libdar::cache Class Reference

the cache class implements a fixed length read/write caching mechanism More...

#include <cache.hpp>

Inheritance diagram for libdar::cache:

Collaboration diagram for libdar::cache:

Public Member Functions
	cache (generic_file &hidden, bool shift_mode, U_I size=102400)
	cache (const cache &ref)=delete
	cache (cache &&ref)=delete
cache &	operator= (const cache &ref)=delete
cache &	operator= (cache &&ref)=delete
void	change_to_read_write ()
virtual bool	skippable (skippability direction, const infinint &amount) override
	whether the implementation is able to skip More...
virtual bool	skip (const infinint &pos) override
	skip at the absolute position More...
virtual bool	skip_to_eof () override
	skip to the end of file
virtual bool	skip_relative (S_I x) override
	skip relatively to the current position
virtual bool	truncatable (const infinint &pos) const override
	whether the implementation is able to truncate to the given position
virtual infinint	get_position () const override
	get the current read/write position
Public Member Functions inherited from libdar::generic_file
	generic_file (gf_mode m)
	main constructor
	generic_file (const generic_file &ref)
	copy constructor
	generic_file (generic_file &&ref) noexcept
	move constructor
generic_file &	operator= (const generic_file &ref)
	assignment operator
generic_file &	operator= (generic_file &&ref) noexcept
	move operator
	~generic_file () noexcept(false)
	virtual destructor, More...
void	terminate ()
	destructor-like call, except that it is allowed to throw exceptions
bool	operator== (generic_file &ref)
bool	operator!= (generic_file &ref)
gf_mode	get_mode () const
	retreive the openning mode for this object
void	read_ahead (const infinint &amount)
void	ignore_read_ahead (bool mode)
virtual U_I	read (char *a, U_I size) override
	read data from the generic_file inherited from proto_generic_file
virtual void	write (const char *a, U_I size) override
	write data to the generic_file inherited from proto_generic_file
void	write (const std::string &arg)
	write a string to the generic_file More...
S_I	read_back (char &a)
	skip back one char, read on char and skip back one char
S_I	read_forward (char &a)
	read one char
virtual void	truncate (const infinint &pos)
	truncate file at the given offset More...
virtual void	copy_to (generic_file &ref)
	copy all data from current position to the object in argument
virtual void	copy_to (generic_file &ref, const infinint &crc_size, crc *&value)
	copy all data from the current position to the object in argument and computes a CRC value of the transmitted data More...
U_32	copy_to (generic_file &ref, U_32 size)
	small copy (up to 4GB) with CRC calculation
infinint	copy_to (generic_file &ref, infinint size)
	copy the given amount to the object in argument
bool	diff (generic_file &f, const infinint &me_read_ahead, const infinint &you_read_ahead, const infinint &crc_size, crc *&value)
	compares the contents with the object in argument More...
bool	diff (generic_file &f, const infinint &me_read_ahead, const infinint &you_read_ahead, const infinint &crc_size, crc *&value, infinint &err_offset)
void	reset_crc (const infinint &width)
	reset CRC on read or writen data More...
bool	crc_status () const
	to known whether CRC calculation is activated or not
crc *	get_crc ()
	get CRC of the transfered date since last reset More...
void	sync_write ()
	write any pending data
void	flush_read ()
	be ready to read at current position, reseting all pending data for reading, cached and in compression engine for example
Public Member Functions inherited from libdar::proto_generic_file
	proto_generic_file (const proto_generic_file &ref)=default
	copy constructor
	proto_generic_file (proto_generic_file &&ref) noexcept=default
	move constructor
proto_generic_file &	operator= (const proto_generic_file &ref)=default
	assignment operator
proto_generic_file &	operator= (proto_generic_file &&ref) noexcept=default
	move operator
virtual	~proto_generic_file () noexcept(false)
	virtual destructor More...

Protected Member Functions
virtual void	inherited_read_ahead (const infinint &amount) override
	tells the object that several calls to read() will follow to probably obtain at least the given amount of data More...
virtual U_I	inherited_read (char *a, U_I size) override
	implementation of read() operation More...
virtual void	inherited_write (const char *a, U_I size) override
	implementation of the write() operation More...
virtual void	inherited_truncate (const infinint &pos) override
	truncate file at the give offset More...
virtual void	inherited_sync_write () override
	write down any pending data More...
virtual void	inherited_flush_read () override
	reset internal engine, flush caches in order to read the data at current position More...
virtual void	inherited_terminate () override
	destructor-like call, except that it is allowed to throw exceptions More...
Protected Member Functions inherited from libdar::generic_file
void	set_mode (gf_mode x)
bool	is_terminated () const

Private Member Functions
bool	need_flush_write () const
void	alloc_buffer (size_t x_size)
	allocate x_size byte in buffer field and set size accordingly
void	release_buffer ()
	release memory set buffer to nullptr and size to zero
void	shift_by_half ()
void	clear_buffer ()
void	flush_write ()
void	fulfill_read ()
U_I	available_in_cache (skippability direction) const

Private Attributes
generic_file *	ref
	underlying file, (not owned by "this', not to be delete by "this")
char *	buffer
	data in transit
U_I	size
	allocated size
U_I	half
	precalculated half = size / 2
U_I	next
	next to read or next place to write to
U_I	last
	first byte of invalid data in the cache. we have: next <= last < size
U_I	first_to_write
	position of the first byte that need to be written. if greater than last, no byte need writing
infinint	buffer_offset
	position of the first byte in buffer
bool	shifted_mode
	whether to half flush and shift or totally flush data
infinint	eof_offset
	size of the underlying file (read-only mode), set to zero if unknown

