PERLAPIO(1) Perl Programmers Reference Guide PERLAPIO(1)
NAME
perlapio - perl's IO abstraction interface.
SYNOPSIS
#define PERLIO_NOT_STDIO 0 /* For co-existence with stdio only */
#include <perlio.h> /* Usually via #include <perl.h> */
PerlIO *PerlIO_stdin(void);
PerlIO *PerlIO_stdout(void);
PerlIO *PerlIO_stderr(void);
PerlIO *PerlIO_open(const char *path,const char *mode);
PerlIO *PerlIO_fdopen(int fd, const char *mode);
PerlIO *PerlIO_reopen(const char *path, /* deprecated */
const char *mode, PerlIO *old);
int PerlIO_close(PerlIO *f);
int PerlIO_stdoutf(const char *fmt,...)
int PerlIO_puts(PerlIO *f,const char *string);
int PerlIO_putc(PerlIO *f,int ch);
SSize_t PerlIO_write(PerlIO *f,const void *buf,size_t numbytes);
int PerlIO_printf(PerlIO *f, const char *fmt,...);
int PerlIO_vprintf(PerlIO *f, const char *fmt, va_list args);
int PerlIO_flush(PerlIO *f);
int PerlIO_eof(PerlIO *f);
int PerlIO_error(PerlIO *f);
void PerlIO_clearerr(PerlIO *f);
int PerlIO_getc(PerlIO *d);
int PerlIO_ungetc(PerlIO *f,int ch);
SSize_t PerlIO_read(PerlIO *f, void *buf, size_t numbytes);
int PerlIO_fileno(PerlIO *f);
void PerlIO_setlinebuf(PerlIO *f);
Off_t PerlIO_tell(PerlIO *f);
int PerlIO_seek(PerlIO *f, Off_t offset, int whence);
void PerlIO_rewind(PerlIO *f);
int PerlIO_getpos(PerlIO *f, SV *save); /* prototype changed */
int PerlIO_setpos(PerlIO *f, SV *saved); /* prototype changed */
int PerlIO_fast_gets(PerlIO *f);
int PerlIO_has_cntptr(PerlIO *f);
SSize_t PerlIO_get_cnt(PerlIO *f);
char *PerlIO_get_ptr(PerlIO *f);
void PerlIO_set_ptrcnt(PerlIO *f, char *ptr, SSize_t count);
int PerlIO_canset_cnt(PerlIO *f); /* deprecated */
void PerlIO_set_cnt(PerlIO *f, int count); /* deprecated */
int PerlIO_has_base(PerlIO *f);
char *PerlIO_get_base(PerlIO *f);
SSize_t PerlIO_get_bufsiz(PerlIO *f);
PerlIO *PerlIO_importFILE(FILE *stdio, const char *mode);
FILE *PerlIO_exportFILE(PerlIO *f, int flags);
FILE *PerlIO_findFILE(PerlIO *f);
void PerlIO_releaseFILE(PerlIO *f,FILE *stdio);
int PerlIO_apply_layers(PerlIO *f, const char *mode,
const char *layers);
int PerlIO_binmode(PerlIO *f, int ptype, int imode,
const char *layers);
void PerlIO_debug(const char *fmt,...)
DESCRIPTION
Perl's source code, and extensions that want maximum portability,
should use the above functions instead of those defined in ANSI C's
stdio.h. The perl headers (in particular "perlio.h") will "#define"
them to the I/O mechanism selected at Configure time.
The functions are modeled on those in stdio.h, but parameter order has
been "tidied up a little".
"PerlIO *" takes the place of FILE *. Like FILE * it should be treated
as opaque (it is probably safe to assume it is a pointer to something).
There are currently three implementations:
1. USE_STDIO
All above are #define'd to stdio functions or are trivial wrapper
functions which call stdio. In this case only PerlIO * is a FILE *.
This has been the default implementation since the abstraction was
introduced in perl5.003_02.
2. USE_PERLIO
Introduced just after perl5.7.0, this is a re-implementation of the
above abstraction which allows perl more control over how IO is
done as it decouples IO from the way the operating system and C
library choose to do things. For USE_PERLIO PerlIO * has an extra
layer of indirection - it is a pointer-to-a-pointer. This allows
the PerlIO * to remain with a known value while swapping the
implementation around underneath at run time. In this case all the
above are true (but very simple) functions which call the
underlying implementation.
This is the only implementation for which "PerlIO_apply_layers()"
does anything "interesting".
The USE_PERLIO implementation is described in perliol.
Because "perlio.h" is a thin layer (for efficiency) the semantics of
these functions are somewhat dependent on the underlying
implementation. Where these variations are understood they are noted
below.
