commoncpp2.texi

贡献一份commoncpp2,有兴趣的可以研究一下

may require within the constructor of your derived thread class, and
to purge or restore any allocated resources in the destructor.  In
most cases, the destructor will be executed after the thread has
terminated, and hence will execute within the context of the thread
that requested a join rather than in the context of the thread that is
being terminated.  Most destructors in derived thread classes should
first call terminate() to make sure the thread has stopped running
before releasing resources.

@cindex thread join
A GNU Common C++ thread is normally canceled by deleting the thread
object.  The process of deletion invokes the thread's destructor, and
the destructor will then perform a "join" against the thread using the
terminate() function.  This behavior is not always desirable since the
thread may block itself from cancellation and block the current
"delete" operation from completing.  One can alternately invoke
terminate() directly before deleting a thread object.

@findex Thread::final
@findex operator new
@cindex detached thread
When a given GNU Common C++ thread exits on it's own through it's
run() method, a "final" method will be called.  This Final method will
be called while the thread is "detached".  If a thread object is
constructed through a "new" operator, it's final method can be used to
"self delete" when done, and allows an independent thread to construct
and remove itself autonomously.

@findex getThread
@cindex pthread_self
A special global function, getThread(), is provided to identify the
thread object that represents the current execution context you are
running under.  This is sometimes needed to deliver signals to the
correct thread.  Since all thread manipulation should be done through
the GNU Common C++ (base) thread class itself, this provides the same
functionality as things like "pthread_self" for GNU Common C++.

GNU Common C++ threads are often aggregated into other classes to
provide services that are "managed" from or operate within the context
of a thread, even within the GNU Common C++ framework itself.  A good
example of this is the TCPSession class, which essentially is a
combination of a TCP client connection and a separate thread the user
can define by deriving a class with a Run() method to handle the
connected service.  This aggregation logically connects the successful
allocation of a given resource with the construction of a thread to
manage and perform operations for said resource.

Threads are also used in "service pools".  In GNU Common C++, a
service pool is one or more threads that are used to manage a set of
resources.  While GNU Common C++ does not provide a direct "pool"
class, it does provide a model for their implementation, usually by
constructing an array of thread "service" objects, each of which can
then be assigned the next new instance of a given resource in turn or
algorithmically.

@findex Thread::signal
@findex Thread::onDisconnect
@findex Thread::onHangup
@cindex SIGPIPE
@cindex SIGHUP
Threads have signal handlers associated with them.  Several signal
types are "predefined" and have special meaning.  All signal handlers
are defined as virtual member functions of the Thread class which are
called when a specific signal is received for a given thread.  The
"SIGPIPE" event is defined as a "onDisconnect" event since it's normally
associated with a socket disconnecting or broken fifo.  The onHangup()
method is associated with the SIGHUP signal.  All other signals are
handled through the more generic signal().

Incidently, unlike Posix, the win32 API has no concept of signals, and
certainly no means to define or deliver signals on a per-thread basis.
For this reason, no signal handling is supported or emulated in the
win32 implementation of GNU Common C++ at this time.

@tindex TCPStream
@tindex TCPSession
In addition to TCPStream, there is a TCPSession class which combines a
thread with a TCPStream object.  The assumption made by TCPSession is
that one will service each TCP connection with a separate thread, and
this makes sense for systems where extended connections may be
maintained and complex protocols are being used over TCP.


@c -----------------------------------------------------------------------
@node Synchronization
@section Synchronization
@cindex Synchronization

Synchronization objects are needed when a single object can be
potentially manipulated by more than one thread (execution) context
concurrently.  GNU Common C++ provides a number of specialized classes
and objects that can be used to synchronize threads.

@tindex Mutex
One of the most basic GNU Common C++ synchronization object is the
Mutex class.  A Mutex only allows one thread to continue execution at
a given time over a specific section of code.  Mutex's have a enter
and leave method; only one thread can continue from the Enter until
the Leave is called.  The next thread waiting can then get through.
Mutex's are also known as "CRITICAL SECTIONS" in win32-speak.

The GNU Common C++ mutex is presumed to support recursive locking.
This was deemed essential because a mutex might be used to block
individual file requests in say, a database, but the same mutex might
be needed to block a whole series of database updates that compose a
"transaction" for one thread to complete together without having to
write alternate non-locking member functions to invoke for each part
of a transaction.

Strangely enough, the original pthread draft standard does not
directly support recursive mutexes.  In fact this is the most common
"NP" extension for most pthread implementations.  GNU Common C++
emulates recursive mutex behavior when the target platform does not
directly support it.