Additional Inherited Members
Public Types inherited from libdar::generic_file
enum	skippability { skip_backward , skip_forward }

Detailed Description

the cache class implements a fixed length read/write caching mechanism

it is intended to reduce context switches when no compression is used and when reading or writing catalogue through a pipe. The catalogue read and write is done by calling dump/constructor methods of the many objects that a catalogue contains. This makes a lot of small reads or writes, which make very poor performances when used over the network through a pipe to ssh. When compression is used, the problem disapears as the compression engine gather these many small reads or writes into much bigger ones. This in only when there is no compression or encryption that this class is useful (and used). Another target of class cache is to provide limited skippability when data is read of written to pipe (which do not have any skippability)

Definition at line 52 of file cache.hpp.

Constructor & Destructor Documentation

cache()

libdar::cache::cache	(	generic_file &	hidden,
		bool	shift_mode,
		U_I	size = 102400
	)

Parameters

hidden	is the file to cache, it is never deleted by the cache object,
shift_mode	if true, when all cached data has been read, half of the data is flushed from the cache, the other half is shifted and new data take place to fill the cache. This is necessary for sequential reading, but has some CPU overhead.
size	is the (fixed) size of the cache

Member Function Documentation

inherited_flush_read()

virtual void libdar::cache::inherited_flush_read ( )

inlineoverrideprotectedvirtual

reset internal engine, flush caches in order to read the data at current position

Note

when the object relies on external object or system object to fetch the data from for reading, when a call to (inherited_)flush_read() occurs, the current object must not assume that any previously read data is still valid if it has internal buffers or the like and it should flush them asap. This call must not propagate the flush_read to any other gneric_file object it could rely on

Implements libdar::generic_file.

Definition at line 82 of file cache.hpp.

inherited_read()

virtual U_I libdar::cache::inherited_read ( char *  a, U_I  size  )

overrideprotectedvirtual

implementation of read() operation

Parameters

[in,out]	a	where to put the data to read
[in]	size	says how much data to read

Returns

the exact amount of data read and put into 'a'

Note

read as much byte as requested, up to end of file stays blocked if not enough data is available and EOF not yet met. May return less data than requested only if EOF as been reached. in other worlds, EOF is reached when returned data is stricly less than the requested data Any problem shall be reported by throwing an exception.

Implements libdar::generic_file.

inherited_read_ahead()

virtual void libdar::cache::inherited_read_ahead ( const infinint &  amount )

overrideprotectedvirtual

tells the object that several calls to read() will follow to probably obtain at least the given amount of data

Parameters

[in]

amount

is the maximum expected amount of data that is known to be read

Note

this call may be implemented as a do-nothing call, its presence is only to allow optimization when possible, like in multi-threaded environment

Implements libdar::generic_file.

inherited_sync_write()

virtual void libdar::cache::inherited_sync_write ( )

inlineoverrideprotectedvirtual

write down any pending data

Note

called after sanity checks from generic_file::sync_write() this method's role is to write down any data pending for writing in the current object it has not to be propagated to other gneric_file object this object could rely on

Implements libdar::generic_file.

Definition at line 81 of file cache.hpp.

inherited_terminate()

virtual void libdar::cache::inherited_terminate ( )

inlineoverrideprotectedvirtual

destructor-like call, except that it is allowed to throw exceptions

Note

this method must never be called directly but using terminate() instead, generic_file class manages it to never be called more than once

Implements libdar::generic_file.

Definition at line 83 of file cache.hpp.

inherited_truncate()

virtual void libdar::cache::inherited_truncate ( const infinint &  pos )

overrideprotectedvirtual

truncate file at the give offset

Note

if pos is greater than the current file size, this call may do nothing (not even enlarging the file)

this call should always fail on a read-only generic_file

implementation must throw exception if truncate is not possible for other reason than read/write access mode

Implements libdar::generic_file.

inherited_write()

virtual void libdar::cache::inherited_write ( const char *  a, U_I  size  )

overrideprotectedvirtual

implementation of the write() operation

Parameters

[in]	a	what data to write
[in]	size	amount of data to write

Note

must either write all data or report an error by throwing an exception

Implements libdar::generic_file.

skip()

virtual bool libdar::cache::skip ( const infinint &  pos )

overridevirtual

skip at the absolute position

Parameters

[in]

pos

the offset in byte where next read/write operation must start

Returns

true if operation was successfull and false if the requested position is not valid (after end of file)

Note

if requested position is not valid the reading/writing cursor must be set to the closest valid position

Implements libdar::generic_file.

Referenced by libdar::cache_global::skip().

skippable()

virtual bool libdar::cache::skippable ( skippability  direction, const infinint &  amount  )

overridevirtual

whether the implementation is able to skip

Note

the capability to skip does not mean that skip_relative() or skip() will succeed, but rather that the inherited class implementation does not by construction forbid the requested skip (like inherited class providing a generic_file interface of an anonymous pipe for example)

Implements libdar::generic_file.

Referenced by libdar::cache_global::skippable().

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The documentation for this class was generated from the following file:

• cache.hpp