Unless otherwise noted, functions return 0 on success, or a negative
value (usually "EOF" which is usually -1) and set "errno" on error.
PerlIO_stdin(), PerlIO_stdout(), PerlIO_stderr()
Use these rather than "stdin", "stdout", "stderr". They are written
to look like "function calls" rather than variables because this
makes it easier to make them function calls if platform cannot
export data to loaded modules, or if (say) different "threads"
might have different values.
PerlIO_open(path, mode), PerlIO_fdopen(fd,mode)
These correspond to fopen()/fdopen() and the arguments are the
same. Return "NULL" and set "errno" if there is an error. There
may be an implementation limit on the number of open handles, which
may be lower than the limit on the number of open files - "errno"
may not be set when "NULL" is returned if this limit is exceeded.
PerlIO_reopen(path,mode,f)
While this currently exists in all three implementations perl
itself does not use it. As perl does not use it, it is not well
tested.
Perl prefers to "dup" the new low-level descriptor to the
descriptor used by the existing PerlIO. This may become the
behaviour of this function in the future.
PerlIO_printf(f,fmt,...), PerlIO_vprintf(f,fmt,a)
These are fprintf()/vfprintf() equivalents.
PerlIO_stdoutf(fmt,...)
This is printf() equivalent. printf is #defined to this function,
so it is (currently) legal to use "printf(fmt,...)" in perl
sources.
PerlIO_read(f,buf,count), PerlIO_write(f,buf,count)
These correspond functionally to fread() and fwrite() but the
arguments and return values are different. The PerlIO_read() and
PerlIO_write() signatures have been modeled on the more sane low
level read() and write() functions instead: The "file" argument is
passed first, there is only one "count", and the return value can
distinguish between error and "EOF".
Returns a byte count if successful (which may be zero or positive),
returns negative value and sets "errno" on error. Depending on
implementation "errno" may be "EINTR" if operation was interrupted
by a signal.
PerlIO_close(f)
Depending on implementation "errno" may be "EINTR" if operation was
interrupted by a signal.
PerlIO_puts(f,s), PerlIO_putc(f,c)
These correspond to fputs() and fputc(). Note that arguments have
been revised to have "file" first.
PerlIO_ungetc(f,c)
This corresponds to ungetc(). Note that arguments have been
revised to have "file" first. Arranges that next read operation
will return the byte c. Despite the implied "character" in the
name only values in the range 0..0xFF are defined. Returns the byte
c on success or -1 ("EOF") on error. The number of bytes that can
be "pushed back" may vary, only 1 character is certain, and then
only if it is the last character that was read from the handle.
PerlIO_getc(f)
This corresponds to getc(). Despite the c in the name only byte
range 0..0xFF is supported. Returns the character read or -1
("EOF") on error.
PerlIO_eof(f)
This corresponds to feof(). Returns a true/false indication of
whether the handle is at end of file. For terminal devices this
may or may not be "sticky" depending on the implementation. The
flag is cleared by PerlIO_seek(), or PerlIO_rewind().
PerlIO_error(f)
This corresponds to ferror(). Returns a true/false indication of
whether there has been an IO error on the handle.
PerlIO_fileno(f)
This corresponds to fileno(), note that on some platforms, the
meaning of "fileno" may not match Unix. Returns -1 if the handle
has no open descriptor associated with it.
PerlIO_clearerr(f)
This corresponds to clearerr(), i.e., clears 'error' and (usually)
'eof' flags for the "stream". Does not return a value.
PerlIO_flush(f)
This corresponds to fflush(). Sends any buffered write data to the
underlying file. If called with "NULL" this may flush all open
streams (or core dump with some USE_STDIO implementations).
Calling on a handle open for read only, or on which last operation
was a read of some kind may lead to undefined behaviour on some
USE_STDIO implementations. The USE_PERLIO (layers) implementation
tries to behave better: it flushes all open streams when passed
"NULL", and attempts to retain data on read streams either in the
buffer or by seeking the handle to the current logical position.
PerlIO_seek(f,offset,whence)
This corresponds to fseek(). Sends buffered write data to the
underlying file, or discards any buffered read data, then positions
the file descriptor as specified by offset and whence (sic). This
is the correct thing to do when switching between read and write on
the same handle (see issues with PerlIO_flush() above). Offset is
of type "Off_t" which is a perl Configure value which may not be
same as stdio's "off_t".
PerlIO_tell(f)
This corresponds to ftell(). Returns the current file position, or
(Off_t) -1 on error. May just return value system "knows" without
making a system call or checking the underlying file descriptor (so
use on shared file descriptors is not safe without a
PerlIO_seek()). Return value is of type "Off_t" which is a perl
Configure value which may not be same as stdio's "off_t".
