path: root/static/unix-v10/man3/sbuf.prot.3

.  \"ident	"%W%"
.  \"Copyright (c) 1984 AT&T
.  \"All Rights Reserved
.  \"THIS IS UNPUBLISHED PROPRIETARY SOURCE CODE OF AT&T
.  \"The copyright notice above does not evidence any
.  \"actual or intended publication of such source code.
.TH SBUF.PROT 3I+ "C++ Stream Library" " "
.SH NAME
streambuf \- interface for derived classes
.SH SYNOPSIS
.nf
.ta1i 2i
.ft B
#include <iostream.h>

typedef long streamoff, streampos;
class ios {
public:
	enum	seek_dir { beg, cur, end };
	enum	open_mode { in, out, ate, app } ;
	// \fIand lots of other stuff ... \fP
	} ;

class streambuf {
public:
		streambuf() ;
		streambuf(char* p, int len, int i=0);

	void	dbp() ;
protected:
	int	allocate();
	char*	base();
	int	blen();
	char*	eback();
	char* 	ebuf();
	char*	egptr();
	char*	epptr();
	void	gbump(int n);
	char*	gptr();
	char*	pbase();
	void	pbump(int n); 
	char*	pptr();
	void	setg(char* eb,char* g, char* eg);
	void	setp(char* p, char* ep);
	void	setb(char* b, char* eb, int a = 0 );
	int	unbuffered();
	void	unbuffered(int);

	virtual int 	doallocate();
	virtual int	pbackfail(int c);
	virtual	~streambuf() ;
public:
	virtual int	overflow(int c=EOF);
	virtual int	underflow();
	virtual streambuf*
		setbuf(char* p, int len);
	virtual streampos
		seekpos(streampos,open_mode=input|output);
	virtual streampos
		seekoff(streamoff,seek_dir,open_mode=input|output);
	virtual int	sync();
};
.fi
.ft R
.SH DESCRIPTION
\f(CWstreambuf\fRs implement the buffer abstraction described in
\fIsbuf.pub\fR(3C++).  But the \f(CWstreambuf\fR class itself contains
only basic members for manipulating the characters and normally
a class derived from \f(CWstreambuf\fR will be used.  This man page
describes the interface needed by programmers who are
coding a derived class.
Broadly speaking there are two kinds of members described here.
The non-virtual functions are provided for manipulating a \fBstreambuf\fR
in ways that are appropriate in a derived class.
Their descriptions reveal details of the implementation that would
be inappropriate in the public interface.
The virtual functions permit the derived class to specialize the
\fBstreambuf\fR class in ways appropriate to the specific sources
and sinks that it is implementing.
The descriptions of virtuals explain the obligations of the
virtuals of the derived class.  If the virtuals behave as specified,
the \fBstreambuf\fR will behave as specified in the
public interface.  However, if the virtuals do not behave as
specified, then the \f(CWstreambuf\fR may not behave properly,
and an \f(CWiostream\fR (or any other code) that relies on proper
behavior of the \f(CWstreambuf\fR may not behave properly either.
.PP
Assume
.br
\(em \fBsb\fR is a \f(CWstreambuf*\fR.
.br
\(em \fBi\fR and \fBn\fR are \f(CWint\fR.
.br
\(em \fBptr\fR, \fBb\fR, \fBeb\fR, \fBp\fR, \fBep\fR, \fBeb\fR, \fBg\fR,
and \fBeg\fR are \f(CWchar*\fR.
.br
\(em \fBc\fR is an \f(CWint\fR character (positive or \f(CWEOF\fR)).
.br
\(em \fBpos\fR is a \f(CWstreampos\fR. (See \fIiostream\fR(3C++).)
.br
\(em \fBoff\fR is a \f(CWstreamoff\fR.
.br
\(em \fBdir\fR is a \f(CWseekdir\fR.
.br
\(em \fBmode\fR is a \f(CWopen_mode\fR.
.PP
Constructors:
.TP
\fBstreambuf()\fR
Constructs
an empty buffer corresponding to an empty sequence.
