PERLIOL(1)

okay bestie, welcome to PERLIOL(1) from the Perl Programmers Reference Guide — part 1 of 4, and we are going to get you fluent in this, no cap. 💖

✨ NAME ✨

perliol — the C API for Perl's implementation of IO in Layers. periodt. 👑

---

🦋 SYNOPSIS 🦋

Here's the vibe when you're defining a layer, bestie:

/* Defining a layer ... */
#include <perliol.h>

---

🌸 DESCRIPTION 🌸

This doc spills the tea on the behavior and implementation of the PerlIO abstraction described in perlapio when USE_PERLIO is defined. 🫶

🎀 History and Background 🎀

The PerlIO abstraction showed up in perl5.003_02, but it lowkey languished as just an abstraction until perl5.7.0. 💀 During that whole time, though, a bunch of perl extensions switched to using it, so the API is mostly fixed to maintain (source) compatibility. no cap.

The aim of the implementation is to provide the PerlIO API in a flexible and platform neutral manner. It's also a trial of an "Object Oriented C, with vtables" approach which may be applied to Raku. slay.

🌈 Basic Structure 🌈

PerlIO is a stack of layers. fr fr.

The low levels of the stack work with the low-level operating system calls (file descriptors in C) getting bytes in and out, while the higher layers of the stack buffer, filter, and otherwise manipulate the I/O, and return characters (or bytes) to Perl. The terms above and below are used to refer to the relative positioning of the stack layers. ✨

A layer contains a "vtable", the table of I/O operations (at C level a table of function pointers), and status flags. The functions in the vtable implement operations like "open", "read", and "write". 👑

When I/O, for example "read", is requested, the request goes from Perl first down the stack using "read" functions of each layer, then at the bottom the input is requested from the operating system services, then the result is returned up the stack, finally being interpreted as Perl data. 🦋

The requests do not necessarily go always all the way down to the operating system: that's where PerlIO buffering comes into play. 💅

When you do an open() and specify extra PerlIO layers to be deployed, the layers you specify are "pushed" on top of the already existing default stack. One way to see it is that "operating system is on the left" and "Perl is on the right". 🫶

What exact layers are in this default stack depends on a lot of things: your operating system, Perl version, Perl compile time configuration, and Perl runtime configuration. See PerlIO, "PERLIO" in perlrun, and open for more information. 🌟

binmode() operates similarly to open(): by default the specified layers are pushed on top of the existing stack. ✨

However, note that even as the specified layers are "pushed on top" for open() and binmode(), this doesn't mean that the effects are limited to the "top": PerlIO layers can be very 'active' and inspect and affect layers also deeper in the stack. As an example there is a layer called "raw" which repeatedly "pops" layers until it reaches the first layer that has declared itself capable of handling binary data. The "pushed" layers are processed in left-to-right order. 👀

sysopen() operates (unsurprisingly) at a lower level in the stack than open(). For example in Unix or Unix-like systems sysopen() operates directly at the level of file descriptors: in the terms of PerlIO layers, it uses only the "unix" layer, which is a rather thin wrapper on top of the Unix file descriptors. 💜

🧚 Layers vs Disciplines 🧚

Initial discussion of the ability to modify IO streams behaviour used the term "discipline" for the entities which were added. This came (I believe) from the use of the term in "sfio", which in turn borrowed it from "line disciplines" on Unix terminals. However, this document (and the C code) uses the term "layer". 🎀

This is, I hope, a natural term given the implementation, and should avoid connotations that are inherent in earlier uses of "discipline" for things which are rather different. 💅

🍓 Data Structures 🍓

The basic data structure is a PerlIOl:

typedef struct _PerlIO PerlIOl;
typedef struct _PerlIO_funcs PerlIO_funcs;
typedef PerlIOl *PerlIO;

struct _PerlIO
{
 PerlIOl *      next;       /* Lower layer */
 PerlIO_funcs * tab;        /* Functions for this layer */
 U32            flags;      /* Various flags for state */
};

A "PerlIOl *" is a pointer to the struct, and the application level "PerlIO *" is a pointer to a "PerlIOl *" — i.e. a pointer to a pointer to the struct. This allows the application level "PerlIO *" to remain constant while the actual "PerlIOl *" underneath changes. (Compare perl's "SV *" which remains constant while its "sv_any" field changes as the scalar's type changes.) An IO stream is then in general represented as a pointer to this linked-list of "layers". 🌈

