C++ Network Programming
Mastering Complexity with
ACE & Patterns
Dr. Douglas C. Schmidt
[email protected]
www.cs.wustl.edu/~schmidt/tutorials-ace.html
Professor of EECS
Vanderbilt University
Nashville, Tennessee
Motivation: Challenges of Networked Applications
Observation
• Building robust, efficient, & extensible
concurrent & networked applications is
hard
• e.g., we must address many complex
topics that are less problematic for nonconcurrent, stand-alone applications
Complexities in networked applications
2
Accidental Complexities
• Low-level APIs
• Poor debugging tools
• Algorithmic decomposition
• Continuous re-invention/discovery of
core concepts & components
Inherent Complexities
• Latency
• Reliability
• Load balancing
• Causal ordering
• Scheduling & synchronization
• Deadlock
Presentation Outline
Cover OO techniques & language features that enhance software quality
• Patterns, which embody
reusable software
architectures & designs
• ACE wrapper facades,
which encapsulate OS
concurrency & network
programming APIs
• OO language features, e.g., classes, dynamic binding &
inheritance, parameterized types
Presentation Organization
1. Background
2. Concurrent & network
challenges & solution
approaches
3. Patterns & wrapper facades in
ACE + applications
3
The Layered Architecture of ACE
www.cs.wustl.edu/~schmidt/ACE.html
•Large open-source user community
• www.cs.wustl.edu/~schmidt/ACE-users.html
14
Features
•Open-source
•200,000+
lines of C++
•40+ personyears of effort
•Ported to
many OS
platforms
•Commercial support by Riverace
• www.riverace.com/
Sidebar: Platforms Supported by ACE
•ACE runs on a wide range of operating systems, including:
•PCs, e.g., Windows (all 32/64-bit versions), WinCE; Redhat, Debian,
and SuSE Linux; & Macintosh OS X;
•Most versions of UNIX, e.g., SunOS 4.x and Solaris, SGI IRIX, HPUX, Digital UNIX (Compaq Tru64), AIX, DG/UX, SCO OpenServer,
UnixWare, NetBSD, & FreeBSD;
•Real-time operating systems, e.g., VxWorks, OS/9, Chorus, LynxOS,
Pharlap TNT, QNX Neutrino and RTP, RTEMS, & pSoS;
•Large enterprise systems, e.g., OpenVMS, MVS OpenEdition,
Tandem NonStop-UX, & Cray UNICOS
•ACE can be used with all of the major C++ compilers on these platforms
•The ACE Web site at http://www.cs.wustl.edu/~schmidt/ACE.html
contains a complete, up-to-date list of platforms, along with instructions
for downloading & building ACE
15
Key Capabilities Provided by ACE
Service Access & Control
Concurrency
16
Event Handling & IPC
Synchronization
The Pattern Language for ACE
Pattern Benefits
• Preserve crucial design
information used by
applications &
middleware frameworks
& components
• Facilitate reuse of
proven software designs
& architectures
• Guide design choices
for application
developers
17
The Frameworks in ACE
ACE Framework
Inversion of Control
Reactor & Proactor
Calls back to application-supplied event handlers to perform
processing when events occur synchronously & asynchronously
Service Configurator
Calls back to application-supplied service objects to initialize,
suspend, resume, & finalize them
Task
Calls back to an application-supplied hook method to perform
processing in one or more threads of control
Acceptor-Connector
Calls back to service handlers to initialize them after they are
connected
Streams
Calls back to initialize & finalize tasks when they are pushed &
popped from a stream
20
Networked Logging Service Example
Key Participants
• Client application
processes
• Generate log records
• Server logging daemon
• Receive, process, &
store log records
C++ code for all logging
service examples are in
• ACE_ROOT/examples/
C++NPv1/
• ACE_ROOT/examples/
C++NPv2/
• The logging server
example in C++NPv2 is
more sophisticated than
the one in C++NPv1
• There’s an extra daemon involved
26
Patterns in the Networked Logging Service
Half-Sync/
Half-Async
Leader/
Followers
Monitor
Object
Active
Object
Reactor
Pipes &
Filters
AcceptorConnector
Component
Configurator
Proactor
Wrapper
Facade
27
Strategized
Locking
Scoped
Locking
Thread-safe
Interface
ACE Basics: Logging
• ACE’s logging facility usually best for diagnostics
• Can customize logging sinks
• Filterable logging severities
• Portable printf()-like format directives (thread/process ID,
date/time, types)
• Serializes output across multiple threads
• ACE propagates settings to threads created via ACE
• Can log across a network
•ACE_Log_Msg class; use thread-specific singleton most of the time, via
ACE_LOG_MSG macro
• Macros encapsulate most usage. Most common:
•ACE_DEBUG ((severity, format [, args…]));
•ACE_ERROR[_RETURN]
((severity, format [,args…])[, return-value]);
• See ACE Programmer’s Guide (APG) tables 3.1 (severities), 3.2
(directives), 3.3 (macros)
28
ACE Logging Usage
•The ACE logging API is similar to printf(), e.g.:
ACE_ERROR ((LM_ERROR, "(%t) fork failed"));
generates:
Oct 31 14:50:13 [email protected]@2766@LM_ERROR@client::(4) fork failed
and
ACE_DEBUG ((LM_DEBUG, "(%t) sending to server %s", host));
generates:
Oct 31 14:50:28 [email protected]@1832@LM_DEBUG@drwho::(6) sending to server tango
29
Format
%l
%N
%n
%P
%p
Action
Displays the line number where the error occurred
Displays the file name where the error occurred
Displays the name of the program
Displays the current process ID
Takes a const char * argument and displays it and the error
string corresponding to errno (similar to perror())
%T
Displays the current time
Logging Severities
• You can control which severities are seen at run time
• Two masks determine whether a message is displayed:
• Process-wide mask (defaults to all severities enabled)
• Per-thread mask (defaults to all severities disabled)
• If logged severity is enabled in either mask, message is displayed
• Set process/instance mask with:
• ACE_Log_Msg::priority_mask (u_long mask, MASK_TYPE which);
•MASK_TYPE is ACE_Log_Msg::PROCESS or ACE_Log_Msg::THREAD.
• Since default is to enable all severities process-wide, all severities are logged
in all threads unless you change it
• Per-thread mask initializer can be adjusted (default is all severities disabled):
• ACE_Log_Msg::disable_debug_messages ();
• ACE_Log_Msg::enable_debug_messages();
• Any set of severities can be specified (OR’d together)
• Note that these methods set and clear a (set of) bits instead of replacing the
mask, as priority_mask() does
30
Logging Severities Example
• To allow threads to decide their own logging, the desired severities
must be:
• Disabled at process level & enabled in the thread(s) to display them.
• e.g.,
ACE_LOG_MSG->priority_mask (0, ACE_Log_Msg::PROCESS);
ACE_Log_Msg::enable_debug_messages ();
ACE_Thread_Manager::instance ()->spawn (service);
ACE_Log_Msg::disable_debug_messages ();
ACE_Thread_Manager::instance ()->spawn_n (3, worker);
• LM_DEBUG severity (only) logged in service thread
• LM_DEBUG severity (and all others) not logged in worker threads
• Note that enable_debug_messages() &
disable_debug_messages() are static methods
31
Redirect Logging to a File
• Default logging sink is stderr. Redirect to a file by setting the
OSTREAM flag and assigning a stream. Can set the flag in two ways:
•ACE_Log_Msg::open (const ACE_TCHAR *prog_name,
u_long options_flags = ACE_Log_Msg::STDERR,
const ACE_TCHAR *logger_key = 0);
•ACE_Log_Msg::set_flags (u_long flags);
• Assign a stream:
•ACE_Log_Msg::msg_ostream (ACE_OSTREAM_TYPE *);
(Optional 2nd arg to tell ACE_Log_Msg to delete the ostream)
•ACE_OSTREAM_TYPE is ofstream where supported, else
FILE*
• To also stop output to stderr, use open() without STDERR flag, or
ACE_Log_Msg::clr_flags (STDERR)
32
Redirect Logging to Syslog
• Redirected log output to ACE_Log_Msg::SYSLOG goes to:
• Windows NT4 and up: system’s Event Log
• UNIX/Linux: syslog facility (uses LOG_USER syslog facility)
• Can’t set this with set_flags/clr_flags; must open. For example:
•ACE_LOG_MSG->open
(argv[0], ACE_Log_Msg::SYSLOG, ACE_TEXT
(“syslogTest”));
• Windows: 3rd arg, if supplied, replaces 1st as program name in event
log
• To turn it off, call open() again with different flag(s). This seems odd,
but you’re effectively resetting the logging… think of it as reopen().
33
Logging Callbacks
• Logging callbacks are useful for adding special processing or
filtering to log output
• Derive a class from ACE_Log_Msg_Callback & reimplement:
•virtual void log (ACE_Log_Record &log_record);
• Use ACE_Log_Msg::msg_callback() to register callback
• Also call ACE_Log_Msg::set_flags() to add
ACE_Log_Msg::MSG_CALLBACK flag
• Beware…
• Callback registration is specific to each ACE_Log_Msg
instance
• Callbacks are not inherited when new threads are created
34
Useful Logging Flags
• There are some other ACE_Log_Msg flags that add useful
functionality to ACE’s logging:
•VERBOSE: Prepends program name, timestamp, host
name, process ID, and message priority to each message
•VERBOSE_LITE: Prepends timestamp and message
priority to each message (this is what ACE test suite uses)
•SILENT: Don’t display any messages of any severity
•LOGGER: Write messages to the local client logger deamon
35
Tracing
• ACE’s tracing facility logs function/method entry & exit
• Uses logging with severity LM_TRACE, so output can be selectively disabled
• Just put ACE_TRACE macro in the function:
#include “ace/Log_Msg.h”
void foo (void)
{
ACE_TRACE (“foo”);
// … do stuff
}
Says:
(1024) Calling foo in file ‘test.cpp’ on line 8
(1024) Leaving foo
• Clever indenting by call depth makes output easier to read
• Huge amount of output, so tracing no-op’d out by default; rebuild with
config.h having: #define ACE_NTRACE 0
36
Networked Logging Service Example
Key Participants
• Client application
processes
• Generate log records
• Server logging daemon
• Receive, process, &
store log records
C++ code for all logging
service examples are in
• ACE_ROOT/examples/
C++NPv1/
• ACE_ROOT/examples/
C++NPv2/
• We’ll develop architecture
similar to ACE’s, but not
same implementation.
37
Network Daemon Design Dimensions
•Communication dimensions address
the rules, form, & level of abstraction
that networked applications use to
interact
•Concurrency dimensions address
the policies & mechanisms governing
the proper use of processes & threads
to represent multiple service instances,
as well as how each service instance
may use multiple threads internally
38
•Service dimensions address key
properties of a networked
application service, such as the
duration & structure of each
service instance
•Configuration dimensions
address how networked services
are identified & the time at which
they are bound together to form
complete applications
Communication Design Dimensions
•Communication is fundamental to networked application
design
•The next three slides present a domain analysis of
communication design dimensions, which address the
rules, form, and levels of abstraction that networked
applications use to interact with each other
•We cover the following communication design dimensions:
•Connectionless versus connection-oriented protocols
•Synchronous versus asynchronous message exchange
•Message-passing versus shared memory
39
Connectionless vs. Connection-oriented Protocols
•A protocol is a set of rules
that specify how control &
data information is
exchanged between
communicating entities
SYN
SYN/ACK
ACK
Connector
Acceptor
3-way handshake in TCP/IP
• Connection-oriented protocols
• Connectionless protocols provide
provide a reliable, sequences, nonduplicated delivery service, which is
useful for applications that can’t
tolerate data loss
• Examples include TCP & ATM
a message-oriented service in
which each message can be routed
and delivered independently
• Examples include UDP & IP
• Connection-oriented applications must address two additional design
issues:
•Data framing strategies, e.g., bytestream vs. message-oriented
•Connection multiplexing (muxing) strategies, e.g., multiplexed vs.
nonmultiplexed
40
Alternative Connection Muxing Strategies
•In multiplexed connections all
client requests emanating from
threads in a single process pass
through one TCP connection to a
server process
•Pros: Conserves OS
communication resources, such
as socket handles and
connection control blocks
•Cons: harder to program, less
efficient, & less deterministic
41
•In nonmultiplexed connections
each client uses a different
connection to communicate with a
peer service
•Pros: Finer control of communication
priorities & low synchronization
overhead since additional locks aren't
needed
•Cons: use more OS resources, &
therefore may not scale well in
certain environments
Sync vs. Async Message Exchange
•Asynchronous
request/response protocols
stream requests from client to
server without waiting for
responses synchronously
•Multiple client requests can
be transmitted before any
responses arrive from a
server
•Synchronous request/response protocols
are the simplest form to implement
•Requests & responses are exchanged
in a lock-step sequence.