@tindex ThreadLock
In addition to the Mutex, GNU Common C++ supports a rwlock class
(ThreadLock).  This implements the X/Open recommended "rwlock".  On
systems which do not support rwlock's, the behavior is emulated with a
Mutex; however, the advantage of a rwlock over a mutex is then
entirely lost.  There has been some suggested clever hacks for
"emulating" the behavior of a rwlock with a pair of mutexes and a
semaphore, and one of these will be adapted for GNU Common C++ in the
future for platforms that do not support rwlock's directly.

@tindex Semaphore
GNU Common C++ also supports "semaphores".  Semaphores are typically
used as a counter for protecting or limiting concurrent access to a
given resource, such as to permitting at most "x" number of threads to
use resource "y", for example.  Semaphore's are also convenient to use
as synchronization objects to rondevous and signal activity and/or
post pending service requests between one thread thread and another.

@tindex Event
In addition to Semaphore objects, GNU Common C++ supports "Event"
objects.  Event objects are triggered "events" which are used to
notify one thread of some event it is waiting for from another thread.
These event objects use a trigger/reset mechanism and are related to
low level conditional variables.

@tindex ThreadKey
@tindex AtomicCounter
@cindex reference counting
A special class, the ThreadKey, is used to hold state information that
must be unique for each thread of context.  Finally, GNU Common C++
supports a thread-safe "AtomicCounter" class.  This can often be used
for reference counting without having to protect the counter with a
separate Mutex counter.  This lends to lighter-weight code.


@c -----------------------------------------------------------------------
@node Sockets
@section Sockets
@cindex Sockets

@cindex Java sockets
GNU Common C++ provides a set of classes that wrap and define the
operation of network "sockets".  Much like with Java, there are also a
related set of classes that are used to define and manipulate objects
which act as "hostname" and "network addresses" for socket
connections.

@tindex InetAddress
@tindex InetHostAddress
@tindex InetMaskAddress
@tindex BroadcastAddress
The network name and address objects are all derived from a common
InetAddress base class. Specific classes, such as InetHostAddress,
InetMaskAddress, etc, are defined from InetAddress entirely so that
the manner a network address is being used can easily be documented
and understood from the code and to avoid common errors and accidental
misuse of the wrong address object.  For example, a "connection" to
something that is declared as a "InetHostAddress" can be kept
type-safe from a "connection" accidently being made to something that
was declared a "BroadcastAddress".

@tindex Socket
@cindex QoS
@cindex sockopt
@cindex Dont-Route
@cindex Keep-Alive
The socket is itself defined in a single base class named, quite
unremarkably, "Socket".  This base class is not directly used, but is
provided to offer properties common to other GNU Common C++ socket
classes, including the socket exception model and the ability to set
socket properties such as QoS, "sockopts" properties like Dont-Route
and Keep-Alive, etc.

@tindex TCPStream
@findex TCPStream::operator<<
@findex TCPStream::operator>>
The first usable socket class is the TCPStream.  Since a TCP
connection is always a "streamed" virtual circuit with flow control,
the standard stream operators ("<<" and ">>") may be used with
TCPStream directly.  TCPStream itself can be formed either by
connecting to a bound network address of a TCP server, or can be
created when "accepting" a network connection from a TCP server.

@cindex TCPSocket
An implicit and unique TCPSocket object exists in GNU Common C++ to
represent a bound TCP socket acting as a "server" for receiving
connection requests.  This class is not part of TCPStream because such
objects normally perform no physical I/O (read or write operations)
other than to specify a listen backlog queue and perform "accept"
operations for pending connections.  The GNU Common C++ TCPSocket
offers a Peek method to examine where the next pending connection is
coming from, and a Reject method to flush the next request from the
queue without having to create a session.

@findex TCPSocket::onAccept
The TCPSocket also supports a "onAccept" method which can be called
when a TCPStream related object is created from a TCPSocket.  By
creating a TCPStream from a TCPSocket, an accept operation
automatically occurs, and the TCPSocket can then still reject the
client connection through the return status of it's OnAccept method.

@tindex UDPSocket
In addition to connected TCP sessions, GNU Common C++ supports UDP
sockets and these also cover a range of functionality.  Like a
TCPSocket, A UDPSocket can be created bound to a specific network
interface and/or port address, although this is not required.  UDP
sockets also are usually either connected or otherwise "associated"
with a specific "peer" UDP socket.  Since UDP sockets operate through
discreet packets, there are no streaming operators used with UDP
sockets.

@tindex UDPBroadcast
In addition to the UDP "socket" class, there is a "UDPBroadcast"
class.  The UDPBroadcast is a socket that is set to send messages to a
subnet as a whole rather than to an individual peer socket that it may
be associated with.