It should be noted that because of the double indirection in a "PerlIO *", a "&(perlio->next)" "is" a "PerlIO *", and so to some degree at least one layer can use the "standard" API on the next layer down. 💅

A "layer" is composed of two parts:

1. The functions and attributes of the "layer class".
2. The per-instance data for a particular handle.

👑 Functions and Attributes 👑

The functions and attributes are accessed via the "tab" (for table) member of "PerlIOl". The functions (methods of the layer "class") are fixed, and are defined by the "PerlIO_funcs" type. They are broadly the same as the public "PerlIO_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. 💅

💜 Per-instance Data 💜

The per-instance data are held in memory beyond the basic PerlIOl struct, by making a PerlIOl the first member of the layer's struct thus:

typedef struct
{
 struct _PerlIO base;       /* Base "class" info */
 STDCHAR *      buf;        /* Start of buffer */
 STDCHAR *      end;        /* End of valid part of buffer */
 STDCHAR *      ptr;        /* Current position in buffer */
 Off_t          posn;       /* Offset of buf into the file */
 Size_t         bufsiz;     /* Real size of buffer */
 IV             oneword;    /* Emergency buffer */
} PerlIOBuf;

In this way (as for perl's scalars) a pointer to a PerlIOBuf can be treated as a pointer to a PerlIOl. ✨

🦋 Layers in action 🦋

                       table           perlio          unix
                   |           |
                   +-----------+    +----------+    +--------+
          PerlIO ->|           |--->|  next    |--->|  NULL  |
                   +-----------+    +----------+    +--------+
                   |           |    |  buffer  |    |   fd   |
                   +-----------+    |          |    +--------+
                   |           |    +----------+

The above attempts to show how the layer scheme works in a simple case. The application's "PerlIO *" points to an entry in the table(s) representing open (allocated) handles. For example the first three slots in the table correspond to "stdin","stdout" and "stderr". The table in turn points to the current "top" layer for the handle — in this case an instance of the generic buffering layer "perlio". That layer in turn points to the next layer down — in this case the low-level "unix" layer. 🌈

The above is roughly equivalent to a "stdio" buffered stream, but with much more flexibility:

• If Unix level "read"/"write"/"lseek" is not appropriate for (say) sockets then the "unix" layer can be replaced (at open time or even dynamically) with a "socket" layer. 🫶
• Different handles can have different buffering schemes. The "top" layer could be the "mmap" layer if reading disk files was quicker using "mmap" than "read". An "unbuffered" stream can be implemented simply by not having a buffer layer. 💅
• Extra layers can be inserted to process the data as it flows through. This was literally the whole driving need for putting the scheme in perl 5.7.0+ — we needed a mechanism that lets data be translated between perl's internal encoding (conceptually at least Unicode as UTF-8) and the "native" format the system uses. That's what the :encoding(xxxx) layer gives you, and it typically sits above the buffering layer. 💅
• A layer can be added that does "\n" to CRLF translation. And bestie, this layer works on any platform, not just the ones that normally do such things. 🌍✨

🌸 Per-instance flag bits 🌸

The generic flag bits are a hybrid situation — part O_XXXXX-style flags deduced from the mode string you passed to PerlIO_open(), and part state bits for typical buffer layers. Two moods, one bitfield. 💅

• PERLIO_F_EOF — End of file. That's it, that's the vibe. 🥀
• 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, we don't talk about that one 🙈).
• PERLIO_F_ERROR — An error has occurred (this is the one for PerlIO_error()).
• PERLIO_F_TRUNCATE — Truncate file, as suggested by the open mode.
• PERLIO_F_APPEND — All writes should be appends.
• PERLIO_F_CRLF — The layer is doing 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 even needs to care about this. PerlIO_binmode() will mess with this flag instead of adding/removing layers if the PERLIO_K_CANCRLF bit is set for the layer's 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 the :utf8 dummy layer. Also set on the :encoding layer.
• PERLIO_F_UNBUF — Layer is unbuffered — i.e. a write to the next layer down should occur for each write to this layer. no hoarding bestie 💖
• PERLIO_F_WRBUF — The buffer for this layer currently holds data written to it but not yet sent to the next layer.
• PERLIO_F_RDBUF — The buffer for this layer currently holds unconsumed data read from the layer below.
• PERLIO_F_LINEBUF — Layer is line buffered. Write data should be passed to the 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. But — 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 gets pushed above a layer that doesn't support the interface. (Perl's sv_gets() does not expect the stream's fast gets behaviour to change during one get.) ⚠️