•Each request must receive a response
synchronously before the next is sent
42
•These protocols therefore
often require a strategy for
detecting lost or failed
requests & resending them
later
Message Passing vs. Shared Memory
• Message passing exchanges data
explicitly via the IPC mechanisms
• Shared memory allows multiple
processes on the same or different
• Application developers generally define the hosts to access & exchange data as
though it were local to the address
protocol for exchanging the data, e.g.:
space of each process
• Format & content of the data
• Applications using native OS shared
• Number of possible participants in each
memory mechanisms must define
exchange (e.g., point-to-point unicast),
how to locate & map the shared
multicast, or broadcast)
memory region(s) & the data
• How participants begin, conduct, & end
structures that are placed in shared
a message-passing session
memory
43
Sidebar: C++ Objects & Shared Memory
Allocating a C++ Object in shared Memory
void *obj_buf = … // Get a pointer to location in shared memory
ABC *abc = new (obj_buf) ABC; // Use C++ placement new operator
• General responsibilities using placement new operator
•Pointer passed to placement new operator must point to a memory region
that is big enough & is aligned properly for the object type being created
•The placed object must be destroyed by explicitly calling the destructor
• Pitfalls initializing C++ objects with virtual functions in shared memory
•The shared memory region may reside at a different virtual memory
location in each process that maps the shared memory
•The C++ compiler/linker need not locate the vtable at the same address in
different processes that use the shared memory
•ACE wrapper façade classes that can be initialized in shared memory must
therefore be concrete data types
•i.e., classes with only non-virtual methods
44
Overview of the Socket API (1/2)
Sockets are the most common network programming
API available on operating system platforms
•Originally developed in
BSD Unix as a C
language API to TCP/IP
protocol suite
•The Socket API has
approximately two dozen
functions classified in five
categories
•Socket is a handle
created by the OS that
associates it with an end
point of a communication
channel
45
•A socket can be bound to a local or remote
address
•In Unix, socket handles & I/O handles can be
used interchangeably in most cases, but this
is not the case for Windows
Overview of the Socket API (2/2)
Local context
management
Connection
establishment &
termination
Data transfer
mechanisms
Options
management
Network
addressing
46
Taxonomy of Socket Dimensions
The Socket API can be decomposed into the following dimensions:
• Type of
communication
service
• e.g., streams versus
datagrams versus
connected datagrams
• Communication &
connection role
• e.g., clients often
initiate connections
actively, whereas
servers often accept
them passively
• Communication
domain
47
• e.g., local host only
versus local or remote
host
Limitations with the Socket APIs (1/2)
Poorly structured, non-uniform, & non-portable
• API is linear rather than hierarchical
• i.e., the API is not structured according to the different phases of
connection lifecycle management and the roles played by the
participants
• No consistency among the names
• Non-portable & error-prone
• Function names: read() & write() used for any I/O handle on
Unix but Windows needs ReadFile() & WriteFile()
• Function semantics: different behavior of same function on different
OS e.g., accept () can take NULL client address parameter on
Unix/Windows, but will crash on some operating systems, such as
VxWorks
• Socket handle representations: different platforms represent
sockets differently e.g., Unix uses unsigned integers whereas
Windows uses pointers
• Header files: Different platforms use different names for header files
for the socket API
48
Limitations with the Socket APIs (2/2)
Lack of type safety
• I/O handles are not amenable to strong type checking at compile
time
• e.g., no type distinction between a socket used for passive
listening & a socket used for data transfer
Steep learning curve due to complex semantics
• Multiple protocol families & address families
• Options for infrequently used features such as broadcasting, async
I/O, non blocking I/O, urgent data delivery
• Communication optimizations such as scatter-read & gather-write
• Different communication and connection roles, such as active &
passive connection establishment, & data transfer
Too many low-level details
• Forgetting to use the network byte order before data transfer
• Possibility of missing a function, such as listen()
• Possibility of mismatch between protocol & address families
• Forgetting to initialize underlying C structures e.g., sockaddr
• Using a wrong socket for a given role
49
Example of Socket API Limitations (1/3)
1 #include <sys/types.h>
2 #include <sys/socket.h>
Possible differences in header
file names
3
4 const int PORT_NUM = 10000;
5
6 int echo_server ()
7 {
50
8
struct sockaddr_in addr;
9
int addr_len;
Forgot to initialize to sizeof
(sockaddr_in)
10
char buf[BUFSIZ];
11
int n_handle;
12
// Create the local endpoint.
Use of non-portable handle type
Example of Socket API Limitations (2/3)
13
int s_handle = socket (PF_UNIX, SOCK_DGRAM, 0);
14
if (s_handle == -1) return -1;
15
16
Use of non-portable return value
17
// Set up address information where server listens.
Protocol and address family
addr.sin_family = AF_INET;
mismatch
18
addr.sin_port = PORT_NUM;
19
20
addr.sin_addr.addr = INADDR_ANY;
Unused structure members not
zeroed out
21
if (bind (s_handle, (struct sockaddr *) &addr,
22
23
24
51
Wrong byte order
sizeof addr) == -1)
return -1;
Missed call to listen()
Example of Socket API Limitations (3/3)
25
// Create a new communication endpoint.
26
if (n_handle = accept (s_handle, (struct sockaddr *) &addr,
27
&addr_len) != -1) {
int n;
29
while ((n = read (s_handle, buf, sizeof buf)) > 0)
30
write (n_handle, buf, n);
31
close (n_handle);
33
}
34
return 0;
35 }
Reading from wrong handle
No guarantee that “n” bytes will be written
32
52
SOCK_DGRAM handle illegal here
28
ACE Socket Wrapper Façade Classes
ACE defines a set of C++
classes that address the
limitations with the Socket
API
• Enhance type-safety
• Ensure portability
• Simplify common use
cases
• Building blocks for
higher-level
abstractions
These classes are
designed in accordance
with the Wrapper
Facade design pattern
53
The Wrapper Façade Pattern (1/2)
Context
•Networked applications must
manage a variety of OS
services, including processes,
threads, socket connections,
virtual memory, & files
Applications
•OS platforms provide low-level
APIs written in C to access
these services
Problem
•The diversity of hardware &
operating systems makes it hard to
build portable & robust networked
application software
•Programming directly to low-level OS
APIs is tedious, error-prone, & non54 portable
Solaris
Win2K
VxWorks
Linux
LynxOS
The Wrapper Façade Pattern (2/2)
Solution
•Apply the Wrapper Facade design pattern (P2) to avoid
accessing low-level operating system APIs directly
Wrapper Facade
calls
data
calls
method1()
…
methodN()
calls
API FunctionA()
calls methods
Application
This pattern
encapsulates data &
functions provided by
existing non-OO APIs
within more concise,
robust, portable,
maintainable, & cohesive
OO class interfaces
void method1(){
functionA();
functionB();
}
: Application
API FunctionB()
API FunctionC()
void methodN(){
functionA();
}
: Wrapper
Facade
: APIFunctionA
: APIFunctionB
method()
functionA()
functionB()
55
ACE Socket Wrapper Façades Taxonomy
•The structure of the
ACE Socket wrapper
facades reflects the
domain of networked
IPC properties
•The ACE Socket
wrapper façade classes
provide the following
capabilities:
•ACE_SOCK_*
classes encapsulate
Internet-domain
Socket API
functionality
•ACE_LSOCK_*
classes encapsulate
UNIX-domain Socket
API functionality
56
•ACE also has wrapper facades for datagrams
•e.g., unicast, multicast, broadcast
Roles in the ACE Socket Wrapper Facade
57
•The active connection role (ACE_SOCK_Connector) is played by
a peer application that initiates a connection to a remote peer
•The passive connection role (ACE_SOCK_Acceptor) is played by
a peer application that accepts a connection from a remote peer &
•The communication role (ACE_SOCK_Stream) is played by both
peer applications to exchange data after they are connected
ACE Socket Addressing Classes (1/2)
Motivation
•Network addressing is a trouble spot in the Socket API
•To minimize the complexity of these low-level details,
ACE defines a hierarchy of classes that provide a
uniform interface for all ACE network addressing
objects
58
ACE Socket Addressing Classes (2/2)
Class Capabilities
•The ACE_Addr class is the
root of the ACE network
addressing hierarchy
•The ACE_INET_Addr
class represents TCP/IP &
UDP/IP addressing
information
•This class eliminates
many subtle sources of
accidental complexity
59
ACE I/O Handle Classes (1/2)
Motivation
•Low-level C I/O handle types are tedious, error-prone, & non-portable
•Even the ACE_HANDLE typedef is still not sufficiently object-oriented &
typesafe
60
int buggy_echo_server (u_short port_num) {
sockaddr_in s_addr;
int acceptor = socket (PF_UNIX, SOCK_DGRAM, 0);
int is not portable to Windows
s_addr.sin_family = AF_INET;
s_addr.sin_port = port_num;
s_addr.sin_addr.s_addr = INADDR_ANY;
bind (acceptor, (sockaddr *) &s_addr, sizeof s_addr);
int handle = accept (acceptor, 0, 0);
for (;;) {
char buf[BUFSIZ];
ssize_t n = read (acceptor, buf, sizeof buf);
if (n <= 0) break;
Reading from wrong handle
write (handle, buf, n);
}
}
ACE I/O Handle Classes (2/2)
Class Capabilities
•ACE_IPC_SAP is the root of
the ACE hierarchy of IPC
wrapper facades
•It provides basic I/O handle
manipulation capabilities to
other ACE IPC wrapper
facades
•ACE_SOCK is the root of the
ACE Socket wrapper facades
& it provides methods to
•Create & destroy socket
handles
•Obtain the network addresses of local & remote peers
•Set/get socket options, such as socket queue sizes,
•Enable broadcast/multicast communication
61
•Disable Nagle‘s algorithm
The ACE_SOCK_Connector Class
Motivation
•There is a confusing asymmetry in the Socket API between (1)
connection roles & (2) socket modes
•e.g., an application may accidentally call recv() or send() on
a data-mode socket handle before it's connected
•This problem can't be detected until run time since C socket
handles are weakly-typed
int buggy_echo_client (u_short port_num, const char *s)
{
int handle = socket (PF_UNIX, SOCK_DGRAM, 0);
write (handle, s, strlen (s) + 1);
sockaddr_in s_addr;
Operations called in
memset (&s_addr, 0, sizeof s_addr);
wrong order
s_addr.sin_family = AF_INET;
s_addr.sin_port = htons (port_num);
connect (handle, (sockaddr *) &s_addr, sizeof s_addr);
}
62
The ACE_SOCK_Connector Class
Class Capabilities
•ACE_SOCK_Connector is factory that establishes a new endpoint of
communication actively & provides capabilities to
•Initiate a connection with a peer acceptor & then to initialize an
ACE_SOCK_Stream object after the connection is established
•Initiate connections in either a blocking, nonblocking, or timed manner
•Use C++ traits to support
generic programming
techniques that enable
wholesale replacement of
IPC functionality
63
Sidebar: Traits for ACE Wrapper Facades (1/2)
•ACE uses the C++ generic programming idiom to define &
combine a set of characteristics to alter the behavior of a
template class
•In C++, the typedef & typename language feature is used to
define a trait
•A trait provides a convenient way to associate related types,
values, & functions with template parameter type without
requiring that they be defined as members of the type
•Traits are used extensively in the C++ Standard Template
Library (STL)
64
Sidebar: Traits for ACE Wrapper Facades (2/2)
•ACE Socket wrapper facades use traits to define the following
associations
•PEER_ADDR – this trait defines the ACE_INET_Addr class associated
with the ACE Socket Wrapper Façade
•PEER_STREAM – this trait defines the ACE_SOCK_Stream data
transfer class associated with the ACE_SOCK_Acceptor &
ACE_SOCK_Connector factories
class ACE_SOCK_Connector {
public:
typedef ACE_INET_Addr
PEER_ADDR;
typedef ACE_SOCK_Stream
PEER_STREAM;
// ...
65
class ACE_TLI_Connector {
public:
typedef ACE_INET_Addr
PEER_ADDR;
typedef ACE_TLI_Stream
PEER_STREAM;
// ...
Using the ACE_SOCK_Connector (1/3)
•This example shows how the ACE_SOCK_Connector can be used
to connect a client application to a Web server
int main (int argc,
char *argv[]) {
const char *pathname =
argc > 1
? argv[1] : “/index.html";
const char *server_hostname =
• Instantiate the connector,
argc > 2
data transfer, & address
? argv[2] : “www.dre.vanderbilt.edu"; objects
typedef ACE_SOCK_Connector CONNECTOR;
CONNECTOR connector;
CONNECTOR::PEER_STREAM peer;
CONNECTOR::PEER_ADDR peer_addr;
if (peer_addr.set (80, server_hostname) == -1) • Block until
connection
return 1;
established or
else if (connector.connect (peer,
peer_addr) == -1) connection request
failure
return 1;
66
Using the ACE_SOCK_Connector (2/3)
// Designate a nonblocking connect.
• Perform a non-blocking
if (connector.connect (peer,
connect
peer_addr,
&ACE_Time_Value::zero) == -1) {
if (errno == EWOULDBLOCK) {
// Do some other work ...
// Now, try to complete connection establishment,
// but don't block if it isn't complete yet.
if (connector.complete (peer,
0,
• If connection not
&ACE_Time_Value::zero) == -1)
established, do other
work & try again without
blocking
// Designate a timed connect.