.TP
\fBstreambuf(b,len,i)\fR
Constructs an empty buffer and then sets up the reserve area
to be the \fBlen\fR bytes starting at \fBb\fR. (\fBi\fR is present
for backward compatibility with the stream package\fR.  The effect
if it is not 0 is undefined.) 
.PP
The protected members of 
\f(CWstreambuf\fR
present an interface to derived classes organized around
three areas (arrays of bytes) managed cooperatively by
the base and derived classes.
They are the get area, the put area, and the reserve area.
The get and the put area are normally disjoint, but they
may both overlap the reserve area, whose primary purpose is
to be a a resource in which
space for the put and get areas can be allocated.  The get and
the put areas are changed as characters are put into and
gotten from the buffer, but the reserve area normally remains
fixed.
The areas are defined by a collection of \f(CWchar*\fR values.
The buffer abstraction is described in terms of pointers that point
between characters, but the \f(CWchar*\fR values must point at
\f(CWchar\fRs.
To establish a correspondence the \f(CWchar*\fR values should be thought
of as pointing just before the byte they really point at.
.PP
Functions to examine the pointers are:
.TP
\fBptr=sb->base()\fR
Returns a pointer to the first byte of the reserve area.
 Space between \fBsb->base()\fR
and \fBsb->ebase()\fR is the reserve area.
.TP
\fBptr=sb->eback()\fR
Returns a pointer to a lower bound on
\fBsb->gptr()\fR.
Space between \fBsb->eback()\fR and \fBsb->gptr()\fR is available
for putback.
.TP
\fBptr=sb->ebuf()\fR
Returns a pointer to the byte after the last byte of the reserve area.
.TP
\fBptr=sb->egptr()\fR
Returns a pointer to the byte after the last byte of the get area.
.TP
\fBptr=sb->epptr()\fR
Returns a pointer to the byte after the last byte of the put area.
.TP
\fBptr=sb->gptr()\fR
Returns a pointer to the first byte of the get area.
The available characters are those between \fBsb->gptr()\fR
and \fBsb->egptr()\fR. The next character fetched will
be \fB*sb->gptr()\fR unless \fBsb->egptr()\fR is less than
or equal to \fBsb->gptr()\fR.
.TP
\fBptr=sb->pbase()\fR
Returns a pointer to the put area base.
Characters between \fBsb->pbase()\fR and \fBsb->pptr()\fR
have been storeded into the buffer and not yet consumed.
.TP
\fBptr=sb->pptr()\fR
Returns a pointer to the first byte of the put area.
The space between \fBsb->pptr()\fR
and \fBsb->epptr()\fR is the put area and characters will be storeed
here.
.PP
The member functions for setting the pointers:
.TP
\fBsb->setb(b,eb,i)\fR
Sets \fBbase\fR and \fBebase\fR to \fBb\fR and \fBeb\fR respectively.
\fBi\fR controls whether the area will be subject to
automatic deletion.
If \fBi\fR is non zero, then
\f(CWdelete\fB b\fR will be done when \fBbase\fR is changed by
another call of \fBsetb\fR, or when the destructor is called for
\fB*sb\fR.
If \fBb\fR and \fBeb\fR
are both null then we say that there is no reserve area.
If \fBb\fR is non-null, there is a reserve area even if
\fBeb\fR is less than \fBb\fR and so the reserve area
has zero length.
.TP
\fBsb->setp(p,ep)\fR
Sets \fBpptr\fR to \fBp\fR, \fBpbase\fR to \fBp\fR, and \fBepptr\R
to \fBep\fR.
.TP
\fBsb->setg(eb,g,eg)\fR
Sets \fBeback\fR to \fBeb\fR, \fBgptr\fR to \fBg\fR, and \fBegptr\fR
to \fBeg\fR.
.PP
Other non-virtual members:
.TP
\fBi=sb->allocate()\fR
Tries to set up a reserve area.
If a reserve area already exists or if \fBsb->unbuffered()\fR
is nonzero returns 0 without doing anything.
If the attempt to allocate space fails \fBallocate\fR
returns \f(CWEOF\fR.  Otherwise (allocation succeeds)
\fBallocate\fR returns 1.  \fBallocate\fR is not called by
any member of \f(CWstreambuf\fR except virtuals.