---

🎀 Methods in Detail 🎀

fsize

            Size_t fsize;

Size of the function table. This gets compared against the value the 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. So like, if the layer is called APR, you'd call:

  open $fh, ">:APR", ...

and Perl knows it has to invoke the PerlIOAPR_open() method implemented by the APR layer. iykyk 💅

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 it assumes the layer's Pushed function will do any required layer stack manipulation — this is used to avoid malloc/free overhead for dummy layers. If the field is non-zero it must be at least the size of PerlIOl, and PerlIO_pushed will allocate memory for the layer's data structures and link the new layer onto the stream's stack. (And if the layer's Pushed method returns an error indication, the layer gets 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. no hand-holding, you're on your own bestie 💅

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 isn't expecting an argument, it need neither save the one passed to it, nor provide Getarg() — it could maybe 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 those 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 whatever 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. It's giving multitasking. 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 layer's information for the opened file, by calling PerlIO_push. The layers is an array of all the layers destined for the PerlIO *, and any arguments passed to them; n is the index into that array of the layer being called. The macro PerlIOArg will return a (possibly NULL) SV * for the argument passed to the layer.

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 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 the 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 the semantics are a little vague. 🤷♀️

If fd is 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. 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 nargs is greater than zero then it gives the number of arguments passed to open, otherwise it will be 1 if for example PerlIO_open was called. In simple cases SvPV_nolen(*args) is the pathname to open.

If a layer provides Open() it should normally call the Open() method of the 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 doesn't, the layer won't be removed and may cause bad problems. 🚨

Returns NULL on failure.

Binmode

            IV        (*Binmode)(pTHX_ PerlIO *f);

Optional. Used when the :raw layer is pushed (explicitly or as a result of binmode(FH)). If not present, the layer will be popped. If present, it should configure the layer as binary (or pop itself) and return 0. If it returns -1 for error, binmode will fail with the layer still on the stack. 💀

Getarg

            SV *      (*Getarg)(pTHX_ PerlIO *f,
                                CLONE_PARAMS *param, int flags);

Optional. If it's there, it should hand back an SV * that is the string argument your layer got when it was pushed. Like, ":encoding(ascii)" would return an SvPV with the value "ascii". (The param and flags arguments can be ignored in most cases, bestie, don't even stress.)

"Dup" uses "Getarg" to retrieve the argument that was originally passed to "Pushed", so you must implement this function if your layer has an extra argument to "Pushed" and is ever gonna get "Dup"ed. periodt. 💅

Fileno

IV        (*Fileno)(pTHX_ PerlIO *f);

Hands back the Unix/Posix numeric file descriptor for the handle. Normally PerlIOBase_fileno() — which literally just asks the next layer down — will be all you need here. ✨

Returns -1 on error, and yes, that includes the case where the layer straight up cannot provide such a file descriptor. 💀

Dup

PerlIO * (*Dup)(pTHX_ PerlIO *f, PerlIO *o,
                CLONE_PARAMS *param, int flags);

XXX: Needs more docs. (iconic honestly, no notes 🙈)

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, bestie. 📖

Typically it 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 the 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 the layer's cached concept of position to avoid overhead. (living rent free in the cache, we love to see it 🦋)

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 the layers below, and then deallocate any data structures (buffers, translation tables, ...) that are not held directly in the data structure.

Returns 0 on success, -1 on failure.

Flush

IV      (*Flush)(pTHX_ PerlIO *f);

Should make the stream's state consistent with the layers below. Translation: any buffered write data should be written, and the 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 the 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 the 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 the 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 a pointer to it. Return NULL on failure.