ACE_Time_Value timeout (10); // 10 second timeout.
if (connector.connect (peer,
• Perform a timed connect
peer_addr,
e.g., 10 seconds in this
&timeout) == -1) {
case
if (errno == ETIME) {
// Timeout, do something else
67
Using the ACE_SOCK_Connector (3/3)
•The ACE_SOCK_Connector can be passed the following values to
control its timeout behavior
68
The ACE_SOCK_Stream Class (1/2)
Motivation
•Developers can misuse sockets in ways that can't be detected during
compilation
•An ACE_SOCK_Stream object can't be used in any role other than
data transfer without violating its (statically type-checked) interface
int buggy_echo_server (u_short port_num) {
sockaddr_in s_addr;
int acceptor = socket (PF_UNIX, SOCK_DGRAM, 0);
s_addr.sin_family = AF_INET;
s_addr.sin_port = port_num;
s_addr.sin_addr.s_addr = INADDR_ANY;
bind (acceptor, (sockaddr *) &s_addr, sizeof s_addr);
int handle = accept (acceptor, 0, 0);
for (;;) {
char buf[BUFSIZ];
ssize_t n = read (acceptor, buf, sizeof buf);
if (n <= 0) break;
Reading from wrong handle
write (handle, buf, n);
}
}
69
The ACE_SOCK_Stream Class (2/2)
Class Capabilities
•Encapsulates data transfer
mechanisms supported by
data-mode sockets to provide
the following capabilities:
•Support for sending &
receiving up to n bytes or
exactly n bytes
•Support for “scatter-read,”
which populate multiple callersupplied buffers instead of a
single contiguous buffer
•Support for ``gather-write'' operations, which transmit the contents of multiple
noncontiguous data buffers in a single operation
•Support for blocking, nonblocking, & timed I/O operations
•Support for generic programming techniques that enable the wholesale
replacement of functionality via C++ parameterized types
70
Using the ACE_SOCK_Stream (1/2)
•This example shows how an ACE_SOCK_Stream can be used to send &
receive data to & from a Web server
// ...Connection code from example in Section 3.5 omitted...
char buf[BUFSIZ];
• Initialize the iovec
iovec iov[3];
vector for scatter-read
& gather-write I/O
iov[0].iov_base = (char *) "GET ";
iov[0].iov_len = 4; // Length of "GET ".
iov[1].iov_base = (char *) pathname;
iov[1].iov_len = strlen (pathname);
iov[2].iov_base = (char *) " HTTP/1.0\r\n\r\n";
iov[2].iov_len = 13; // Length of " HTTP/1.0\r\n\r\n";
if (peer.sendv_n (iov, 3) == -1)
return 1;
• Perform blocking gather-
write on ACE_SOCK_Stream
for (ssize_t n; (n = peer.recv (buf, sizeof buf)) > 0; )
ACE::write_n (ACE_STDOUT, buf, n);
return peer.close () == -1 ? 1 : 0;
}
71
• Perform blocking read on
ACE_SOCK_Stream
Using the ACE_SOCK_Stream (2/2)
•Blocking & non-blocking I/O semantics can be controlled via the
ACE_SOCK_STREAM enable() & disable() methods, e.g.,
•peer.enable (ACE_NONBLOCK); // enables non blocking
peer.disable (ACE_NONBLOCK); // disable non blocking
• If the I/O operation blocks, it returns a -1 & errno is set to
EWOULDBLOCK
•I/O operations can involve timeouts, e.g.,
ACE_Time_Value timeout (10); // 10 second timeout
If (peer.sendv_n (iov, 3, &timeout) == -1) {
// check if errno is set to ETIME,
// which indicates a timeout
}
// similarly use timeout for receiving data
72
Sidebar: Working with (& Around) Nagle’s Algorithm
Nagle’s Algorithm
• Problem: Need to tackle the send-side silly window syndrome, where small
data payloads, such as a keystroke, result in transmissions of large packets
& causing unnecessary waste of network resources & congestion
• Solution: The OS kernel buffers a # of small-sized application messages &
concatenates them into a larger size packet that can then be transmitted
• Consequences: Although network congestion is minimized, it can lead to
higher & unpredictable latencies, as well as lower throughput
Controlling Nagle’s Algorithm via ACE
• Use the set_option() method of the ACE_SOCK class e.g.,
int nodelay = 1; // Disable Nagle’s algorithm
ACE_SOCK_Stream option_setter (handle);
if (-1 == option_setter.set_option (ACE_IPPROTO_TCP,
TCP_NODELAY,
&nodelay,
sizeof (nodelay)))
...
73
The ACE_SOCK_Acceptor Class (1/2)
Motivation
•The C functions in the Socket API are weakly typed, which makes it easy
to apply them incorrectly in ways that can’t be detected until run-time
•The ACE_SOCK_Acceptor class ensures type errors are detected at
compile-time
74
int buggy_echo_server (u_short port_num) {
sockaddr_in s_addr;
int acceptor = socket (PF_UNIX, SOCK_DGRAM, 0);
s_addr.sin_family = AF_INET;
s_addr.sin_port = port_num;
s_addr.sin_addr.s_addr = INADDR_ANY;
bind (acceptor, (sockaddr *) &s_addr, sizeof s_addr);
int handle = accept (acceptor, 0, 0);
for (;;) {
char buf[BUFSIZ];
ssize_t n = read (acceptor, buf, sizeof buf);
if (n <= 0) break;
Reading from wrong handle
write (handle, buf, n);
}
}
The ACE_SOCK_Acceptor Class (2/2)
Class Capabilities
•This class is a factory
that establishes a new
endpoint of
communication
passively & provides
the following
capabilities:
•It accepts a connection from a peer connector & then initializes an
ACE_SOCK_Stream object after the connection is established
•Connections can be accepted in either a blocking, nonblocking, or
timed manner
•C++ traits are used to support generic programming techniques that
enable the wholesale replacement of functionality via C++
parameterized types
75
Using the ACE_SOCK_Acceptor
• This example shows how an ACE_SOCK_Acceptor & ACE_SOCK_Stream can
be used to accept connections & send/receive data to/from a web client
extern char *get_url_pathname (ACE_SOCK_Stream *);
int main ()
• Instantiate the acceptor, data transfer, & address objects
{
ACE_INET_Addr server_addr;
• Initialize a passive
ACE_SOCK_Acceptor acceptor;
mode endpoint to
ACE_SOCK_Stream peer;
listen for connections
on port 80
if (server_addr.set (80) == -1) return 1;
if (acceptor.open (server_addr) == -1) return 1;
• Accept a new connection
for (;;) {
if (acceptor.accept (peer) == -1) return 1;
peer.disable (ACE_NONBLOCK); // Ensure blocking <send_n>.
ACE_Auto_Array_Ptr<char *> pathname (get_url_pathname (peer));
ACE_Mem_Map mapped_file (pathname.get ());
• Send the
}
}76
requested data
if (peer.send_n (mapped_file.addr (),
mapped_file.size ()) == -1) return 1;
peer.close ();
• Close the connection to the sender
return acceptor.close () == -1 ? 1 : 0;
• Stop receiving any
connections
Sidebar: The ACE_Mem_Map Class
Memory Mapped Files
ACE_Mem_Map Class
•Many modern operating systems provide a
mechanism for mapping a file’s contents
directly into a process’s virtual address
space
•A wrapper façade that
encapsulates the memory
mapped file system
mechanisms on different
operating systems
•This memory-mapped file mechanism can
be read from or written to directly by
referencing the virtual memory
•e.g., via pointers instead of using less
efficient I/O functions
•The file manager defers all read/write
operations to the virtual memory manager
•Contents of memory mapped files can be
shared by multiple processes on the same
machine
•It can also be used to provide a persistent
backing store
77
•Relieves application
developers from having to
manually perform
bookkeeping tasks
•e.g., explicitly opening
files or determining their
lengths
•The ACE_Mem_Map class
offers multiple constructors
with several signature
variants
The ACE_Message_Block Class (1/2)
MESSAGES
BUFFERED FOR
TRANSMISSION
MESSAGES IN TRANSIT
MESSAGES
BUFFERED
AWAITING
PROCESSING
Motivation
•Many networked applications require a means to manipulate messages
efficiently, e.g.:
•Storing messages in buffers as they are received from the network or
from other processes
•Adding/removing headers/trailers from messages as they pass through a
user-level protocol stack
•Fragmenting/reassembling messages to fit into network MTUs
•Storing messages in buffers for transmission or retransmission
•Reordering messages that were received out-of-sequence
78
The ACE_Message_Block Class (2/2)
Class Capabilities
•This class is a composite that enables efficient manipulation of messages via
the following operations:
•Each ACE_Message_Block contains a pointer to a reference-counted
ACE_Data_Block which in turn points to the actual data associated with a
message
•It allows multiple
messages to be
chained together into a
composite message
•It allows multiple
messages to be joined
together to form an
ACE_Message_Queue
79
•It treats
synchronization &
memory management
properties as aspects
Two Kinds of Message Blocks
• Simple messages contain a • Composite messages contain multiple
one ACE_Message_Block
ACE_Message_Blocks
• These blocks are linked together in accordance with
• An ACE_Message_Block
the Composite pattern
points to an
• Composite messages often consist of a control
ACE_Data_Block
message that contains bookkeeping information
• An ACE_Data_Block
• e.g., destination addresses, followed by one or
points to the actual data
more data messages that contain the actual
payload
contents of the message
•ACE_Data_Blocks can be referenced counted
80
Using the ACE_Message_Block (1/2)
•The following program reads all data from standard input into a singly linked
list of dynamically allocated ACE_Message_Blocks
•These ACE_Message_Blocks are chained together by their continuation
pointers
• Allocate an ACE_Message_Block
int main (int argc, char *argv[])
whose payload is of size BUFSIZ
{
ACE_Message_Block *head = new ACE_Message_Block (BUFSIZ);
ACE_Message_Block *mblk = head; • Read data from standard input
into the message block starting
at write pointer (wr_ptr ())
for (;;) {
ssize_t nbytes = ACE::read_n (ACE_STDIN,
mblk->wr_ptr (),
mblk->size ());
if (nbytes <= 0)
break; // Break out at EOF or error.
mblk->wr_ptr (nbytes);
81
• Advance write pointer by the number
of bytes read to end of buffer
Using the ACE_Message_Block (2/2)
• Allocate a new ACE_Message_Block of size BUFSIZ &
chain it to the previous one at the end of the list
mblk->cont (new ACE_Message_Block (BUFSIZ));
mblk = mblk->cont ();
• Advance mblk to point to the newly
}
allocated ACE_Message_Block
// Print the contents of the list to the standard output.
for (mblk = head; mblk != 0; mblk = mblk->cont ())
ACE::write_n (ACE_STDOUT, mblk->rd_ptr (), mblk->length ());
• For every message block, print mblk->length() amount
of contents starting at the read pointer (rd_ptr ())
• Can also use ACE::write_n (head) to write entire
chain…
head->release (); // Release all the memory in the chain.
return 0;
}82
ACE CDR Streams
Motivation
• Networked applications that send/receive messages often require support for
• Linearization
• To handle the
conversion of richly
typed data to/from
raw memory buffers
•(De)marshaling
• To interoperate with
heterogeneous
compiler alignments
& hardware
instructions with
different byte-orders
• The ACE_OutputCDR & ACE_InputCDR classes provide a highly optimized,
portable, & convenient means to marshal & demarshal data using the
standard CORBA Common Data Representation (CDR)
•ACE_OutputCDR creates a CDR buffer from a data structure (marshaling)
83 •ACE_InputCDR extracts data from a CDR buffer (demarshaling)
The ACE_OutputCDR & ACE_InputCDR Classes
Class Capabilities
•ACE_OutputCDR & ACE_InputCDR support the following features:
• They provide operations to (de)marshal the following types:
• Primitive types, e.g., booleans; 16-, 32-, & 64-bit integers; 8-bit octets;
single & double precision floating point numbers; characters; & strings
• Arrays of primitive types
• The insertion (<<) and extraction (>>) operators can marshal & demarshal
primitive types, using the same syntax as the C++ iostream components
•ACE_Message_Block chains are used internally to minimize mem copies
• They take advantage of CORBA CDR alignment & byte-ordering rules to
avoid memory copying & byte-swapping operations, respectively
• They provide optimized byte swapping code that uses inline assembly
language instructions for common hardware platforms (such as Intel x86) &
standard hton*()& ntoh*() macros/functions on other platforms
• They support zero copy marshaling & demarshaling of octet buffers
• Users can define custom character set translators for platforms that do not
84 use ASCII or Unicode as their native character sets
Sidebar: Log Record Message Structure
ACE_Log_Record is a type that
ACE uses internally to keep track
of the fields in a log record
• This example uses a 8-byte,
CDR encoded header followed
by the payload
• Header includes byte order,
payload length, & other fields
class ACE_Log_Record
{
private:
ACE_UINT type_;
ACE_UINT pid_;
ACE_Time_Value timestamp_;
char msg_data_[ACE_MAXLOGMSGLEN];
public:
ACE_UINT type () const;
ACE_UINT pid () const;
const ACE_Time_Value timestamp () const;
const char *msg_data () const;
};
85
Using ACE_OutputCDR
• We show the ACE CDR insertion (operator<<) & extraction (operator>>)
operators for ACE_Log_Record that's used by client application & logging server
int operator<< (ACE_OutputCDR &cdr,
const ACE_Log_Record &log_record)
{
size_t msglen = log_record.msg_data_len ();
// Insert each <log_record> field into the output CDR stream.
cdr << ACE_CDR::Long (log_record.type ());
cdr << ACE_CDR::Long (log_record.pid ());
cdr << ACE_CDR::Long (log_record.time_stamp ().sec ());
cdr << ACE_CDR::Long (log_record.time_stamp ().usec ());
cdr << ACE_CDR::ULong (msglen);
cdr.write_char_array (log_record.msg_data (), msglen);
return cdr.good_bit ();
}
86
After marshaling all the fields of the log record into
the CDR stream, return the success/failure status
Using ACE_InputCDR
int operator>> (ACE_InputCDR &cdr,
ACE_Log_Record &log_record)
{ ACE_CDR::Long type;
Temporaries used during
ACE_CDR::Long pid;
demarshaling (not always
ACE_CDR::Long sec, usec;
necessary)
ACE_CDR::ULong buffer_len;
// Extract each field from input CDR stream into <log_record>.
if ((cdr >> type) && (cdr >> pid) && (cdr >> sec)
&& (cdr >> usec) && (cdr >> buffer_len)) {
ACE_TCHAR log_msg[ACE_Log_Record::MAXLOGMSGLEN + 1];
log_record.type (type);
log_record.pid (pid);
log_record.time_stamp (ACE_Time_Value (sec, usec));
cdr.read_char_array (log_msg, buffer_len);
log_msg[buffer_len] = '\0';
log_record.msg_data (log_msg);
}
return cdr.good_bit (); After demarshaling all the fields of the log record
from the CDR stream, return the success/failure
}
status
87
Implementing the Client Application (1/6)
•The following client application
illustrates how to use the ACE
C++ Socket wrapper facades &
CDR streams to establish
connections, marshal log
records, & send the data to our
logging server
88
•This example behaves as follows:
1.Reads lines from standard input
2.Sends each line to the logging
server in a separate log record &
3.Stops when it reads EOF from
standard input
class Logging_Client {
Header file: “Logging_Client.h”
public:
// Send <log_record> to the server.
int send (const ACE_Log_Record &log_record);
// Accessor method.
