PERLAPIO(1) Perl Programmers Reference Guide PERLAPIO(1)
NAME
perlapio - perl's IO abstraction interface.
SYNOPSIS
#define PERLIONOTSTDIO 0 /* For co-existence with stdio only */
#include /* Usually via #include */
PerlIO *PerlIOstdin(void);
PerlIO *PerlIOstdout(void);
PerlIO *PerlIOstderr(void);
PerlIO *PerlIOopen(const char *path,const char *mode);
PerlIO *PerlIOfdopen(int fd, const char *mode);
PerlIO *PerlIOreopen(const char *path, const char *mode, PerlIO *old); /* deprecated */
int PerlIOclose(PerlIO *f);
int PerlIOstdoutf(const char *fmt,...)
int PerlIOputs(PerlIO *f,const char *string);
int PerlIOputc(PerlIO *f,int ch);
int PerlIOwrite(PerlIO *f,const void *buf,sizet numbytes);
int PerlIOprintf(PerlIO *f, const char *fmt,...);
int PerlIOvprintf(PerlIO *f, const char *fmt, valist args);
int PerlIOflush(PerlIO *f);
int PerlIOeof(PerlIO *f);
int PerlIOerror(PerlIO *f);
void PerlIOclearerr(PerlIO *f);
int PerlIOgetc(PerlIO *d);
int PerlIOungetc(PerlIO *f,int ch);
int PerlIOread(PerlIO *f, void *buf, sizet numbytes);
int PerlIOfileno(PerlIO *f);
void PerlIOsetlinebuf(PerlIO *f);
Offt PerlIOtell(PerlIO *f);
int PerlIOseek(PerlIO *f, Offt offset, int whence);
void PerlIOrewind(PerlIO *f);
int PerlIOgetpos(PerlIO *f, SV *save); /* prototype changed */
int PerlIOsetpos(PerlIO *f, SV *saved); /* prototype changed */
int PerlIOfastgets(PerlIO *f);
int PerlIOhascntptr(PerlIO *f);
int PerlIOgetcnt(PerlIO *f);
char *PerlIOgetptr(PerlIO *f);
void PerlIOsetptrcnt(PerlIO *f, char *ptr, int count);
int PerlIOcansetcnt(PerlIO *f); /* deprecated */
void PerlIOsetcnt(PerlIO *f, int count); /* deprecated */
int PerlIOhasbase(PerlIO *f);
char *PerlIOgetbase(PerlIO *f);
int PerlIOgetbufsiz(PerlIO *f);
PerlIO *PerlIOimportFILE(FILE *stdio, const char *mode);
FILE *PerlIOexportFILE(PerlIO *f, int flags);
FILE *PerlIOfindFILE(PerlIO *f);
void PerlIOreleaseFILE(PerlIO *f,FILE *stdio);
int PerlIOapplylayers(PerlIO *f, const char *mode, const char *layers);
int PerlIObinmode(PerlIO *f, int ptype, int imode, const char *layers);
void PerlIOdebug(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. USESTDIO
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 perl5.00302.
2. USESFIO
A "legacy" implementation in terms of the "sfio" library. Used for
some specialist applications on Unix machines ("sfio" is not widely
ported away from Unix). Most of above are #define'd to the sfio
functions. PerlIO * is in this case Sfiot *.
3. USEPERLIO
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 USEPERLIO 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 imple-
mentation 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 "PerlIOapplylayers()"
does anything "interesting".
The USEPERLIO 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 implementa-
tion. 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.
PerlIOstdin(), PerlIOstdout(), PerlIOstderr()
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.
PerlIOopen(path,, mode), PerlIOfdopen(fd,,mode)
These correspond to fopen()/fdopen() and the arguments are the
same. Return "NUL" 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 "NUL" is returned if this limit is exceeded.
PerlIOreopen(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 descrip-
tor used by the existing PerlIO. This may become the behaviour of
this function in the future.
PerlIOprintf(f,,fmt,,...), PerlIOvprintf(f,,fmt,,a)
These are fprintf()/vfprintf() equivalents.
PerlIOstdoutf(fmt,,...)
This is printf() equivalent. printf is #defined to this function,
so it is (currently) legal to use "printf(fmt,...)" in perl
sources.
PerlIOread(f,,buf,,count), PerlIOwrite(f,,buf,,count)
These correspond functionally to fread() and fwrite() but the argu-
ments and return values are different. The PerlIOread() and Per-
lIOwrite() 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 distin-
guish 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.
PerlIOclose(f)
Depending on implementation "errno" may be "EINTR" if operation was
interrupted by a signal.
PerlIOputs(f,,s), PerlIOputc(f,,c)
These correspond to fputs() and fputc(). Note that arguments have
been revised to have "file" first.
PerlIOungetc(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..0xF 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.
PerlIOgetc(f)
This corresponds to getc(). Despite the c in the name only byte
range 0..0xF is supported. Returns the character read or -1
("EOF") on error.
PerlIOeof(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 PerlIOseek(), or PerlIOrewind().
PerlIOerror(f)
This corresponds to ferror(). Returns a true/false indication of
whether there has been an IO error on the handle.
PerlIOfileno(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.
PerlIOclearerr(f)
This corresponds to clearerr(), i.e., clears 'error' and (usually)
'eof' flags for the "stream". Does not return a value.
PerlIOflush(f)
This corresponds to fflush(). Sends any buffered write data to the
underlying file. If called with "NUL" this may flush all open
streams (or core dump with some USESTDIO implementations). Call-
ing 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
USESTDIO implementations. The USEPERLIO (layers) implementation
tries to behave better: it flushes all open streams when passed
"NUL", and attempts to retain data on read streams either in the
buffer or by seeking the handle to the current logical position.
