top: PerlIO layers can be very active and inspect and affect layers also deeper in the stack. As an example there is a layer called raw which repeatedly pops layers until it reaches the first layer that has declared itself capable of handling binary data. The pushed layers are processed in left-to-right order.
sysopen() operates (unsurprisingly) at a lower level in the stack than open(). For example in UNIX or UNIX-like systems sysopen() operates directly at the level of file descriptors: in the terms of PerlIO layers, it uses only the unix layer, which is a rather thin wrapper on top of the UNIX file descriptors.
Layers vs Disciplines
Initial discussion of the ability to modify IO streams behaviour used the term discipline for the entities which were added. This came (I believe) from the use of the term in sfio, which in turn borrowed it from line disciplines on Unix terminals. However, this document (and the C code) uses the term layer.
This is, I hope, a natural term given the implementation, and should avoid connotations that are inherent in earlier uses of discipline for things which are rather different.
Data Structures
The basic data structure is a PerlIOl:
typedef struct _PerlIO PerlIOl;
typedef struct _PerlIO_funcs PerlIO_funcs;
typedef PerlIOl *PerlIO;
struct _PerlIO
{
PerlIOl * next; /* Lower layer */
PerlIO_funcs * tab; /* Functions for this layer */
IV flags; /* Various flags for state */
};
A
PerlIOl *is a pointer to the struct, and the application level
PerlIO *is a pointer to a
PerlIOl *- i.e. a pointer to a pointer to the struct. This allows the application level
PerlIO *to remain constant while the actual
PerlIOl *underneath changes. (Compare perls
SV *which remains constant while its
sv_anyfield changes as the scalars 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_funcstype. They are broadly the same as the public
PerlIO_xxxxxfunctions:
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);
/* Perls 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
mallocfor 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 Perls traditional fast access to the buffer. |
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 layers 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 perls 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 applications
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,
stdoutand
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:
o |
If Unix level read/ write/ lseekis 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 mmapthan 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 perls 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 \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_XXXXXstyle 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_CANCRLFbit 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_FASTGETSfor 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. (Perls sv_gets()does not expect the streams fast getsbehaviour 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_pusheddoes not malloc anything and assumes layers 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_pushedwill allocate memory for the layers data structures and link new layer onto the streams stack. (If the layers Pushed method returns an error indication the layer is popped again.) | ||||||||||
kind |
IV kind;
| ||||||||||
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 modeargument may be NULL if this occurs post-open. The argwill be non- NULLif an argument string was passed. In most cases this should call PerlIOBase_pushed()to convert modeinto the appropriate PERLIO_F_XXXXXflags 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_warnthat 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 layers 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 perls open, PerlIO_open, perls sysopen, PerlIO_fdopenand 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 PerlIOArgwill 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 Iprefix is used during creation of stdin.. stderrvia special PerlIO_fdopencalls; the #prefix means that this is sysopenand that imode and perm should be passed to PerlLIO_open3; rmeans read, wmeans write and ameans append. The +suffix means that both reading and writing/appending are permitted. The bsuffix means file should be binary, and tmeans it is text. (Almost all layers should do the IO in binary mode, and ignore the b/t bits. The :crlflayer should be pushed to handle the distinction.) If old is not NULLthen 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_openwas 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_pushwas performed and open has failed, it must PerlIO_popitself, since if its not, the layer wont be removed and may cause bad problems. Returns NULLon failure. | ||||||||||
Binmode |
IV (*Binmode)(pTHX_ PerlIO *f);
Optional. Used when :rawlayer 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 binmodewill 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)
Dupuses Getargto retrieve the argument originally passed to Pushed, so you must implement this function if your layer has an extra argument to Pushedand will ever be Duped. | ||||||||||
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, NULLon failure. | ||||||||||
Read |
SSize_t (*Read)(pTHX_ PerlIO *f, void *vbuf, Size_t count);
Basic read operation. Typically will call Filland 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 stdios 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 Flushmethod and then the Seekmethod 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 streams 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 PerlIOBufs 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_ERRORis 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_XXXXXflags, 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 layers 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 ptrand/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:
o |
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. |
o |
Perl implementations
PerlIO::via::QuotedPrint in the Perl core and PerlIO::via::* on CPAN. |
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.cprovides 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 systems stdio does not permit perls fast gets access, and which do not distinguish between O_TEXTand O_BINARY. |
stdio |
A layer which provides the PerlIO API via the layer scheme, but
implements it by calling systems stdio. This is (currently) the default
if systems stdio provides sufficient access to allow perls fast gets
access and which do not distinguish between O_TEXTand 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_TEXTand O_BINARYopens. (At some point unix will be replaced by a native Win32 IO layer on that platform, as Win32s 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 layers 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_RAWbit 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_UTF8flag on the layer which was (and now is once more) the top of the stack. |
PerlIOBase_xxxx()functions which are intended to be used in the table slots of classes which do not need to do anything special for a particular method.
Extension Layers
Layers can made available by extension modules. When an unknown layer is encountered the PerlIO code will perform the equivalent of :
use PerlIO layer;
Where layer is the unknown layer. PerlIO.pm will then attempt to:
require PerlIO::layer;
If after that process the layer is still not defined then the
openwill 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 $scalarstarts 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.
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 itd 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). |