.TP
\fBi=sb->blen()\fR
Returns the current size (in chars) of the current reserve area.
.TP
\fBdbp()\fR
Writes directly on file descriptor 1 
information in ASCII about the state of the
buffer.  It is intended for debugging and nothing
is specified about the form of the output.  It is considered part
of the protected interface because the information it prints can
only be understood in relation to that interface, but it is a public
function so that it can be called anywhere during debugging.  
.TP
\fBsb->gbump(n)\fR
Increments \fBgptr\fR by \fBn\fR
which may be positive or negative.
No checks are made on whether the new
value of \fBgptr\fR is in bounds.
.TP
\fBsb->pbump(n)\fR
Increments \fBpptr\fR by \fBn\fR
which may be positive or negative.
No checks are made on whether the new
value of \fBpptr\fR is in bounds.
.sp
.nf
.in -.5i
\fBsb->unbuffered(i)\fR
\fBi=sb->unbuffered()\fR
.in
.fi
There is a private variable known as \fBsb\fR's buffering state.
\fBsb->unbuffered(i)\fR sets the value of this variable
to \fBi\fR and \fBsb->unbuffered()\fR returns the current value.
This state is independent of the actual
allocation of a reserve area.  Its primary purpose is to
control whether a reserve area is allocated automatically
by \fBallocate\fR.
.PP
Virtual functions must be redefined in
derived classes to specialize the behavior of \fBstreambuf\fRs:
.TP
\fBi=sb->doallocate()\fR
Is called when \fBallocate\fR determines
that space is needed.
\fBdoallocate\fR is required to call \fBsetb\fR to provide a reserve
area or to return \f(CWEOF\fR if it cannot.  It is only called
if \fBsb->unbuffered()\fR is non-zero and \fBsb->base()\fR is non-zero.
.TP
\fBi=overflow(c)\fR
Is called to consume characters.  If \fBc\fR is not \f(CWEOF\fR
it also must either save \fBc\fR or consume it.  
Usually it is called when the put area is full and
an attempt is being made to store a new character, but
it can be called at other times.
The normal action is to consume the characters between \fBpbase\fR
and \fBpptr\fR, call \fBsetp\fR to establish a new put area, and
if \fBc\f(CW!=EOF\fR store it (using \fBsputc\fR).
If \fBsb->unbuffered()\fR is non-zero,
\fBoverflow\fR is not allowed to call \fBsetp\fR and 
so must consume \fBn\fR
\fBsb->overflow\fR
should return \fBEOF\fR to indicate an error; otherwise it should
return something else.
.TP
\fBi=sb->pbackfail(c)
Is called when \fBeback\fR equals \fBgptr\fR and an attempt 
has been made to putback \fBc\fR.
If this situation can be dealt with (e.g., by repositioning
an external file), \fBpbackfail\fR should return \fBc\fR;
otherwise it should return \f(CWEOF\fR.
.TP
\fBpos=sb->seekoff(off,dir,mode)\fR
Repositions the get and/or put pointers  (i.e., the abstract
get and put pointers, not \fBpptr\fR and \fBgptr\fR).  The
meanings of \fBoff\fR and \fBdir\fR
are discussed in 
\fIsbuf.pub\fR(3C++).
\fBmode\fR specifies whether the put pointer (\fBoutput\fR bit set) or
the get pointer (\fBinput\fR bit set) is to be modified.  Both bits
may be set in which case both pointers should be affected.
A class derived from \fBstreambuf\fR is not required to
support repositioning.  \fBseekoff\fR should return \f(CWEOF\fR if 
the class does not support repositioning.  If the class does
support repositioning, \fBseekoff\fR should return the new
position or \f(CWEOF\fR on error.
.TP
\fBpos=sb->seekpos(pos,mode)\fR
Repositions the streambuf get and/or put pointer to \fBpos\fR.
\fBmode\fR specifies which pointers are affected as for \fBseekoff\fR.