@tindex UDPDuplex
UDP sockets are often used for building "realtime" media streaming
protocols and full duplex messaging services.  When used in this
manner, typically a pair of UDP sockets are used together; one socket
is used to send and the other to receive data with an associated pair
of UDP sockets on a "peer" host.  This concept is represented through
the GNU Common C++ UDPDuplex object, which is a pair of sockets that
communicate with another UDPDuplex pair.

@tindex SocketPort
@tindex SocketService
Finally, a special set of classes, "SocketPort" and "SocketService",
exist for building realtime streaming media servers on top of UDP and
TCP protocols.  The "SocketPort" is used to hold a connected or
associated TCP or UDP socket which is being "streamed" and which
offers callback methods that are invoked from a "SocketService"
thread.  SocketService's can be pooled into logical thread pools that
can service a group of SocketPorts.  A millisecond accurate "timer" is
associated with each SocketPort and can be used to time synchronize
SocketPort I/O operations.


@c -----------------------------------------------------------------------
@node Serial I/O
@section Serial I/O
@cindex Serial I/O

GNU Common C++ serial I/O classes are used to manage serial devices
and implement serial device protocols.  From the point of view of GNU
Common C++, serial devices are supported by the underlying Posix
specified "termios" call interface.

The serial I/O base class is used to hold a descriptor to a serial
device and to provide an exception handling interface for all serial
I/O classes.  The base class is also used to specify serial I/O
properties such as communication speed, flow control, data size, and
parity.  The "Serial" base class is not itself directly used in
application development, however.

GNU Common C++ Serial I/O is itself divided into two conceptual modes;
frame oriented and line oriented I/O.  Both frame and line oriented
I/O makes use of the ability of the underlying tty driver to buffer
data and return "ready" status from when select either a specified
number of bytes or newline record has been reached by manipulating
termios c_cc fields appropriately.  This provides some advantage in
that a given thread servicing a serial port can block and wait rather
than have to continually poll or read each and every byte as soon as
it appears at the serial port.

@tindex TTYStream
@findex TTYStream::operator<<
@findex TTYStream::operator>>
@tindex ttystream
The first application relevant serial I/O class is the TTYStream
class.  TTYStream offers a linearly buffered "streaming" I/O session
with the serial device.  Furthermore, traditional C++ "stream"
operators (<< and >>) may be used with the serial device.  A more
"true" to ANSI C++ library format "ttystream" is also available, and
this supports an "open" method in which one can pass initial serial
device parameters immediately following the device name in a single
string, as in "/dev/tty3a:9600,7,e,1", as an example.

@tindex TTYSession
The TTYSession aggragates a TTYStream and a GNU Common C++ Thread
which is assumed to be the execution context that will be used to
perform actual I/O operations.  This class is very anagolous to
TCPSession.

@tindex TTYPort
@tindex TTYService
The TTYPort and TTYService classes are used to form thread-pool
serviced serial I/O protocol sets.  These can be used when one has a
large number of serial devices to manage, and a single (or limited
number of) thread(s) can then be used to service the tty port objects
present.  Each tty port supports a timer control and several virtual
methods that the service thread can call when events occur.  This
model provides for "callback" event management, whereby the service
thread performs a "callback" into the port object when events occur.
Specific events supported include the expiration of a TTYPort timer,
pending input data waiting to be read, and "sighup" connection breaks.


@c -----------------------------------------------------------------------
@node Block I/O
@section Block I/O
@cindex Block I/O

@tindex RandomFile
GNU Common C++ block I/O classes are meant to provide more convenient
file control for paged or random access files portably, and to answer
many issues that ANSI C++ leaves untouched in this area.  A common
base class, RandomFile, is provided for setting descriptor attributes
and handling exceptions.  From this, three kinds of random file access
are supported.

@tindex ThreadFile
@findex pwread
@findex pwwrite
ThreadFile is meant for use by a threaded database server where
multiple threads may each perform semi-independent operations on a
given database table stored on disk.  A special "fcb" structure is
used to hold file "state", and pread/pwrite is used whenever possible
for optimized I/O.  On systems that do not offer pwread/pwrite, a
Mutex lock is used to protect concurrent lseek and read/write
operations.  ThreadFile managed databases are assumed to be used only
by the local server and through a single file descriptor.

@tindex SharedFile
SharedFile is used when a database may be shared between multiple
processes.  SharedFile automatically applies low level byte-range
"file locks", and provides an interface to fetch and release
byte-range locked portions of a file.

@tindex MappedFile
@findex MappedFile::sync
The MappedFile class provides a portable interface to memory mapped
file access.  One can map and unmap portions of a file on demand, and
update changed memory pages mapped from files immediately through
sync().


@c -----------------------------------------------------------------------