ACE_SOCK_Stream &peer () { return logging_peer_; }
// Close the connection to the server.
~Logging_Client () { logging_peer_.close (); }
private:
ACE_SOCK_Stream logging_peer_; // Connected to server.
};
Implementing the Client Application (2/6)
The Logging_Client::send() method behaves as follows:
1.Computes the size of the payload (lines 2 – 8)
2.Marshals the header & data into an output CDR (lines 10 – 16) &
3.Sends it to the logging server (lines 18 – 24)
1 int Logging_Client::send (const ACE_Log_Record &log_record) {
2
const size_t max_payload_size =
3
4 // type()
4
+ 8 // timestamp
5
+ 4 // process id
6
+ 4 // data length
7
+ ACE_Log_Record::ACE_MAXLOGMSGLEN // data
8
+ ACE_CDR::MAX_ALIGNMENT; // padding;
9
10
ACE_OutputCDR payload (max_payload_size);
11
payload << log_record;
12
ACE_CDR::ULong length = payload.total_length ();
13
First marshal the payload to contain the linearized ACE_Log_Record
89
Implementing the Client Application (3/6)
4. Then marshal the header info that includes byte order & payload
length
14
15
16
17
18
19
20
21
22
23
24
25 }
ACE_OutputCDR header (ACE_CDR::MAX_ALIGNMENT + 8);
header << ACE_OutputCDR::from_boolean (ACE_CDR_BYTE_ORDER);
header << ACE_CDR::ULong (length);
5. Construct an iovec of size 2 with header & payload info
iovec iov[2];
iov[0].iov_base
iov[0].iov_len
iov[1].iov_base
iov[1].iov_len
=
=
=
=
header.begin ()->rd_ptr ();
8;
payload.begin ()->rd_ptr ();
length;
return logging_peer_.sendv_n (iov, 2);
6. Send entire message to
logging server
90
Since TCP/IP is a bytestream protocol
(i.e., without any message boundaries) the
logging service uses CDR as a message
framing protocol to delimit log records
Implementing the Client Application (4/6)
1 int main (int argc, char *argv[])
The Logging_Client
2 {
3
u_short logger_port =
main program
4
argc > 1 ? atoi (argv[1]) : 0;
5
const char *logger_host =
6
argc > 2 ? argv[2] : ACE_DEFAULT_SERVER_HOST;
7
int result;
8
9
ACE_INET_Addr server_addr;
10
11
if (logger_port != 0)
12
result = server_addr.set (logger_port, logger_host);
13
else
14
result = server_addr.set ("ace_logger", logger_host);
15
if (result == -1)
16
ACE_ERROR_RETURN((LM_ERROR,
17
"lookup %s, %p\n",
18
logger_port == 0 ? "ace_logger" : argv[1],
19
logger_host), 1);
20
91
Sidebar: ACE Debugging & Error Macros
• Consolidates printing of debug and error messages via a printf ()-like
format e.g., ACE_DEBUG, ACE_ERROR (& their *_RETURN counterparts) that
encapsulate the ACE_Log_Msg::log() method
• Arguments are enclosed in a double set of parentheses to make it appear as
one argument to the C++ preprocessor
• First argument is the severity code; second one is a format string supporting
a superset of printf() conversion specifiers
Format
92
Action
%l
Displays the line number where the error occurred
%N
Displays the file name where the error occurred
%n
Displays the name of the program
%P
Displays the current process ID
%p
Takes a const char * argument and displays it and the
error string corresponding to errno (similar to perror())
%T
Displays the current time
%t
Displays the calling thread’s ID
Implementing the Client Application (5/6)
Use the ACE_SOCK_Connector wrapper façade to connect to the logging
server
21
22
23
24
25
26
27
28
29
30
31
32
ACE_SOCK_Connector connector;
Logging_Client logging_client;
if (connector.connect (logging_client.peer (),
server_addr) < 0)
ACE_ERROR_RETURN ((LM_ERROR,
"%p\n",
"connect()"),
1);
// Limit the number of characters read on each record.
cin.width (ACE_Log_Record::MAXLOGMSGLEN);
Contents of the message to be sent to logging server are obtained from
standard input
93
Implementing the Client Application (6/6)
33
for (;;) {
34
std::string user_input;
35
getline (cin, user_input, '\n');
36
Create log_record
37
if (!cin || cin.eof ()) break;
38
39
ACE_Time_Value now (ACE_OS::gettimeofday ());
40
ACE_Log_Record log_record (LM_INFO, now,
41
ACE_OS::getpid ());
42
log_record.msg_data (user_input.c_str ());
43
44
if (logging_client.send (log_record) == -1)
45
ACE_ERROR_RETURN ((LM_ERROR,
46
"%p\n", "logging_client.send()"),
1);
47
} Send log_record to logging server
48
49
return 0; // Logging_Client destructor
50
// closes TCP connection.
51 }
94
The Logging_Server Classes
The figure below illustrates our Logging_Server abstract base class, the
Logging_Handler class we'll describe shortly, & the concrete logging server
classes that we'll develop in subsequent sections of the tutorial
95
Implementing the Logging_Server (1/5)
•This example uses the ACE_Message_Block & ACE CDR classes in a
common base class that simplifies logging server implementations in
the examples
// Forward declaration.
class ACE_SOCK_Stream;
Header file “Logging_Server.h”
class Logging_Server
{
public:
// Template Method that runs logging server's event loop.
virtual int run (int argc, char *argv[]);
protected:
// The following four methods are ``hooks'' that can be
// overridden by subclasses.
virtual int open (u_short logger_port = 0);
virtual int wait_for_multiple_events () { return 0; }
virtual int handle_connections () = 0;
virtual int handle_data (ACE_SOCK_Stream * = 0) = 0;
96
Sidebar: Template Method Pattern
• Intent
Abstract Class
• Define the skeleton of an
template_method ();
algorithm in an operation,
hook_method_1();
deferring some steps to
hook_method_1();
...
subclasses
hook_method_1();
...
• Context
hook_method_2();
...
• You have a fixed algorithm
structure with variations
possible for individual steps
Concrete Class 1
• Problem
• You want to plug in & out steps hook_method_1();
of the algorithm without
hook_method_2();
changing the algorithm itself
• Solution
Concrete Class 2
• Define a fixed base class
function that calls virtual “hook”
hook_method_2();
methods that derived classes
can override
97
Implementing the Logging_Server (2/5)
Header file “Logging_Server.h” (cont’d)
// This helper method can be used by the hook methods.
int make_log_file (ACE_FILE_IO &, ACE_SOCK_Stream * = 0);
// Close the socket endpoint and shutdown ACE.
virtual ~Logging_Server () {
acceptor_.close ();
}
// Accessor.
ACE_SOCK_Acceptor &acceptor () {
return acceptor_;
}
private:
// Socket acceptor endpoint.
ACE_SOCK_Acceptor acceptor_;
};
98
Implementing the Logging_Server (3/5)
Implementation file “Logging_Server.cpp”
#include
#include
#include
#include
#include
#include
"ace/FILE_Addr.h"
• Template method providing the
"ace/FILE_Connector.h"
skeleton of the algorithm to use
"ace/FILE_IO.h"
• Hook methods will be overridden by
"ace/INET_Addr.h"
subclasses unless default is ok to
"ace/SOCK_Stream.h"
use
"Logging_Server.h"
int Logging_Server::run (int argc, char *argv[])
{
if (open (argc > 1 ? atoi (argv[1]) : 0) == -1)
return -1;
Three hook methods that can be overridden in subclasses
for (;;) {
if (wait_for_multiple_events () == -1) return -1;
if (handle_connections () == -1) return -1;
if (handle_data () == -1) return -1;
}
return 0;
}
99
Implementing the Logging_Server (4/5)
Initialize the acceptor so it can accept connections from any server
network interface
int Logging_Server::open (u_short logger_port)
{
// Raises the number of available socket handles to
// the maximum supported by the OS platform.
ACE::set_handle_limit ();
ACE_INET_Addr server_addr;
int result;
if (logger_port != 0)
result = server_addr.set (logger_port, INADDR_ANY);
else
result = server_addr.set ("ace_logger", INADDR_ANY);
if (result == -1) return -1;
// Start listening and enable reuse of listen address
// for quick restarts.
return acceptor_.open (server_addr, 1);
100
}
Implementing the Logging_Server (5/5)
int Logging_Server::make_log_file (ACE_FILE_IO &logging_file,
ACE_SOCK_Stream *logging_peer)
{
std::string filename (MAXHOSTNAMELEN, ’\0’);
if (logging_peer != 0) { // Use client host name as file name.
ACE_INET_Addr logging_peer_addr;
logging_peer->get_remote_addr (logging_peer_addr);
logging_peer_addr.get_host_name (filename.c_str (),
filename.size ());
filename += ".log";
} else filename = "logging_server.log";
ACE_FILE_Connector connector;
return connector.connect (logging_file,
ACE_FILE_Addr (filename.c_str ()),
0, // No time-out.
ACE_Addr::sap_any, // Ignored.
Create the log file using the
0, // Don't try to reuse the addr.
ACE_FILE_Connector factory
O_RDWR|O_CREAT|O_APPEND,
ACE_DEFAULT_FILE_PERMS);
101
}
Sidebar: The ACE File Wrapper Facades
• ACE file wrapper facades encapsulate platform mechanisms for
unbuffered file operations
• The design of these wrapper facades is very similar to ACE IPC
wrapper facades
• The ACE File classes decouple:
•Initialization factories: e.g., ACE_FILE_Connector, which
opens and/or creates files
•Data transfer classes: e.g., ACE_FILE_IO, which applications
use to read/write data from/to files opened using
ACE_FILE_Connector
• This generality in ACE’s design of wrapper facades helps strategize
higher-level ACE framework components
•e.g., ACE_Acceptor, ACE_Connector, & ACE_Svc_Handler
102
Implementing the Logging_Handler (1/6)
Header file “Logging_Handler.h”
#include "ace/FILE_IO.h"
#include "ace/SOCK_Stream.h"
class ACE_Message_Block; // Forward declaration.
class Logging_Handler
{
protected:
// Reference to a log file.
ACE_FILE_IO &log_file_;
// Connected to the client.
ACE_SOCK_Stream logging_peer_;
103
This class is used by the
logging server to encapsulate
the I/O & processing of log
records
Implementing the Logging_Server (2/6)
Header file “Logging_Handler.h” cont’d
// Receive one log record from a connected client. <mblk>
// contains the hostname, <mblk->cont()> contains the log
// record header (the byte order and the length) and the data.
int recv_log_record (ACE_Message_Block *&mblk);
// Write one record to the log file. The <mblk> contains the
// hostname and the <mblk->cont> contains the log record.
int write_log_record (ACE_Message_Block *mblk);
// Log one record by calling <recv_log_record> and
// <write_log_record>.
int log_record ();
};
When a log record is
received it is stored as
an ACE_Message_Block
chain
104
Implementing the Logging_Server (3/6)
1. Receive incoming data & use the input CDR class to parse header
2. Then payload based on the framing protocol &
3. Finally save it in an ACE_Message_Block chain
1 int Logging_Handler::recv_log_record (ACE_Message_Block *&mblk)
2 {
First save the peer hostname
3
ACE_INET_Addr peer_addr;
4
logging_peer_.get_remote_addr (peer_addr);
5
mblk = new ACE_Message_Block (MAXHOSTNAMELEN + 1);
6
peer_addr.get_host_name (mblk->wr_ptr (), MAXHOSTNAMELEN);
7
mblk->wr_ptr (strlen (mblk->wr_ptr ()) + 1); // Go past name.
8
9
ACE_Message_Block *payload =
10
new ACE_Message_Block (ACE_DEFAULT_CDR_BUFSIZE);
11
// Align Message Block for a CDR stream.
Force proper alignment
12
ACE_CDR::mb_align (payload);
13
14
if (logging_peer_.recv_n (payload->wr_ptr (), 8) == 8) {
15
payload->wr_ptr (8);
// Reflect addition of 8 bytes.
16
Receive the header info (byte
17
ACE_InputCDR cdr (payload); order & length)
18
105
Implementing the Logging_Server (4/6)
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39 }
106
ACE_CDR::Boolean byte_order; Demarshal header info
// Use helper method to disambiguate booleans from chars.
cdr >> ACE_InputCDR::to_boolean (byte_order);
cdr.reset_byte_order (byte_order);
ACE_CDR::ULong length; Resize message block to be the right size for
payload & that’s aligned properly
cdr >> length;
payload->size (8 + ACE_CDR::MAX_ALIGNMENT + length);
if (logging_peer_.recv_n (payload->wr_ptr(), length) > 0) {
payload->wr_ptr (length);
// Reflect additional bytes.
mblk->cont (payload); // Chain the header and payload.
return length; // Return length of the log record.
}
}
payload->release ();
mblk->release ();
payload = mblk = 0;
return -1;
On error, free up allocated message
blocks
Implementing the Logging_Server (5/6)
1. Send the message block chain to the log file, which is stored in binary format
2. If debug flag is set, print contents of the message
1 int Logging_Handler::write_log_record (ACE_Message_Block *mblk)
2 {
3
if (log_file_->send_n (mblk) == -1) return -1;
4
5
if (ACE::debug ()) {
6
ACE_InputCDR cdr (mblk->cont ());
7
ACE_CDR::Boolean byte_order;
8
ACE_CDR::ULong length;
9
cdr >> ACE_InputCDR::to_boolean (byte_order);
10
cdr.reset_byte_order (byte_order);
11
cdr >> length;
12
ACE_Log_Record log_record;
13
cdr >> log_record; // Extract the <ACE_log_record>.