PerlIO_getpos(f,p), PerlIO_setpos(f,p)
These correspond (loosely) to fgetpos() and fsetpos(). Rather than
stdio's Fpos_t they expect a "Perl Scalar Value" to be passed. What
is stored there should be considered opaque. The layout of the data
may vary from handle to handle. When not using stdio or if
platform does not have the stdio calls then they are implemented in
terms of PerlIO_tell() and PerlIO_seek().
PerlIO_rewind(f)
This corresponds to rewind(). It is usually defined as being
PerlIO_seek(f,(Off_t)0L, SEEK_SET);
PerlIO_clearerr(f);
PerlIO_tmpfile()
This corresponds to tmpfile(), i.e., returns an anonymous PerlIO or
NULL on error. The system will attempt to automatically delete the
file when closed. On Unix the file is usually "unlink"-ed just
after it is created so it does not matter how it gets closed. On
other systems the file may only be deleted if closed via
PerlIO_close() and/or the program exits via "exit". Depending on
the implementation there may be "race conditions" which allow other
processes access to the file, though in general it will be safer in
this regard than ad. hoc. schemes.
PerlIO_setlinebuf(f)
This corresponds to setlinebuf(). Does not return a value. What
constitutes a "line" is implementation dependent but usually means
that writing "\n" flushes the buffer. What happens with things
like "this\nthat" is uncertain. (Perl core uses it only when
"dumping"; it has nothing to do with $| auto-flush.)
Co-existence with stdio
There is outline support for co-existence of PerlIO with stdio.
Obviously if PerlIO is implemented in terms of stdio there is no
problem. However in other cases then mechanisms must exist to create a
FILE * which can be passed to library code which is going to use stdio
calls.
The first step is to add this line:
#define PERLIO_NOT_STDIO 0
before including any perl header files. (This will probably become the
default at some point). That prevents "perlio.h" from attempting to
#define stdio functions onto PerlIO functions.
XS code is probably better using "typemap" if it expects FILE *
arguments. The standard typemap will be adjusted to comprehend any
changes in this area.
PerlIO_importFILE(f,mode)
Used to get a PerlIO * from a FILE *.
The mode argument should be a string as would be passed to
fopen/PerlIO_open. If it is NULL then - for legacy support - the
code will (depending upon the platform and the implementation)
either attempt to empirically determine the mode in which f is
open, or use "r+" to indicate a read/write stream.
Once called the FILE * should ONLY be closed by calling
"PerlIO_close()" on the returned PerlIO *.
The PerlIO is set to textmode. Use PerlIO_binmode if this is not
the desired mode.
This is not the reverse of PerlIO_exportFILE().
PerlIO_exportFILE(f,mode)
Given a PerlIO * create a 'native' FILE * suitable for passing to
code expecting to be compiled and linked with ANSI C stdio.h. The
mode argument should be a string as would be passed to
fopen/PerlIO_open. If it is NULL then - for legacy support - the
FILE * is opened in same mode as the PerlIO *.
The fact that such a FILE * has been 'exported' is recorded,
(normally by pushing a new :stdio "layer" onto the PerlIO *), which
may affect future PerlIO operations on the original PerlIO *. You
should not call "fclose()" on the file unless you call
"PerlIO_releaseFILE()" to disassociate it from the PerlIO *. (Do
not use PerlIO_importFILE() for doing the disassociation.)
Calling this function repeatedly will create a FILE * on each call
(and will push an :stdio layer each time as well).
PerlIO_releaseFILE(p,f)
Calling PerlIO_releaseFILE informs PerlIO that all use of FILE * is
complete. It is removed from the list of 'exported' FILE *s, and
the associated PerlIO * should revert to its original behaviour.
Use this to disassociate a file from a PerlIO * that was associated
using PerlIO_exportFILE().
PerlIO_findFILE(f)
Returns a native FILE * used by a stdio layer. If there is none, it
will create one with PerlIO_exportFILE. In either case the FILE *
should be considered as belonging to PerlIO subsystem and should
only be closed by calling "PerlIO_close()".
"Fast gets" Functions
In addition to standard-like API defined so far above there is an
"implementation" interface which allows perl to get at internals of
PerlIO. The following calls correspond to the various FILE_xxx macros
determined by Configure - or their equivalent in other implementations.
This section is really of interest to only those concerned with
detailed perl-core behaviour, implementing a PerlIO mapping or writing
code which can make use of the "read ahead" that has been done by the
IO system in the same way perl does. Note that any code that uses these
interfaces must be prepared to do things the traditional way if a
handle does not support them.