Get_bufsiz

Size_t  (*Get_bufsiz)(pTHX_ PerlIO *f);

Return the number of bytes that the 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 the 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. lowkey simple like that.)
• PerlIOBase(PerlIO *f) returns the "Base" pointer — or in other words, the PerlIOl* pointer.
• PerlIOSelf(PerlIO* f, type) returns 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, sets errno to EBADF and returns 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, sets errno to EINVAL. Or if the f is invalid, sets errno to EBADF and returns 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, sets 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, sets errno to EINVAL. Or if the f is invalid, sets errno to EBADF.

Implementing PerlIO Layers

If you find the implementation document unclear or not sufficient, go look at the existing PerlIO layer implementations, which include:

• C implementations
  • The perlio.c and perliol.h in the Perl core implement the "unix", "perlio", "stdio", "crlf", "utf8", "byte", "raw", "pending" layers, and also the "mmap" and "win32" layers if applicable. (The "win32" is currently unfinished and unused — to see what is used instead in Win32, see "Querying the layers of filehandles" in PerlIO.)
  • PerlIO::encoding, PerlIO::scalar, PerlIO::via in the Perl core.
  • PerlIO::gzip and APR::PerlIO (mod_perl 2.0) on CPAN.
• Perl implementations
  • PerlIO::via::QuotedPrint in the Perl core and PerlIO::via::* on CPAN.

If you are creating a PerlIO layer, you may want to be lazy — in other words, implement only the methods that interest you. 😌 The other methods you can either replace with the "blank" methods

PerlIOBase_noop_ok
PerlIOBase_noop_fail

(which do nothing, and return zero and -1, respectively) or, for certain methods, you may assume a default behaviour by using a NULL method. The Open method looks for help in the 'parent' layer. The following table summarizes the behaviour:

method      behaviour with NULL

Clearerr    PerlIOBase_clearerr
Close       PerlIOBase_close
Dup         PerlIOBase_dup
Eof         PerlIOBase_eof
Error       PerlIOBase_error
Fileno      PerlIOBase_fileno
Fill        FAILURE
Flush       SUCCESS
Getarg      SUCCESS
Get_base    FAILURE
Get_bufsiz  FAILURE
Get_cnt     FAILURE
Get_ptr     FAILURE
Open        INHERITED
Popped      SUCCESS
Pushed      SUCCESS
Read        PerlIOBase_read
Seek        FAILURE
Set_cnt     FAILURE
Set_ptrcnt  FAILURE
Setlinebuf  PerlIOBase_setlinebuf
Tell        FAILURE
Unread      PerlIOBase_unread
Write       FAILURE

FAILURE        Set errno (to EINVAL in Unixish, to LIB$_INVARG in VMS)
               and return -1 (for numeric return values) or NULL (for
               pointers)
INHERITED      Inherited from the layer below
SUCCESS        Return 0 (for numeric return values) or a pointer

Core Layers

The file perlio.c provides the following layers:

"unix"

A basic non-buffered layer which calls Unix/POSIX read(), write(), lseek(), close(). No buffering. Even on platforms that distinguish between O_TEXT and O_BINARY, this layer is always O_BINARY. 🔥

"perlio"

A very complete generic buffering layer which provides the whole of PerlIO API. It is also intended to be used as a "base class" for other layers. (For example its Read() method is implemented in terms of the Get_cnt()/Get_ptr()/Set_ptrcnt() methods.)

"perlio" over "unix" provides a complete replacement for stdio as seen via PerlIO API. This is the default for USE_PERLIO when system's stdio does not permit perl's "fast gets" access, and which do not distinguish between "O_TEXT" and "O_BINARY".

"stdio"

A layer which provides the PerlIO API via the layer scheme, but implements it by calling system's stdio. This is (currently) the default if system's stdio provides sufficient access to allow perl's "fast gets" access and which do not distinguish between "O_TEXT" and "O_BINARY".

