PERLIOL(1) Perl Programmers Reference Guide PERLIOL(1) #
PERLIOL(1) Perl Programmers Reference Guide PERLIOL(1)
NNAAMMEE #
perliol - C API for Perl's implementation of IO in Layers.
SSYYNNOOPPSSIISS #
/* Defining a layer ... */
#include <perliol.h>
DDEESSCCRRIIPPTTIIOONN #
This document describes the behavior and implementation of the PerlIO
abstraction described in perlapio when "USE_PERLIO" is defined.
HHiissttoorryy aanndd BBaacckkggrroouunndd 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 Raku.
BBaassiicc SSttrruuccttuurree 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 _a_b_o_v_e and _b_e_l_o_w 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 ooppeenn(()) 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.
bbiinnmmooddee(()) operates similarly to ooppeenn(()): 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
ooppeenn(()) and bbiinnmmooddee(()), 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.
ssyyssooppeenn(()) operates (unsurprisingly) at a lower level in the stack than
ooppeenn(()). For example in Unix or Unix-like systems ssyyssooppeenn(()) 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.
LLaayyeerrss vvss DDiisscciipplliinneess 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.
DDaattaa SSttrruuccttuurreess 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 _a_p_p_l_i_c_a_t_i_o_n 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.
FFuunnccttiioonnss aanndd AAttttrriibbuutteess 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,
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.
PPeerr--iinnssttaannccee DDaattaa 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.
LLaayyeerrss iinn aaccttiioonn.. 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.
PPeerr--iinnssttaannccee ffllaagg bbiittss 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".)
MMeetthhooddss iinn DDeettaaiill fsize Size_t fsize;
Size of the function table. This is compared against the value PerlIO
code "knows" as a compatibility check. Future versions _m_a_y be able to
tolerate layers compiled against an old version of the headers.
name
char * name;
The name of the layer whose ooppeenn(()) method Perl should invoke on
ooppeenn(()). For example if the layer is called APR, you will call:
open $fh, ">:APR", ...
and Perl knows that it has to invoke the PPeerrllIIOOAAPPRR__ooppeenn(()) 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 ooppeenn(()) 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 _t_r_u_e then
_p_e_r_l_i_o_._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 _f_a_l_s_e.
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 _l_a_y_e_r_s 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.
Where a layer opens or takes ownership of a file descriptor, that
layer is responsible for getting the file descriptor's close-on-exec
flag into the correct state. The flag should be clear for a file
descriptor numbered less than or equal to "PL_maxsysfd", and set for
any file descriptor numbered higher. For thread safety, when a layer
opens a new file descriptor it should if possible open it with the
close-on-exec flag initially set.
The _m_o_d_e 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 _i_m_o_d_e and _p_e_r_m should be passed to
"PerlLIO_open3"; 'r' means rread, 'w' means wwrite and 'a' means
aappend. 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 _o_l_d is not "NULL" then this is a "PerlIO_reopen". Perl itself does
not use this (yet?) and semantics are a little vague.
If _f_d not negative then it is the numeric file descriptor _f_d, which
will be open in a manner compatible with the supplied mode string,
the call is thus equivalent to "PerlIO_fdopen". In this case _n_a_r_g_s
will be zero. The file descriptor may have the close-on-exec flag
either set or clear; it is the responsibility of the layer that takes
ownership of it to get the flag into the correct state.
If _n_a_r_g_s 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". (_p_a_r_a_m
and _f_l_a_g_s 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
___r_e_a_d 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.)
UUttiilliittiieess 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 _c_a_l_l_b_a_c_k from the functions of the layer _f (just by the name of
the IO function, like "Read") with the _a_r_g_s, or if there is no such
callback, calls the _b_a_s_e version of the callback with the same args, or
if the f is invalid, set errno to EBADF and return _f_a_i_l_u_r_e.
Perl_PerlIO_or_fail(PerlIO* f, callback, failure, args) either calls the
_c_a_l_l_b_a_c_k of the functions of the layer _f with the _a_r_g_s, or if there is no
such callback, set errno to EINVAL. Or if the f is invalid, set errno to
EBADF and return _f_a_i_l_u_r_e.
Perl_PerlIO_or_Base_void(PerlIO* f, callback, base, args) either calls
the _c_a_l_l_b_a_c_k of the functions of the layer _f with the _a_r_g_s, or if there
is no such callback, calls the _b_a_s_e 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
_c_a_l_l_b_a_c_k of the functions of the layer _f with the _a_r_g_s, or if there is no
such callback, set errno to EINVAL. Or if the f is invalid, set errno to
EBADF. #
IImmpplleemmeennttiinngg PPeerrllIIOO LLaayyeerrss If you find the implementation document unclear or not sufficient, look at the existing PerlIO layer implementations, which include:
• C implementations
The _p_e_r_l_i_o_._c and _p_e_r_l_i_o_l_._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
CCoorree LLaayyeerrss 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 mmmmaapp(())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
UUnnrreeaadd(()) 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 _p_e_r_l_i_o_._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.
EExxtteennssiioonn LLaayyeerrss 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 _l_a_y_e_r is the unknown layer. _P_e_r_l_I_O_._p_m 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 _P_e_r_l_I_O_._p_m "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( $fh, "+<:scalar", \$scalar );
When a handle is so opened, then reads get bytes from the string
value of _$_s_c_a_l_a_r, and writes change the value. In both cases the
position in _$_s_c_a_l_a_r starts as zero but can be altered via "seek", and
determined via "tell".
Please note that this layer is implied when calling ooppeenn(()) 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.
TTOODDOO #
Things that need to be done to improve this document.
• Explain how to make a valid fh without going through ooppeenn(())(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 SSEETTEERRRRNNOO(()) 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).
perl v5.36.3 2023-02-15 PERLIOL(1)