PerlIO_fast_gets(f)
Returns true if implementation has all the interfaces required to
allow perl's "sv_gets" to "bypass" normal IO mechanism. This can
vary from handle to handle.
PerlIO_fast_gets(f) = PerlIO_has_cntptr(f) && \
PerlIO_canset_cnt(f) && \
'Can set pointer into buffer'
PerlIO_has_cntptr(f)
Implementation can return pointer to current position in the
"buffer" and a count of bytes available in the buffer. Do not use
this - use PerlIO_fast_gets.
PerlIO_get_cnt(f)
Return count of readable bytes in the buffer. Zero or negative
return means no more bytes available.
PerlIO_get_ptr(f)
Return pointer to next readable byte in buffer, accessing via the
pointer (dereferencing) is only safe if PerlIO_get_cnt() has
returned a positive value. Only positive offsets up to value
returned by PerlIO_get_cnt() are allowed.
PerlIO_set_ptrcnt(f,p,c)
Set pointer into buffer, and a count of bytes still in the buffer.
Should be used only to set pointer to within range implied by
previous calls to "PerlIO_get_ptr" and "PerlIO_get_cnt". The two
values must be consistent with each other (implementation may only
use one or the other or may require both).
PerlIO_canset_cnt(f)
Implementation can adjust its idea of number of bytes in the
buffer. Do not use this - use PerlIO_fast_gets.
PerlIO_set_cnt(f,c)
Obscure - set count of bytes in the buffer. Deprecated. Only
usable if PerlIO_canset_cnt() returns true. Currently used in only
doio.c to force count less than -1 to -1. Perhaps should be
PerlIO_set_empty or similar. This call may actually do nothing if
"count" is deduced from pointer and a "limit". Do not use this -
use PerlIO_set_ptrcnt().
PerlIO_has_base(f)
Returns true if implementation has a buffer, and can return pointer
to whole buffer and its size. Used by perl for -T / -B tests.
Other uses would be very obscure...
PerlIO_get_base(f)
Return start of buffer. Access only positive offsets in the buffer
up to the value returned by PerlIO_get_bufsiz().
PerlIO_get_bufsiz(f)
Return the total number of bytes in the buffer, this is neither the
number that can be read, nor the amount of memory allocated to the
buffer. Rather it is what the operating system and/or
implementation happened to "read()" (or whatever) last time IO was
requested.
Other Functions
PerlIO_apply_layers(f,mode,layers)
The new interface to the USE_PERLIO implementation. The layers
":crlf" and ":raw" are only ones allowed for other implementations
and those are silently ignored. (As of perl5.8 ":raw" is
deprecated.) Use PerlIO_binmode() below for the portable case.
PerlIO_binmode(f,ptype,imode,layers)
The hook used by perl's "binmode" operator. ptype is perl's
character for the kind of IO:
'<' read
'>' write
'+' read/write
imode is "O_BINARY" or "O_TEXT".
layers is a string of layers to apply, only ":crlf" makes sense in
the non USE_PERLIO case. (As of perl5.8 ":raw" is deprecated in
favour of passing NULL.)
Portable cases are:
PerlIO_binmode(f,ptype,O_BINARY,NULL);
and
PerlIO_binmode(f,ptype,O_TEXT,":crlf");
On Unix these calls probably have no effect whatsoever. Elsewhere
they alter "\n" to CR,LF translation and possibly cause a special
text "end of file" indicator to be written or honoured on read. The
effect of making the call after doing any IO to the handle depends
on the implementation. (It may be ignored, affect any data which is
already buffered as well, or only apply to subsequent data.)
PerlIO_debug(fmt,...)
PerlIO_debug is a printf()-like function which can be used for
debugging. No return value. Its main use is inside PerlIO where
using real printf, warn() etc. would recursively call PerlIO and be
a problem.
PerlIO_debug writes to the file named by $ENV{'PERLIO_DEBUG'} or
defaults to stderr if the environment variable is not defined.
Typical use might be
Bourne shells (sh, ksh, bash, zsh, ash, ...):
PERLIO_DEBUG=/tmp/perliodebug.log ./perl -Di somescript some args
Csh/Tcsh:
setenv PERLIO_DEBUG /tmp/perliodebug.log
./perl -Di somescript some args
If you have the "env" utility:
env PERLIO_DEBUG=/tmp/perliodebug.log ./perl -Di somescript args
Win32:
set PERLIO_DEBUG=perliodebug.log
perl -Di somescript some args
On a Perl built without "-DDEBUGGING", or when the "-Di" command-
line switch is not specified, or under taint, PerlIO_debug() is a
no-op.
perl v5.26.3 2018-03-01 PERLAPIO(1)
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