PerlIOseek(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 desciptor 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 PerlIOflush() above). Offset is
of type "Offt" which is a perl Configure value which may not be
same as stdio's "offt".
PerlIOtell(f)
This corresponds to ftell(). Returns the current file position, or
(Offt) -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 Per-
lIOseek()). Return value is of type "Offt" which is a perl Con-
figure value which may not be same as stdio's "offt".
PerlIOgetpos(f,,p), PerlIOsetpos(f,,p)
These correspond (loosely) to fgetpos() and fsetpos(). Rather than
stdio's Fpost 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 plat-
form does not have the stdio calls then they are implemented in
terms of PerlIOtell() and PerlIOseek().
PerlIOrewind(f)
This corresponds to rewind(). It is usually defined as being
PerlIOseek(f,(Offt)0L, SEKSET);
PerlIOclearerr(f);
PerlIOtmpfile()
This corresponds to tmpfile(), i.e., returns an anonymous PerlIO or
NUL 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 Per-
lIOclose() 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.
PerlIOsetlinebuf(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. Obvi-
ously 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 PERLIONOTSTDIO 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 * argu-
ments. The standard typemap will be adjusted to comprehend any changes
in this area.
PerlIOimportFILE(f,,mode)
Used to get a PerlIO * from a FILE *.
The mode argument should be a string as would be passed to
fopen/PerlIOopen. If it is NUL 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 "Per-
lIOclose()" on the returned PerlIO *.
The PerlIO is set to textmode. Use PerlIObinmode if this is not
the desired mode.
This is not the reverse of PerlIOexportFILE().
PerlIOexportFILE(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/Per-
lIOopen. If it is NUL 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, (nor-
mally 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 "Per-
lIOreleaseFILE()" to disassociate it from the PerlIO *. (Do not
use PerlIOimportFILE() 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).
PerlIOreleaseFILE(p,,f)
Calling PerlIOreleaseFILE 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 PerlIOexportFILE().
PerlIOfindFILE(f)
Returns a native FILE * used by a stdio layer. If there is none, it
will create one with PerlIOexportFILE. In either case the FILE *
should be considered as belonging to PerlIO subsystem and should
only be closed by calling "PerlIOclose()".
""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 FILExxx 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 han-
dle does not support them.
PerlIOfastgets(f)
Returns true if implementation has all the interfaces required to
allow perl's "svgets" to "bypass" normal IO mechanism. This can
vary from handle to handle.
PerlIOfastgets(f) = PerlIOhascntptr(f) && \
PerlIOcansetcnt(f) && \
`Can set pointer into buffer'
PerlIOhascntptr(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 PerlIOfastgets.
PerlIOgetcnt(f)
Return count of readable bytes in the buffer. Zero or negative
return means no more bytes available.
PerlIOgetptr(f)
Return pointer to next readable byte in buffer, accessing via the
pointer (dereferencing) is only safe if PerlIOgetcnt() has
returned a positive value. Only positive offsets up to value
returned by PerlIOgetcnt() are allowed.
PerlIOsetptrcnt(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 pre-
vious calls to "PerlIOgetptr" and "PerlIOgetcnt". The two val-
ues must be consistent with each other (implementation may only use
one or the other or may require both).
PerlIOcansetcnt(f)
Implementation can adjust its idea of number of bytes in the
buffer. Do not use this - use PerlIOfastgets.
PerlIOsetcnt(f,,c)
Obscure - set count of bytes in the buffer. Deprecated. Only
usable if PerlIOcansetcnt() returns true. Currently used in only
doio.c to force count less than -1 to -1. Perhaps should be Per-
lIOsetempty or similar. This call may actually do nothing if
"count" is deduced from pointer and a "limit". Do not use this -
use PerlIOsetptrcnt().
PerlIOhasbase(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...
PerlIOgetbase(f)
Return start of buffer. Access only positive offsets in the buffer
up to the value returned by PerlIOgetbufsiz().
PerlIOgetbufsiz(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 implementa-
tion happened to "read()" (or whatever) last time IO was requested.
Other Functions
PerlIOapplylayers(f,mode,layers)
The new interface to the USEPERLIO implementation. The layers
":crlf" and ":raw" are only ones allowed for other implementations
and those are silently ignored. (As of perl5.8 ":raw" is depre-
cated.) Use PerlIObinmode() below for the portable case.
PerlIObinmode(f,ptype,imode,layers)
The hook used by perl's "binmode" operator. ptype is perl's char-
acter for the kind of IO:
'<' read
'>' write
']' read/write
imode is "OBINARY" or "OTEXT".
layers is a string of layers to apply, only ":crlf" makes sense in
the non USEPERLIO case. (As of perl5.8 ":raw" is deprecated in
favour of passing NUL.)
Portable cases are:
PerlIObinmode(f,ptype,OBINARY,Nullch);
and
PerlIObinmode(f,ptype,OTEXT,":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.)
PerlIOdebug(fmt,...)
PerlIOdebug 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.
PerlIOdebug writes to the file named by $ENV{'PERLIODEBUG'} typi-
cal use might be
Bourne shells (sh, ksh, bash, zsh, ash, ...):
PERLIODEBUG=/dev/tty ./perl somescript some args
Csh/Tcsh:
setenv PERLIODEBUG /dev/tty
./perl somescript some args
If you have the "env" utility:
env PERLIODEBUG=/dev/tty ./perl somescript some args
Win32:
set PERLIODEBUG=CON
perl somescript some args
If $ENV{'PERLIODEBUG'} is not set PerlIOdebug() is a no-op.
perl v5.8.6 2004-11-05 PERLAPIO(1)
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