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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 the perlapio manpage when USE_PERLIO is defined (and
USE_SFIO is not).
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.
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 the PerlIO manpage,
PERLIO in the perlrun manpage, and the open manpage 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.
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.
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 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:
-
The functions and attributes of the "layer class".
-
The per-instance data for a particular handle.
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:
-
Opening and setup functions
-
Basic IO operations
-
Stdio class buffering options.
-
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.
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.
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:
-
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.
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".)
- 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,
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);
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