14
log_record.print (mblk->rd_ptr (), 1, cerr);
15
}
16
17
return mblk->total_length ();
18
}
107
Implementing the Logging_Server (6/6)
1. Receives the message
2. Demarshals it into a ACE_Message_Block &
3. Writes it to the log file
int Logging_Handler::log_record ()
{
ACE_Message_Block *mblk = 0;
if (recv_log_record (mblk) == -1)
return -1;
else {
int result = write_log_record (mblk);
mblk->release (); // Free up the entire contents.
return result == -1 ? -1 : 0;
}
}
Later on we’ll see the virtue of splitting the recv_log_record()
& write_log_record() logic into two methods …
108
Iterative Logging Server
•This is the simplest possible logging server implementation
•The iterative server
implementation simply
accepts & processes
one client connection
at a time
•Clearly, this approach
does not scale up for
non-trivial applications
of the logging service!!!
•Subsequent
implementations will
enhance this version,
yet still use the logging
server framework
109
ACE_SOCK_Acceptor
ACE_SOCK_Stream
Only one client is
accepted/processed
at a time
Implementing the Iterative_Logging_Server (1/3)
#include
#include
#include
#include
#include
"ace/FILE_IO.h"
"ace/INET_Addr.h"
"ace/Log_Msg.h"
"Logging_Server.h"
"Logging_Handler.h"
Header file:
Iterative_Logging_Server.h
class Iterative_Logging_Server : public Logging_Server
{
public:
Iterative_Logging_Server (): logging_handler_ (log_file_) {}
Logging_Handler &logging_handler () {
return logging_handler_;
}
protected:
ACE_FILE_IO log_file_;
Logging_Handler logging_handler_;
// Other methods shown below...
};
110
Implementing the Iterative_Logging_Server (2/3)
virtual int open (u_short logger_port) {
if (make_log_file (log_file_) == -1)
ACE_ERROR_RETURN ((LM_ERROR, "%p\n", "make_log_file()"),
-1);
return Logging_Server::open (logger_port);
}
Override & “decorate” the Logging_Server::open() method
virtual int handle_connections () {
ACE_INET_Addr logging_peer_addr;
Override the
handle_connections()
hook method to handle one
connection at a time
if (acceptor ().accept (logging_handler_.peer (),
&logging_peer_addr) == -1)
ACE_ERROR_RETURN ((LM_ERROR, "%p\n", "accept()"), -1);
ACE_DEBUG ((LM_DEBUG, "Accepted connection from %s\n",
logging_peer_addr.get_host_name ()));
return 0;
}
111
Implementing the Iterative_Logging_Server (3/3)
virtual int handle_data (ACE_SOCK_Stream *) {
while (logging_handler_.log_record () != -1)
continue;
Delegate I/O to Logging_Handler
logging_handler_.close (); // Close the socket handle.
return 0;
}
Main program of iterative logging server
#include "ace/Log_Msg.h"
#include "Iterative_Logging_Server.h"
int main (int argc, char *argv[])
{
Iterative_Logging_Server server;
if (server.run (argc, argv) == -1)
ACE_ERROR_RETURN ((LM_ERROR, "%p\n", "server.run()"), 1);
return 0;
}
112
Concurrency Design Dimensions
•Concurrency is essential to develop scalable & robust
networked applications, particularly servers
•The next group of slides present a domain analysis of
concurrency design dimensions that address the policies &
mechanisms governing the proper use of processes, threads, &
synchronizers
•We cover the following design dimensions in this chapter:
•Iterative versus concurrent versus reactive servers
•Processes versus threads
•Process/thread spawning strategies
•User versus kernel versus hybrid threading models
•Time-shared versus real-time scheduling classes
•Task- versus message-based architectures
113
Iterative vs. Concurrent Servers
•Iterative/reactive servers handle
each client request in its entirety
before servicing subsequent
requests
•Best suited for short-duration or
114 infrequent services
•Concurrent servers handle multiple
requests from clients simultaneously
•Best suited for I/O-bound services or
long-duration services
•Also good for busy servers
Multiprocessing vs. Multithreading
•A process provides the context for
executing program instructions
•Each process manages certain
resources (such as virtual memory,
I/O handles, and signal handlers) &
is protected from other OS
processes via an MMU
•IPC between processes can be
complicated & inefficient
115
•A thread is a sequence of instructions
in the context of a process
•Each thread manages certain
resources (such as runtime stack,
registers, signal masks, priorities, &
thread-specific data)
•Threads are not protected from other
threads
•IPC between threads can be more
efficient than IPC between processes
Thread Pool Eager Spawning Strategies
•This strategy prespawns one or more OS processes or threads at server
creation time
•These``warm-started'' execution resources form a pool that improves response
time by incurring service startup overhead before requests are serviced
•Two general types of eager spawning strategies are shown below:
•These strategies based on Half-Sync/Half-Async & Leader/Followers patterns
116
The Half-Sync/Half-Async Pattern
•The Half-Sync/Half-Async
pattern decouples async
& sync service processing
in concurrent systems to
simplify programming
without unduly reducing
performance
Sync
Service
Layer
Sync Service 1
Sync Service 2
<<read/write>>
<<read/write>>
Queueing
Layer
Async
Service
Layer
Sync Service 3
Queue
<<dequeue/enqueue>>
Async Service
<<read/write>>
<<interrupt>>
External
Event Source
This pattern yields two primary benefits:
1.Threads can be mapped to separate CPUs to scale up server
performance via multi-processing
2.Each thread blocks independently, which prevents a flowcontrolled connection from degrading the QoS that other clients
receive
117
Half-Sync/Half-Async Pattern Dynamics
: External Event
Source
: Async Service
: Queue
: Sync Service
notification
read()
work()
message
message
enqueue()
notification
read()
work()
message
•This pattern defines two service
processing layers—one async &
one sync—along with a queueing
layer that allows services to
exchange messages between
the two layers
118
•The pattern allows sync services
(such as processing log records
from different clients) to run
concurrently, relative both to each
other & to async/reactive services
(such as event demultiplexing)
Drawbacks with Half-Sync/Half-Async
Problem
•Although Half-Sync/Half-Async
threading model is more
scalable than the purely reactive
model, it is not necessarily the
most efficient design
•e.g., passing a request
between the async thread &
a worker thread incurs:
•Dynamic memory (de)allocation,
•Synchronization operations,
•A context switch, &
•CPU cache updates
•This overhead can make server
latency unnecessarily high
119
Worker
Thread 1
Worker
Thread 2
Worker
Thread 3
<<get>>
<<get>>
Request Queue
<<get>>
<<put>>
handlers
acceptor
Event source
Solution
•Apply the Leader/Followers
architectural pattern to minimize
server threading overhead
The Leader/Followers Pattern
demultiplexes
•The Leader/Followers architectural
pattern is more efficient than HalfSync/Half-Async
Thread Pool
synchronizer
join()
promote_new_leader()
•Multiple threads take turns sharing
event sources to detect, demux,
dispatch, & process service requests
that occur on the event sources
*
Event Handler
Handle
uses
*
•This pattern eliminates the need for—&
the overhead of—a separate Reactor
thread & synchronized request queue
used in the Half-Sync/Half-Async
pattern
Handle Set
handle_events()
deactivate_handle()
reactivate_handle()
select()
handle_event ()
get_handle()
Iterative Handles
Concrete Event
Handler A
Handles
Concurrent Handles
Handle Sets
Concurrent
Handle Sets
Iterative
Handle Sets
120
handle_event ()
get_handle()
UDP Sockets +
TCP Sockets +
WaitForMultipleObjects()
WaitForMultpleObjects()
UDP Sockets +
select()/poll()
TCP Sockets +
select()/poll()
Concrete Event
Handler B
handle_event ()
get_handle()
Leader/Followers Pattern Dynamics
Thread 1
1.Leader
thread
demuxing
Thread 2
: Thread
Pool
: Handle
Set
: Concrete
Event Handler
join()
handle_events()
join()
event
handle_event()
2.Follower
thread
promotion
3.Event
handler
demuxing &
event
processing
4.Rejoining the
thread pool
121
thread 2 sleeps
until it becomes
the leader
thread 2
waits for a
new event,
thread 1
processes
current
event
join()
thread 1 sleeps
until it becomes
the leader
deactivate_
handle()
promote_
new_leader()
handle_
events()
reactivate_
handle()
event
handle_event()
deactivate_
handle()
Thread-per-Request On-demand Spawning Strategy
•On-demand spawning creates a new process or thread in response to the
arrival of client connection and/or data requests
•Typically used to implement the thread-per-request and thread-perconnection models
•The primary benefit of on-demand spawning strategies is their reduced
consumption of resources
•The drawbacks, however, are that these strategies can degrade
performance in heavily loaded servers & determinism in real-time systems
due to costs of spawning processes/threads and starting services
122
The N:1 & 1:1 Threading Models
•OS scheduling ensures applications use host CPU resources suitably
•Modern OS platforms provide various models for scheduling threads
•A key difference between the models is the contention scope in which threads
compete for system resources, particularly CPU time
•The two different contention scopes are shown below:
123
• Process contention scope (aka “user
threading”) where threads in the same
process compete with each other (but not
directly with threads in other processes)
• System contention scope (aka “kernel
threading”) where threads compete
directly with other system-scope threads,
regardless of what process they’re in
The N:M Threading Model
•Some operating systems
(such as Solaris) offer a
combination of the N:1 &
1:1 models, referred to as
the ``N:M'‘ hybridthreading model
•When an application
spawns a thread, it can
indicate in which
contention scope the
thread should operate
•The OS threading library
creates a user-space
thread, but only creates a
kernel thread if needed or
if the application explicitly
requests the system
contention scope
124
•When the OS kernel blocks an LWP, all user
threads scheduled onto it by the threads
library also block
•However, threads scheduled onto other
LWPs in the process can continue to make
progress
Task- vs. Message-based Concurrency Architectures
•A concurrency architecture is a binding between:
•CPUs, which provide the execution context for application code
•Data & control messages, which are sent & received from one or
more applications & network devices
•Service processing tasks, which perform services upon messages as
they arrive & depart
•Task-based concurrency
architectures structure
multiple CPUs according
to units of service
functionality in an
application
•Message-based
concurrency architectures
structure the CPUs
according to the messages
received from applications
& network devices
125
Overview of OS Concurrency Mechanisms
•Networked applications, particularly servers, must often
process requests concurrently to meet their quality of
service requirements
•This section presents an overview of the
•Synchronous event demultiplexing
•Multiprocessing
•Multithreading &
•Synchronization
mechanisms available to implement those designs
•We also discuss common portability & programming
problems that arise when networked applications are
developed using native C-level concurrency APIs
126
Synchronous Event Demultiplexing
• Synchronous event demuxers wait for certain events to occur on a set of event
sources, where the caller is returned the thread of control whenever one or more event
sources become active
• e.g., poll() on System V UNIX, WaitForMultipleObjects() on Win32, &
select()
•select() is the most common event demultiplexing API for I/O handles
int select (int width,
// Maximum handle plus 1
fd_set *read_fds,
// Set of "read" handles
fd_set *write_fds,
// Set of "write" handles
fd_set *except_fds,
// Set of "exception" handles
struct timeval *timeout);// Time to wait for events
•fd_set is a structure •select() modifies the fd_set depending on the
representing the set
active/inactive handles as follows:
of handles to check
• If a handle is not active in the fd_set, it is ignored & select
for I/O events, such
will keep it inactive in the fd_set
as ready for reading,
• If a handle is active, select() will determine if there are
writing, or exception
pending events. If there is one, the appropriate fd_set has
events
the handle activated else its value is made inactive in the
returned fd_set
127
Multiprocessing Mechanisms
• Multiprocessing mechanisms include the features provided by the OS for creating &
managing the execution of multiple processes, e.g.,
• Process lifetime operations – such as fork() & exec*() on POSIX &
CreateProcess() on Win32 create a new process address space for programs to
run
• The initiating process can set command line arguments, environment variables
and working directories for the new process
• The new process can terminate voluntarily by reaching the end of its execution or
be involuntarily killed via signals (in POSIX) or TerminateProcess() (in Win32)
• Process synchronization options – provided by the OS to retain the identity and
exit status of a process and report it to the parent, e.g.,
• POSIX wait() & waitpid()
• Win32 WaitForSingleObject() & WaitForMultipleObjects()
• Process property operations – used to get/set process properties, such as default
file access permissions, user identification, resource limits, scheduling priority and
current working directory
128
Multithreading Mechanisms (1/2)
• Multithreading mechanisms are provided by the OS to handle thread lifetime
management, synchronization, priorities, & thread specific storage
• Thread lifetime operations – include operations to create threads, e.g.,
pthread_create() (PThreads) & CreateThread() (Win32)
• Thread termination is achieved in the following manner:
• Voluntarily – by reaching the end point of the thread entry function or
calling pthread_exit() (Pthreads) or ExitThread() (Win32)
• Involuntarily – by being killed via a signal or an aynchronous thread
cancelation operations, such as pthread_cancel() (Pthreads) and
TerminateThread() (Win32)
• Thread synchronization operations – that allow created threads to be
• Detached – where the OS reclaims storage used for the thread’s state & exit
status after it has exit
• Joinable – where the OS retains identity & exit status of a terminating thread
so other threads can synchronize with it
• Other operations that allow threads to suspend & resume each other, or send
signals to other threads
129
Multithreading Mechanisms (2/2)
• Thread property operations – includes operations to set and get thread
properties, such as priority and scheduling class.
• Thread-specific storage – is similar to global data except that the data is
global in the scope of the executing thread.