Returns \fBpos\fR (the argument) or \f(CWEOF\fR if the class does
not support repositioning or an error occurs.
.TP
\fBsb=sb->setbuf(ptr,len)\fR
Offers the array at \fBptr\fR with \fBlen\fR bytes should
be used as a reserve
area.   The normal interpretation is that
if \fBptr\fR or \fBlen\fR are zero then this is a request
to make the \fBsb\fR unbuffered.
The derived class may use this area or not as it chooses.
If may accept or ignore the request for unbuffered state as it
chooses.
\fBsetbuf\fR should return \fBsb\fR if it honors the request.
Otherwise it should return 0.
.TP
\fBi=sb->sync()\fR
Is called to give the derived class
a chance to
look at the state of the areas, and synchronize
them with any external representation.
Normally \fBsync\fR should
consume any characters that have been storeed into the put area,
and if possible give back to the source any characters in the get area
that have not been fetched.  When \fBsync\fR returns there should not
be any unconsumed characters, and the get area should be empty.
\fBsync\fR should return \fBEOF\fR if some kind of failure occurs.
.TP
\fBi=sb->underflow()\fR
Is called to supply characters for fetching, i.e.,
to create a condition in which the get area is not empty.
If it is called when there are characters in the get area
it should return the first character.  If the get area is empty
it should create a nonempty get area 
and return the next character (which it should also
leave in the get area).
If there are no more characters available
\fBunderflow\fR
should return \f(CWEOF\fR and leave an empty put area.
.PP
The default definitions of the virtual functions:
.TP
\fBi=sb->streambuf::doallocate()\fR
Attempts to allocate a reserve area using \f(CWoperator new\fR.
.TP
\fBi=sb->streambuf::overflow(n)\fR
Is compatible
with the old stream package, but that behavior is not
considered part of the specification of the iostream package.
So \fBstreambuf::overflow\fR should be treated as if 
it had undefined behavior.  That is, derived classes should
always define it.
.TP
\fBi=sb->streambuf::pbackfail(n)
Returns \f(CWEOF\fR.
.TP
\fBpos=sb->streambuf::seekpos(pos,mode)\fR
Returns \fBsb->seekoff(streamoff(pos),seek_beg,mode)\fR.
Thus to define seeking in a derived class, it is frequently
only necessary to define 
\fBseekoff\fR and use the inherited \fBstreambuf::seekpos\fR.
.TP
\fBpos=sb->streambuf::seekoff(off,dir,mode)\fR
Returns \f(CWEOF\fR.
.TP
\fBsb=sb->streambuf::setbuf(ptr,len)\fR
Will honor the request when ever there is no reserve area.
.TP
\fBi=sb->streambuf::sync()\fR
Returns 0 if the get area is empty and there are no unconsumed
characters.  Otherwise it returns \f(CWEOF\fR.
.TP
\fBi=sb->streambuf::underflow()\fR
Is compatible
with the old stream package, but that behavior is not
considered part of the specification of the iostream package.
So \fBstreambuf::underflow\fR should be treated as if 
it had undefined behavior.  That is, it should always be defined
in derived classes.
.SH CAVEATS
The constructors are public for compatibility with the
old stream package.
They ought to be protected.
.PP
The interface for unbuffered actions is awkward.
It's hard to write \fBunderflow\fR and \fBoverflow\fR
virtuals that behave properly
for unbuffered \f(CWstreambuf\fRs without special casing.
Also there is no way for the virtuals to react sensibly to
multi character gets or puts.
.PP
Although the public interface to \f(CWstreambuf\fRs
deals in characters and bytes,
the interface to derived classes deals in \f(CWchar\fRs.
Since a decision had to be made on the types of the real data
pointers, it seemed easier to reflect that choice in the
types of the protected members than to duplicate all
the members with both plain and unsigned char versions.
But perhaps all these uses of \f(CWchar*\fR ought to have been
with a typedef.
.PP 
The implementation contains a variant
of  \fBsetbuf\fR that accepts a third argument.
It is present only for compatibility
with the old stream package.
.SH SEE ALSO
sbuf.pub(3C++)
streambuf(3C++)
iostream(3C++)