"crlf"

A layer that's derived using "perlio" as a base class 💖 It gives you that Win32-flavored translation of "\n" into CR,LF. It can either sit above "perlio" or just be the buffer layer itself, no biggie. "crlf" on top of "unix" is the default whenever your system is the kind that cares about O_TEXT vs O_BINARY opens. (Heads up bestie: at some point "unix" is gonna get swapped out for a "native" Win32 IO layer on that platform, because Win32's read/write layer has, um, various drawbacks 😬) Also the "crlf" layer is a totally reasonable model if you wanna write a layer that transforms data in some way ✨

"mmap"

If Configure detects mmap() functions, this layer shows up — it's got "perlio" as a "base" and it does its "read" operations by mmap()ing the file 🌸 Performance improvement is honestly marginal on modern systems so it's mainly vibing there as a proof of concept. It's likely to be unbundled from the core at some point too. Still, the "mmap" layer is a reasonable model for a minimalist "derived" layer 💅

"pending"

An "internal" derivative of "perlio" that can provide an Unread() function for layers that have no buffer or just can't be bothered 🙈 (Basically this layer's Fill() pops itself off the stack and so resumes reading from the layer below.)

"raw"

A dummy layer that never actually exists on the layer stack, iconic behavior honestly. Instead, when it gets "pushed" it actually pops the stack and removes itself, then calls the Binmode function table entry on all the layers in the stack. Normally that (via PerlIOBase_binmode) nukes any layers that don't have the PERLIO_K_RAW bit set 🔥 Layers can change that behavior by defining their own Binmode entry.

"utf8"

Another dummy layer, she's a ghost 👻 When pushed, it pops itself and sets the PERLIO_F_UTF8 flag on the layer that was (and now, once more, is) the top of the stack.

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In addition, perlio.c also provides a bunch of PerlIOBase_xxxx() functions that are meant to be used in the table slots of classes that don't need to do anything special for a particular method 🫶

Extension Layers

Layers can be made available by extension modules. When an unknown layer gets encountered, the PerlIO code will do the equivalent of:

use PerlIO 'layer';

where layer is the unknown layer. Then PerlIO.pm will attempt to:

require PerlIO::layer;

And if after all that the layer still isn't defined, then the "open" is gonna fail, periodt 💀

The following extension layers are bundled with perl:

":encoding"

use Encoding;

makes this layer available, even though PerlIO.pm "knows" where to find it. It's an example of a layer that takes an argument, because it gets called like this:

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 opened like this, reads get bytes from the string value of $scalar, and writes change that value. In both cases the position in $scalar starts at zero, but you can alter it via "seek", and figure out where it's at via "tell" ✨

Please note that this layer is implied when you call open() like this: open( $fh, "+<", \$scalar );

":via"

Provided so layers can be implemented as Perl code 🎀 For instance:

use PerlIO::via::StripHTML;
open( my $fh, "<:via(StripHTML)", "index.html" );

See PerlIO::via for details 👑

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🎀 TODO 🎀

Things that need to be done to improve this document, aka the to-do list of dreams 💜

• 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 an fh, like it was opened by Perl.

How PerlIO_apply_layera fits in, where its docs are, was it made public?

Currently the example could be something like this:

PerlIO *foo_to_PerlIO(pTHX_ char *mode, ...)
{
    char *mode; /* "w", "r", etc */
    const char *layers = ":APR"; /* the layer name */
    PerlIO *f = PerlIO_allocate(aTHX);
    if (!f) {
        return NULL;
    }

    PerlIO_apply_layers(aTHX_ f, mode, layers);

    if (f) {
        PerlIOAPR *st = PerlIOSelf(f, PerlIOAPR);
        /* fill in the st struct, as in _open() */
        st->file = file;
        PerlIOBase(f)->flags |= PERLIO_F_OPEN;

        return f;
    }
    return NULL;
}

• fix/add the documentation in places marked as XXX 👀
• The handling of errors by the layer is not specified. e.g. when $! should be set explicitly, when the error handling should be just delegated to the top layer.

Probably give some hints on using SETERRNO() or pointers to where they can be found.

• I think it would help to give some concrete examples to make it easier to understand the API. Of course I agree that the API has to be concise, but since there is no second document that is more of a guide, I think that it'd make it easier to start with the doc which is an API, but has examples in it in places where things are unclear, to a person who is not a PerlIO guru (yet) 😭

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And that's the whole page, bestie!! 💖✨ This was perl v5.42.3, dated 2026-08-03, and it's PERLIOL(1) — go forth and layer responsibly 🦋🌸💅