• Each thread has its own copy of a TSS data e.g., errno
• Each TSS item has a key that is global to all threads within a process
• A thread uses this key to access its copy of the TSS data
• Keys are created by factory functions, such as pthread_key_create()
(Pthreads) or TlsAlloc() (Win32).
• Key/pointer relationships are managed by TSS set/get functions, such as
pthread_getspecific() & pthread_setspecific() (Pthreads) or
TlsGetValue() & TlsSetValue() (Win32)
130
Synchronization Mechanisms (1/2)
• Synchronization mechanisms allow processes and threads to
coordinate their execution order and the order in which they access
shared resources, such as files, network devices, database records, and
shared memory
• Mutexes – serialize execution of multiple threads by defining a critical
section of code that can be executed by only one thread at a time. A
thread owning a mutex must release it
• There are two kinds of mutexes:
• Nonrecursive mutex – that will deadlock or fail if the thread currently
owning the mutex tries to reacquire it without first releasing it
• Recursive mutex – that will allow the thread owning the mutex to
reacquire it without deadlocking
• The owner thread is responsible to release it the same number of
times it has acquired it
131
Synchronization Mechanisms (2/2)
• Readers/writer locks – allows access to a shared resource by either
multiple threads simultaneously having read-only access or only one thread
at a time having a read-write access
• They help improve performance for applications where resources are read
more often than modified
• They can be implemented to give more preference to either the readers or
the writer
• Semaphores – is a non negative integer that can be incremented and
decremented atomically
• A thread blocks when it tried to decrement a semaphore whose value is 0
• A block thread makes progress when another thread increments the value
of the semaphore
• Usually implemented using sleep locks, that trigger a context switch
• Condition variables – allows a thread to coordinate & schedule its own
processing
• A thread can wait on complex expressions to attain a desired state
• Used to build higher level patterns such as active object & monitor objects
132
Sidebar: Evaluating Synchronization Mechanisms
• Performance of synchronization mechanisms depends on the OS
implementation and hardware
• Some general issues to keep in mind:
• Condition variables & semaphores – generally have a higher overhead
than mutexes
• Native OS implementations usually perform better than emulated
behavior
• Mutexes versus Reader/Writer Locks – mutexes generally have the lower
overhead than reader/writer locks.
• On a multiprocessor platform, reader/writer locks scale well since multiple
readers can execute in parallel
• Nonrecursive mutexes – are more efficient than recursive mutexes
• Moreover, subtle errors can be caused using recursive mutexes due to
mismatch in the number of lock & unlock operations
133
Sidebar: ACE API Error Propagation Strategies
• Error reporting strategies usually differ across different concurrency
APIs & OS
•e.g., UI & Pthreads return 0 on success and a non-zero number on
failure whereas Win32 returns 0 on failure and conveys the error
value via thread specific storage
• This makes code non-portable and filled with accidental complexities
• ACE concurrency wrapper facades solve this problem by returning -1
on error and setting the errno variable in thread specific storage
134
The ACE Event Demuxing Wrapper Facades
•The reactive server model can be thought of as “lightweight multitasking,”
where a single-threaded server communicates with multiple clients in a
round-robin manner without introducing the threading & synchronization
overhead & complexity
•This server concurrency strategy uses an event loop that examines &
reacts to events from its clients continuously
•An event loop demultiplexes input from various event sources so they can
be processed in an orderly way
•Event sources in networked applications are primarily socket handles
•The most popular event demultipelxing function is select(), which
provides the basis for the ACE classes described below
135
The ACE_Handle_Set Class (1/2)
Motivation
•The fd_set represents a source of accidental complexity in the following
areas:
•The code to scan for active handles is often a “hot spot” since it
executes continually in a tight loop
•The macros supplied to manipulate & scan an fd_set must be used
carefully to avoid processing handles that aren't active & to avoid
corrupting an fd_set
•The fd_set is defined in system-supplied header files whose
representation is exposed to programmers
•There are subtle nonportable aspects of fd_set when used in
conjunction with select()
136
The ACE_Handle_Set Class (2/2)
Class Capabilities
•The ACE_Handle_Set class uses the Wrapper Façade pattern to
encapsulate fd_sets & provide the following capabilities:
•It enhances portability, ease of use, & type safety of event-driven
applications that use of fd_set & select() across OS platforms
•It tracks & adjusts the fd_set size-related values automatically as handles
are added & removed
•ACE_Handle_Set_Iterator is an optimized iterator for ACE_Handle_Set
137
Reactive Logging Server Version 1
•This example enhances the earlier iterative logging server implementation by
using select() together with the ACE_Handle_Set &
ACE_Handle_Set_Iterator classes
This server
demultiplexes the
following two types of
events:
• Arrival of new
connections from
clients
• Arrival of log
records on client
connections
138
Using the ACE_Handle_Set (1/4)
• We use a pair of ACE_Handle_Set data members since
select() modifies the fd_set parameters passed to it, so
we need to keep a master copy.
class Reactive_Logging_Server
: public Iterative_Logging_Server
{
protected:
// Keeps track of the acceptor socket handle and all the
// connected stream socket handles.
ACE_Handle_Set master_handle_set_;
// Keep track of handles marked as active by <select>.
ACE_Handle_Set active_handles_;
typedef Iterative_Logging_Server PARENT;
// Other methods shown below...
};
139
Using the ACE_Handle_Set (2/4)
virtual int open (u_short logger_port) {
PARENT::open (logger_port);
master_handle_set_.set_bit (acceptor ().get_handle ());
acceptor ().enable (ACE_NONBLOCK);
return 0; •open() method uses ACE_Handle_Set to track the
}
acceptor’s handle in fd_set
• Note the use of non-blocking acceptor...
virtual int wait_for_multiple_events () {
active_handles_ = master_handle_set_;
140
• Override hook method
if (select (active_handles_.max_set () + 1,
active_handles_.fdset (),
0,
// no write_fds
0,
// no except_fds • Use select() to determine
0) == -1) // no timeout
all active handles
return -1;
active_handles_.sync
((ACE_HANDLE) active_handles_.max_set () + 1);
return 0; sync() resets handle count in ACE_Handle_Set after the
select() call
}
Sidebar: Motivation for Non-blocking Acceptors
Context
• An acceptor socket is passed to select() & gets marked as active
when a connection is received
• Many servers use this event to call accept() without blocking
Problem
• Client disconnects exactly within the time interval between the server
making the select() & accept() calls
• Possible race condition due to asynchronous behavior of TCP/IP
leading to accept() call being blocked forever & hanging the
application
Solution
• Acceptor sockets should always be set in non-blocking mode
• Achieved portably in ACE via the enable()method of
ACE_IPC_SAP class passing it the ACE_NONBLOCK flag
141
Using the ACE_Handle_Set (3/4)
•Override the hook method
virtual int handle_connections () {
if (active_handles_.is_set (acceptor ().get_handle ())) {
while (acceptor ().accept (logging_handler ().peer ()) == 0)
master_handle_set_.set_bit
(logging_handler ().peer ().get_handle ());
// Remove acceptor handle from further consideration.
active_handles_.clr_bit (acceptor ().get_handle ());
}
return 0;
}
142
•If the acceptor() handle is
active, iteratively accept all the
connections & save them in
master_handle_set_
Using the ACE_Handle_Set (4/4)
Reactive logging server main program
#include "ace/Log_Msg.h"
#include "Reactive_Logging_Server.h"
int main (int argc, char *argv[])
{
Reactive_Logging_Server server;
if (server.run (argc, argv) == -1)
ACE_ERROR_RETURN ((LM_ERROR, "%p\n", "server.run()"), 1);
return 0;
}
handle_data() method shown later using
ACE_Handle_Set_Iterator class
143
Using the ACE_Handle_Set_Iterator (1/2)
This method has problems. Do not copy this example!
virtual int handle_data (ACE_SOCK_Stream *) {
for (ACE_HANDLE handle = acceptor ().get_handle () + 1;
handle < active_handles_.max_set () + 1;
handle++) {
• Non-portable assumption that socket handles are a contiguous range integers
• Cannot assume acceptor socket has lowest number
if (active_handles_.is_set (handle)) {
logging_handler ().peer ().set_handle (handle);
if (logging_handler ().log_record () == -1) {
// Handle connection shutdown or comm failure.
master_handle_set_.clr_bit (handle);
logging_handler ().close ();
}
}
It’s inefficient to sequentially search
}
for active handles in the large, but
return 0;
sparsely populated handle set
144
}
Using the ACE_Handle_Set_Iterator (2/2)
Overcoming drawbacks outlined before in handle_data() method by using
the ACE_Handle_Set_Iterator class
virtual int handle_data (ACE_SOCK_Stream *) {
ACE_Handle_Set_Iterator peer_iterator (active_handles_);
for (ACE_HANDLE handle;
(handle = peer_iterator ()) != ACE_INVALID_HANDLE;
) {
logging_handler ().peer ().set_handle (handle);
if (logging_handler ().log_record () == -1) {
// Handle connection shutdown or comm failure.
master_handle_set_.clr_bit (handle);
logging_handler ().close ();
}
}
}
•ACE_Handle_Set_Iterator optimizes searching for active handles based on
the underlying platform’s representation of fd_set
• These optimizations are encapsulated within a common interface, making the
145 class portable
Reactive Logging Server Version 2
•We now extend our reactive server example to write log records from
different clients to different log files, one for each connected client
•This reactive server
implementation
maintains a map
container that allows a
logging server to keep
separate log files for
each of its clients
•The figure also shows
how we use the
ACE::select()
wrapper method & the
ACE_Handle_Set
class to service
multiple clients via a
reactive server model
146
Implementing the Reactive Logging Server (1/6)
class ACE {
public:
static int select (int width, ACE_Handle_Set &rfds,
const ACE_Time_Value *tv = 0);
static int select (int width, ACE_Handle_Set *rfds,
ACE_Handle_Set *wfds = 0, ACE_Handle_Set *efds = 0,
const ACE_Time_Value *tv = 0);
// ... Other methods omitted ...
};
#include
#include
#include
#include
#include
#include
"ace/ACE.h"
Reactive logging server using
"ace/Handle_Set.h"
"ace/Hash_Map_Manager.h" ACE::select() & hash map
container classes
"ace/Synch.h"
"Logging_Server.h"
"Logging_Handler.h"
typedef ACE_Hash_Map_Manager<ACE_HANDLE,
ACE_FILE_IO *,
ACE_Null_Mutex> LOG_MAP;
147
Implementing the Reactive Logging Server (2/6)
Association between each connected peer &
its log file is maintained in a hash map
class Reactive_Logging_Server_Ex : public Logging_Server
{
protected:
// Associate an active handle to an <ACE_FILE_IO> pointer.
LOG_MAP log_map_;
// Keep track of acceptor socket and all the connected
// stream socket handles.
ACE_Handle_Set master_handle_set_;
// Keep track of read handles marked as active by <select>.
ACE_Handle_Set active_read_handles_;
typedef Logging_Server PARENT;
// Other methods shown below...
};
148
Implementing the Reactive Logging Server (3/6)
open() method similar to previous one for
first version of reactive logging server
virtual int open (u_short logger_port) {
PARENT::open (logger_port);
master_handle_set_.set_bit (acceptor ().get_handle ());
acceptor ().enable (ACE_NONBLOCK);
return 0;
}
virtual int wait_for_multiple_events () {
active_read_handles_ = master_handle_set_;
int width = (int) active_read_handles_.max_set () + 1;
return ACE::select (width, active_read_handles_);
}
Note use of ACE::select(), which calls active_read_handles_.sync()
automatically
149
Implementing the Reactive Logging Server (4/6)
• This version of reactive logging server creates a log file when a peer establishes
connection
• The association between the log file & the handle is maintained in the hash map
virtual int handle_connections () {
ACE_SOCK_Stream logging_peer;
while (acceptor ().accept (logging_peer) != -1) {
ACE_FILE_IO *log_file = new ACE_FILE_IO;
// Use the client's hostname as the logfile name.
make_log_file (*log_file, &logging_peer);
// Add the new <logging_peer>'s handle to the map and
// to the set of handles we <select> for input.
log_map_.bind (logging_peer.get_handle (), log_file);
master_handle_set_.set_bit (logging_peer.get_handle ());
}
// Remove acceptor handle from further consideration...
active_read_handles_.clr_bit (acceptor ().get_handle ());
return 0;
150
}
Implementing the Reactive Logging Server (5/6)
virtual int handle_data (ACE_SOCK_Stream *) {
ACE_Handle_Set_Iterator peer_iterator (active_read_handles_);
for (ACE_HANDLE handle;
(handle = peer_iterator ()) != ACE_INVALID_HANDLE;
) {
Identify the log file corresponding to the peer
ACE_FILE_IO *log_file; who sent the data
log_map_.find (handle, log_file);
Logging_Handler logging_handler (*log_file, handle);
if (logging_handler.log_record () == -1) {
logging_handler.close ();
master_handle_set_.clr_bit (handle);
log_map_.unbind (handle);
log_file->close ();
delete log_file;
Free up resources on error
}
}
return 0;
151
}
Implementing the Reactive Logging Server (6/6)
Reactive logging server main program (this
should start looking rather familiar by now… ;-))
int main (int argc, char *argv[])
{
Reactive_Logging_Server_Ex server;
if (server.run (argc, argv) == -1)
ACE_ERROR_RETURN ((LM_ERROR, "%p\n", "server.run()"), 1);
return 0;
}
152
The ACE Process Wrapper Facades
OS multiprocessing support helps to
• Enable concurrency by scheduling & running separate processes on different CPUs
• Increase robustness by using memory-management unit (MMU) hardware to
protect separate process address spaces from accidental or malicious corruption by
other active processes in the system
• Enhance security by allowing each process to verify or control per-user or persession security & authentication information
We now cover the following ACE classes that can use to spawn & manage one or more
processes:
These classes are related as follows:
153
The ACE_Process Class (1/2)
Motivation
UNIX
•OS process management
mechanisms differ
syntactically & semantically
•e.g., the UNIX fork()
system function is very
different than Windows
CreateProcess()
•Addressing these platform
variations in each
application is difficult,
tedious, error prone, &
unnecessary since ACE
provides the ACE_Process
class
154
Windows
The ACE_Process Class (2/2)
Class Capabilities
•This class encapsulates
the variation among
different OS
multiprocessing APIs in
accordance with the
Wrapper Facade pattern
by defining methods that
provide the following
capabilities:
•Spawn & terminate processes
•Synchronize on process exit
•Access process properties, such as process ID
•Most methods are portable, though some are platform-specific
155
Sidebar: POSIX Portability Challenges
Context
• Many Unix applications use POSIX fork() system call to build concurrent
process-based servers
Problem
• Portability is a concern if we rely on fork() to both:
•Have a parent process fork() a child process with a duplicate of the
parent’s address space and all I/O handles
•Have both parent and child return from fork() with the same location
but with different return values, which can then run different parts of the
program
Forces
•fork() is the only system call that allows duplicating an address space
• I/O handles of the parent are often required in the child process
Solution
• Use the ACE_Process_Options class that provides methods such as
pass_handle(), working_directory(), setenv() & others
156
Using ACE_Process
• This program uses ACE_Process to computes factorials “recursively”
int main (int argc, char *argv[])
{ ACE_Process_Options options;
char *n_env = 0; int n;
if (argc == 1) { // Top-level process.
n_env = ACE_OS::getenv ("FACTORIAL");
n = n_env == 0 ? 0 : atoi (n_env);
options.command_line ("%s %d", argv[0], n == 0 ? 10 : n);
}
else if (atoi (argv[1]) == 1) return 1; // Base case.
else {
n = atoi (argv[1]);
options.command_line ("%s %d", argv[0], n - 1);
}
Pass command line for spawned child to use
ACE_Process child;
child.spawn (options); // Make the ``recursive'' call.
child.wait ();
return n * child.exit_code (); // Compute n factorial.
