Provided by: perl-doc_5.40.0-8_all 
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_fill(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);
Size_t PerlIO_unread(PerlIO *f,const void *vbuf, size_t count
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, const char *mode);
FILE *PerlIO_findFILE(PerlIO *f);
void PerlIO_releaseFILE(PerlIO *f,FILE *stdio);
int PerlIO_apply_layers(pTHX_ PerlIO *f, const char *mode,
const char *layers);
int PerlIO_binmode(pTHX_ 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 two 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 both 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_fill(f)
Fills the buffer associated with "f" with data from the layer below. "PerlIO_read"
calls this as part of its normal operation. Returns 0 upon success; -1 on failure.
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_unread(f,buf,count)
This allows one to unget more than a single byte. It effectively unshifts "count"
bytes onto the beginning of the buffer "buf", so that the next read operation(s) will
return them before anything else that was in the buffer.
Returns the number of unread bytes.
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(aTHX_ f,mode,layers)
The new interface to the USE_PERLIO implementation. The layers ":crlf" and ":raw" are
the 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(aTHX_ 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(aTHX_ f,ptype,O_BINARY,NULL);
and
PerlIO_binmode(aTHX_ 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.