}
157
The ACE_Process_Options Class (2/2)
Class Capabilities
•This class unifies how process properties are passed to ACE_Process
& ACE_Process_Manager to provide the following capabilities:
•Enable an application to specify desired process control information
•Allow process control items to be expanded as platforms change
•Provide a decoupling mechanism that enables ACE to offer these
capabilities without varying the process creation interface
158
The ACE_Process_Options Class (1/2)
Motivation
•Operating systems provide various methods for setting the
properties of newly-created processes, including
• Program image, which program should the new process
execute?
• Open I/O handles, e.g., should the child process inherit
open I/O handles or other OS objects; should it close
some or all of its inherited open handles?
• Working directory, e.g., should the child process run in
the same or different directory as its parent?
• Process relationship, e.g., should the child process run
in the background as an independent daemon or as part
of a related group?
159
Using ACE_Process_Options (1/2)
•This example demonstrates how to use ACE_Process_Options to pass
environment, working directory, & command line to spawned child
process
int main (int argc, char *argv[])
{
ACE_Process_Options options;
FILE *fp = 0;
char *n_env = 0;
int n;
if (argc == 1) { // Top-level process.
n_env = ACE_OS::getenv ("FACTORIAL");
n = n_env == 0 ? 0 : atoi (n_env);
options.command_line ("%s %d", argv[0], n == 0 ? 10 : n);
const char *working_dir = ACE_OS::getenv ("WORKING_DIR");
if (working_dir) options.working_directory (working_dir);
fp = fopen ("factorial.log", "a");
options.setenv ("PROGRAM=%s", ACE::basename (argv[0]));
}
160
Using ACE_Process_Options (2/2)
else {
fp = fopen ("factorial.log", "a");
if (atoi (argv[1]) == 1) {
fprintf (fp, "[%s|%d]: base case\n",
ACE_OS::getenv ("PROGRAM"), ACE_OS::getpid ());
fclose (fp);
return 1; // Base case.
} else {
n = atoi (argv[1]);
options.command_line ("%s %d", argv[0], n - 1);
}
Wait for child to exit so we can get its exit status (which
}
will be a factorial value it has computed for “n-1”)
ACE_Process child;
child.spawn (options); // Make the ``recursive'' call.
child.wait ();
int factorial = n * child.exit_code (); // Compute n factorial.
fprintf (fp, "[%s|%d]: %d! == %d\n",
ACE_OS::getenv ("PROGRAM"), ACE_OS::getpid (),
n, factorial);
fclose (fp);
return factorial;
161
}
The ACE_Process_Manager Class (1/2)
Motivation
•Complex networked applications often require groups of processes to
coordinate to provide a particular service, e.g.,
•A multistage workflow automation application may spawn multiple
processes to work on different parts of a large problem
•One master process may wait for the entire group of worker processes to
exit before proceeding with the next stage in the workflow
162
The ACE_Process_Manager Class (2/2)
Class Capabilities
•This class uses the Wrapper Facade pattern to combine the portability
& power of ACE_Process with the ability to manage groups of
processes as a unit & provide the following capabilities:
•It provides internal record keeping to manage & monitor groups of
processes that are spawned by the ACE_Process class
•It allows one process to spawn a group of process & wait for them to
exit before proceeding with its own processing
163
Multiprocessing Logging Server
•This revision of the logging server uses a process-per connection
concurrency model
•The master process spawns a new worker process for each accepted
connection to the logging service port
•The master process then continues to accept new connections
•Each worker process handles all logging requests sent by a client across one
connection; the process exits when this connection closes
164
Using ACE_Process_Manager (1/6)
class Process_Per_Connection_Logging_Server
: public Logging_Server
{
public:
•Process-per-connection
// … Methods shown below …
logging server that uses the
protected:
ACE_Process_Manager
std::string prog_name_;
};
virtual int run (int argc, char *argv[]) {
prog_name_ = argv[0];
// Ensure NUL-termination.
prog_name_[prog_name_.size () - 1] = '\0';
if (argc == 3)
return run_worker (argc, argv); // Only on Win32.
else
return run_master (argc, argv);
}
165
Process Creation on POSIX & Windows
UNIX
•Master/Worker Process
Creation Sequence for
Windows
•Child doesn’t automatically
inherit the parent process’s
image, so we must take other
steps…
•e.g., pass the socket handle
166
•Master/Worker
Process Creation
Sequence for POSIX
•Note use of child
copy of parent’s
process image
Windows
Using ACE_Process_Manager (2/6)
int run_master (int argc, char *argv[]) {
u_short logger_port = 0;
if (argc == 2) logger_port = atoi (argv[1]);
if (open (logger_port) == -1) return -1;
for (;;)
if (handle_connections () == -1) return -1;
return 0;
}
Method only gets invoked for Win32
int run_worker (int argc, char *argv[]) {
ACE_HANDLE socket_handle =
ACE_static_cast (ACE_HANDLE, atoi (argv[2]));
ACE_SOCK_Stream logging_peer (socket_handle);
handle_data (&logging_peer);
logging_peer.close ();
return 0;
}
167
Sidebar: Portable Casting on C++ Compilers
Context
• C language cast operator used frequently for type casting
Problem
• Can lead to subtle errors due to type violations caused by misuse
• Although C++ introduces several keywords to allow robust type casting, they are not
supported across the spectrum of C++ compilers
Solution
• Use ACE cast macros shown below
ACE Cast Macro
C++ Cast Used if Available
ACE_const_cast (TYPE, EXPR)
const_cast<TYPE> (EXPR)
ACE_static_cast (TYPE, EXPR)
static_cast<TYPE> (EXPR)
ACE_dynamic_cast (TYPE, EXPR)
dynamic_cast<TYPE> (EXPR)
ACE_reinterpret_cast (TYPE, EXPR)
reinterpret_cast<TYPE> (EXPR)
• These methods are largely deprecated in ACE since new C++ compilers are better
168
Using ACE_Process_Manager (3/6)
1 virtual int handle_connections () {
2
ACE_SOCK_Stream logging_peer;
3
if (acceptor ().accept (logging_peer) == -1)
4
return -1;
Create a Logger_Process that the process manager will manage
5
6
Logging_Process *logger =
7
new Logging_Process (prog_name_, logging_peer);
8
ACE_Process_Options options;
Note use of Singleton pattern
9
pid_t pid = ACE_Process_Manager::instance ()->spawn
10
(logger, options);
spawn() invokes hook methods on logger
11
if (pid == 0) {
12
acceptor().close (); to initialize options properly
13
handle_data (&logging_peer);
14
delete logger;
On POSIX, this will be a child, so close
the acceptor, handle the data, & release
15
ACE_OS::exit (0);
resources
16
}
169
On Win32, the child process created after the spawn call above will end up
running the run_worker() method shown earlier instead of lines 11 thru 16
Singleton Pattern
• Intent
• Ensure a class has only one
instance & provide a global
point of access to it
• Context
• Need to address initialization
versus usage ordering
• Problem
• Want to ensure a single
instance of a class, shared
by all uses throughout a
program
class Singleton {
public:
static Singleton *instance (){
if (instance_ == 0) {
instance_ = new Singleton;
}
return instance_;
}
void method_1 ();
// … Other methods omitted …
private:
// Private constructor
Singleton ();
// Initialized to 0 by linker.
static Singleton *instance_;
};
• Solution
• Provide a global access method (static in C++)
• First use of the access method instantiates the class
• Constructors for instance are made private
170
Using ACE_Process_Manager (4/6)
17
logging_peer.close ();
Parent does not need the peer
18
if (pid == -1)
19
ACE_ERROR_RETURN ((LM_ERROR, "%p\n", "spawn()"), -1);
20
21
return ACE_Process_Manager::instance ()->wait
22
(0, ACE_Time_Value::zero);
23 }
virtual int handle_data (ACE_SOCK_Stream *logging_peer) {
// Ensure blocking <recv>s.
logging_peer->disable (ACE_NONBLOCK);
ACE_FILE_IO log_file;
make_log_file (log_file, logging_peer);
Logging_Handler logging_handler (log_file, *logging_peer);
while (logging_handler.log_record () != -1) continue;
log_file.close ();
return 0;
handle_data() method similar to before (uses blocking I/O)
}
171
Using ACE_Process_Manager (5/6)
class Logging_Process : public ACE_Process
{
Logging_Process class extends ACE_Process
private:
Logging_Process (); // Force desired constructor to be used.
std::string prog_name_;
ACE_SOCK_Stream logging_peer_;
public:
Logging_Process (const std::string &prog_name,
const ACE_SOCK_Stream &logging_peer)
: logging_peer_ (logging_peer.get_handle ())
{ prog_name_ (prog_name); }
prepare() hook method is invoked by spawn() prior to the actual spawning
virtual int prepare (ACE_Process_Options &options) {
if (options.pass_handle (logging_peer_.get_handle ()) == -1)
ACE_ERROR_RETURN ((LM_ERROR, "%p\n", "pass_handle()"), -1);
pass_handle() adds “+H <handle>” to command line on Windows
options.command_line ("%s", prog_name_.c_str ());
options.avoid_zombies (1); Don’t exec() after fork() on POSIX
options.creation_flags (ACE_Process_Options::NO_EXEC);
return 0;
172
}
Using ACE_Process_Manager (6/6)
virtual void unmanage () {
Invoked by process manager as it removes the
delete this;
managed process
}
static void sigterm_handler (int /* signum */) { /* No-op. */ }
int main (int argc, char *argv[])
{
// Register to receive the <SIGTERM> signal.
ACE_Sig_Action sa (sigterm_handler, SIGTERM);
Process_Per_Connection_Logging_Server server;
if (server.run (argc, argv) == -1 && errno != EINTR)
ACE_ERROR_RETURN ((LM_ERROR, "%p\n", "server.run()"), 1);
// Barrier synchronization.
return ACE_Process_Manager::instance ()->wait ();
}
173
The ACE Threading Wrapper Facades
• Multithreading is useful for servers that manage connection-oriented or connectionless
network associations for many clients simultaneously
• Today's powerful OS support for multithreading helps networked applications to:
• Leverage hardware advances, such as symmetric multiprocessing that enables
true execution parallelism
• Increase performance by overlapping computation & communication
• Improve response time for GUIs & network servers to ensure that time-sensitive
tasks are scheduled as needed
• Simplify program structure by enabling applications to use intuitive synchronous
programming mechanisms, rather than more complex asynchronous programming
mechanisms
• Below we describe the following ACE classes that networked applications can use to
spawn & manage one or more threads of control within a process:
174
The ACE_Thread_Manager Class (1/2)
Motivation
•Different operating systems use different
APIs to create & manage threads, which
causes two types of non-portable
variability:
•Syntactic, e.g., the Win32
CreateThread() & the Pthreads
pthread_create() functions provide
similar thread creation capabilities, even
though their APIs differ syntactically
•Semantic, e.g., both Pthreads and UI
threads support detached threads,
whereas Win32 does not, & VxWorks
supports only detached threads
175
The ACE_Thread_Manager Class (2/2)
Class Capabilities
•This class uses the Wrapper
Facade pattern to encapsulate
variation among different OS
multithreading APIs & provide the
following portable capabilities:
•Spawns one or more threads,
each running an applicationdesignated function concurrently
•Alters the most common thread
attributes, for example,
scheduling priority & stack size,
for each of the spawned threads
•Spawns & manages a set of threads as a cohesive collection, called a
thread group
•Manages the threads in an ACE_Task (discussed in C++NPv2 tutorial)
•Facilitates cooperative cancelation of threads,
176 •Waits for one or more threads to exit
ACE_Thread_Manager::spawn() Options
The ACE_Thread_Manager::spawn() method can be passed a set of
flags to specify the properties of the created thread, whose value is a bitwise inclusive ``or'' of the flags shown in the following table:
177
Thread-per-Connection Logging Server
•This example illustrates the thread-per-connection concurrency model
•The master thread runs continuously and plays the role of a factory that
1.Accepts a connection & creates an ACE_SOCK_Stream object dynamically
2.Spawns a worker thread that uses this object to handle this client's logging
•The worker thread performs all subsequent log record processing on the
ACE_SOCK_Stream & destroys it when the connection is closed
Thread_Per_
Connection_
Logging_
Server
178
Sidebar: Traps & Pitfalls of Mixing Objects & Threads
Context
•Lifecycle of objects passed as parameters to thread entry point functions
must be managed carefully
Problem
•Programmers forget to make the following distinction
•A thread is a unit of execution
•An object is a chunk of memory with associated methods
•There is no implicit connection between a thread & any object that the
thread accesses during execution
Forces
•A thread should not be able access an object after the latter has been
deleted
Solution
•Dynamically allocate the object passing it to the thread function as parameter
& let the latter delete it
179
Sidebar: How Threads are Spawned in ACE
The figure below show the calls that occur when ACE_Thread_Manager::
spawn() is invoked on a platform configuration that uses the UI Threads
thr_create() system function:
Regardless of OS, the following steps occur to spawn a thread:
•The OS creates a thread execution context
•The OS allocates memory for the thread's stack
•The new thread's register set is prepared so that when it's scheduled into
execution, it will call the thread entry point function supplied as a
parameter to spawn()
•The thread is marked runnable so the OS can start executing it
180
Using ACE_Thread_Manager (1/5)
•This example uses the ACE_Thread_Manager to
implement our first multithreaded logging server based
on a thread-per-connection concurrency model
class Thread_Per_Connection_Logging_Server : public Logging_Server
{
private:
class Thread_Args {
public:
Thread_Args (Thread_Per_Connection_Logging_Server *lsp)
: this_ (lsp) {}
Thread_Per_Connection_Logging_Server *this_;
ACE_SOCK_Stream logging_peer_;
};
// Passed as a parameter to <ACE_Thread_Manager::spawn>.
static void *run_svc (void *arg);
protected:
// Other methods shown below...
};
181
Using ACE_Thread_Manager (2/5)
virtual int handle_connections () {
auto_ptr<Thread_Args> thread_args (new Thread_Args (this));
if (acceptor ().accept (thread_args->logging_peer_) == -1)
return -1;
if (ACE_Thread_Manager::instance ()->spawn (
// Pointer-to-function entry point.
Thread_Per_Connection_Logging_Server::run_svc,
// <run_svc> parameter.
Arguments to pass to thread entry point function
ACE_static_cast (void *, thread_args.get ()),
Create a detached kernel thread
THR_DETACHED | THR_SCOPE_SYSTEM) == -1)
return -1;
thread_args.release ();
// Spawned thread now owns memory
return 0;
}
182
Using ACE_Thread_Manager (3/5)
• Thread entry point “adapter” method that handles incoming data from peer
• This method will also clean up resources, such as closing the peer
connection
void *Thread_Per_Connection_Logging_Server::run_svc (void *arg)
{
auto_ptr<Thread_Args>
thread_args (ACE_static_cast (Thread_Args *, arg));
We’re responsible for deleting this dynamically allocated
memory when we’re done, so we use an auto_ptr
thread_args->this_->handle_data (&thread_args->logging_peer_);
thread_args->logging_peer_.close ();
return 0;
// Return value is ignored.
}
183
Using ACE_Thread_Manager (4/5)
virtual int handle_data (ACE_SOCK_Stream *client) {
ACE_FILE_IO log_file;
// Client's hostname used as logfile name.
make_log_file (log_file, client);
// Place the connection into blocking mode.
client->disable (ACE_NONBLOCK);
Logging_Handler logging_handler (log_file, *client);
ACE_Thread_Manager *tm = ACE_Thread_Manager::instance ();
ACE_thread_t me = ACE_OS::thr_self ();
Note the use of the Singleton pattern
// Keep handling log records until client closes connection
// or this thread is asked to cancel itself.
while (!tm->testcancel (me)
&& logging_handler.log_record () != -1) continue;
log_file.close ();
return 0;
}
184
Note the use of cooperative thread cancellation
Sidebar: Serializing Singletons in ACE
• As discussed earlier, the Singleton pattern ensures that a class has only one instance
& provides a global point of access to it
• ACE singletons use the Double-Checked Locking Optimization pattern to reduce
contention and synchronization overhead when critical sections of code must acquire
locks in a thread-safe manner just once during program execution
ACE_Thread_Manager *ACE_Thread_Manager::instance () {
if (ACE_Thread_Manager::thr_mgr_ == 0) {
ACE_GUARD_RETURN (ACE_Recursive_Thread_Mutex, ace_mon,
*ACE_Static_Object_Lock::instance(),
0));
if (ACE_Thread_Manager::thr_mgr_ == 0)
ACE_Thread_Manager::thr_mgr_ = new ACE_Thread_Manager;
}
return ACE_Thread_Manager::thr_mgr_;
}
185
ACE_Static_Object_Lock instance is created
prior to execution of main() program
Using ACE_Thread_Manager (5/5)
Thread-per-connection logging server main program
int main (int argc, char *argv[])
{
// Register to receive the <SIGTERM> signal.
ACE_Sig_Action sa (sigterm_handler, SIGTERM);
Thread_Per_Connection_Logging_Server server;
if (server.run (argc, argv) == -1)
ACE_ERROR_RETURN ((LM_ERROR, "%p\n", "server.run()"), 1);
// Cooperative thread cancelation.
ACE_Thread_Manager::instance ()->cancel_all ();
// Barrier synchronization, wait no more than a minute.
ACE_Time_Value timeout (ACE_OS::gettimeofday ());
timeout += 60;
return ACE_Thread_Manager::instance ()->wait (&timeout);
}
186
The ACE_Sched_Params Class (1/2)
Motivation
•Certain types of
networked applications,
particularly those with
real-time requirements,
need strict control over
their thread priorities
•It’s important that this
control be as portable
as possible
187
The ACE_Sched_Params Class (2/2)
Class Capabilities
•This class uses the Wrapper
Facade pattern to
encapsulate OS scheduling
class APIs & can be used
with the
ACE_OS::sched_params()
method to provide the
following capabilities:
•A portable means to specify scheduling policies, such as FIFO
•A way to specify a time-slice quantum for the round-robin scheduling
policy
•A way to specify the scope in which the policy applies, for example, to the
current process or the current thread
•A consistent representation of scheduling priorities, which is necessary
since higher scheduling priorities are indicated by lower priority values on
some OS platforms
188
Using ACE_Sched_Params (1/2)
•This example demonstrates a real-time thread-per-connection logging
server whose scheduling parameters can be controlled via
ACE_Sched_Params
class RT_Thread_Per_Connection_Logging_Server
: public Thread_Per_Connection_Logging_Server
{
public:
virtual int open (u_short port) {
ACE_Sched_Params fifo_sched_params
(ACE_SCHED_FIFO,
ACE_Sched_Params::priority_min (ACE_SCHED_FIFO),
ACE_SCOPE_PROCESS);
if (ACE_OS::sched_params (fifo_sched_params) == -1) {
if (errno == EPERM || errno == ENOTSUP)
ACE_DEBUG ((LM_DEBUG,
"Warning: user's not superuser, so "
"we'll run in the time-shared class\n"));
else
189
Using ACE_Sched_Params (2/2)
ACE_ERROR_RETURN ((LM_ERROR,
"%p\n", "ACE_OS::sched_params()"), -1);
}
// Initialize the parent classes.
return Thread_Per_Connection_Logging_Server::open (port);
}
virtual int handle_data (ACE_SOCK_Stream *logging_client) {
int prio =
ACE_Sched_Params::next_priority
(ACE_SCHED_FIFO,
ACE_Sched_Params::priority_min (ACE_SCHED_FIFO),
ACE_SCOPE_THREAD);
Data handling is done at a higher priority than connections
ACE_OS::thr_setprio (prio);
return Thread_Per_Connection_Logging_Server::handle_data
(logging_client);
}
};
190
The ACE_TSS Class (1/2)
Motivation
•While C++ global variables can be useful, their potential for harmful
side-effects & undefined initialization semantics are problematic
•These problems are exacerbated in multithreaded applications,
e.g., when multiple threads access unsynchronized global
variables simultaneously
•The use of synchronizers in these situations can often cause more
problems than they solve…
•The classic example is errno
// ...
if (recv (……) == -1 &&
errno != EWOULDBLOCK)
ACE_ERROR ((LM_ERROR, “recv failed, %p”, errno));
};
191
The ACE_TSS Class (2/2)
Class Capabilities
•This class implements the
Thread-Specific Storage pattern,
which encapsulates & enhances
the native OS Thread-Specific
Storage (TSS) APIs to provide
the following capabilities:
•It supports data that are ``physically'' thread specific, that is, private
to a thread, but allows them to be accessed as though they were
``logically'' global to a program
•It uses the C++ delegation operator, operator->(), to provide
thread-specific smart pointers
•It encapsulates the management of the keys associated with TSS
objects
•For platforms that lack adequate TSS support natively (such as
VxWorks) ACE_TSS emulates TSS efficiently
192
The Thread-Specific Storage Pattern
• The Thread-Specific Storage pattern allows multiple
threads to use one ‘logically global’ access point to
retrieve an object that is local to a thread, without incurring
locking overhead on each object access
Thread-Specific
Object Set
manages
thread 1
The application thread identifier,
thread-specific object set, &
proxy cooperate to obtain the
correct thread-specific object
thread m
key 1
Thread-Specific
Object Proxy
Thread-Specific
Object
[k,t]
accesses
key n
: Application
Thread
: Thread-Specific
Object Proxy
method()
: Key
Factory
: Thread-Specific
Object Set
create_key()
key
: Thread-Specific
Object
TSObject
193
key
set()
Using ACE_TSS (1/3)
• This example illustrates how to
implement & apply ACE_TSS to our
thread-per-connection logging
server
• In this implementation, each
thread gets its own request count
that resides in thread-specific
storage to alleviate race
conditions on the request count
without requiring a mutex
template <class TYPE> TYPE *
ACE_TSS<TYPE>::operator-> () {
if (once_ == 0) {
// Ensure that we're serialized.
ACE_GUARD_RETURN (ACE_Thread_Mutex, guard, keylock_, 0);
if (once_ == 0) {
ACE_OS::thr_keycreate (&key_, &ACE_TSS<TYPE>::cleanup);
once_ = 1;
}
}
We used the double-checked locking optimization pattern here
194
Using ACE_TSS (2/3)
TYPE *ts_obj = 0;
// Initialize <ts_obj> from thread-specific storage.
ACE_OS::thr_getspecific (key_, (void **) &ts_obj);
// Check if this method's been called in this thread.
if (ts_obj == 0) {
// Allocate memory off the heap and store it in a pointer.
ts_obj = new TYPE;
// Store the dynamically allocated pointer in TSS.
ACE_OS::thr_setspecific (key_, ts_obj);
}
return ts_obj;
}
195
Using ACE_TSS (3/3)
class Request_Count {
public:
Request_Count (): count_ (0) {}
void increment () { ++count_; }
int value () const { return count_; }
private:
int count_;
};
static ACE_TSS<Request_Count> request_count;
virtual int handle_data (ACE_SOCK_Stream *) {
while (logging_handler_.log_record () != -1)
// Keep track of number of requests.
request_count->increment ();
This call increments variable in thread-specific storage
ACE_DEBUG ((LM_DEBUG, "request_count = %d\n",
request_count->value ()));
}
196
The ACE Synchronization Wrapper Facades
•Different operating systems provide different synchronization mechanisms
with different semantics using different APIs
•Some of these APIs conform to international standards, such as Pthreads
•Other APIs conform to de facto standards, such as Win32
•Below we describe the following ACE classes that networked applications
can use to synchronize threads and/or processes portably
197
The ACE_Lock* Pseudo-Class
•The ACE mutex, readers/writer, semaphore, & file lock
mechanisms all support the ACE_LOCK* interface shown below
•ACE_LOCK* is a “pseudo-class,” i.e., it's not a real C++ class in
ACE
198
•We use it to illustrate the uniformity of the signatures supported by
many of the ACE synchronization classes
•e.g., ACE_Thread_Mutex, ACE_Process_Mutex, &
ACE_Thread_Semaphore
The ACE_Guard Classes
Motivation
•When acquiring and releasing
locks explicitly, it can be hard
to ensure that all paths
through the code release the
lock, especially when C++
exceptions are thrown
•ACE provides the
ACE_Guard class & its
associated subclasses to
help assure that locks are
acquired & released properly
Class Capabilities
•These classes implement the Scoped Locking idiom, which leverages the
semantics of C++ class constructors & destructors to ensure a lock is
acquired & released automatically upon entry to and exit from a block of
C++ code, respectively
199
The Scoped Locking Idiom
Motivation
•Code that shouldn’t execute concurrently must be protected by some type of
lock that is acquired/released when control enters/leaves a critical section
•If programmers must acquire & release locks explicitly, it is hard to ensure that
the locks are released in all paths through the code
•e.g., in C++ control can leave a scope due to a return, break, continue, or
goto statement, as well as from an unhandled exception being propagated
out of the scope
void method () {
lock_.acquire ();
// The implementation may return prematurely…
lock_.release ();
}
•The Scoped Locking idiom defines a guard class whose constructor
automatically acquires a lock when control enters a scope & whose
destructor automatically releases the lock when control leaves the scope
200
void method () {
ACE_Guard <ACE_Thread_Mutex> guard (lock_);
// The lock is released when the method returns
}
Implementing Scoped Locking in ACE
template <class LOCK>
class ACE_Guard {
Generic ACE_Guard Wrapper Facade
public:
// Store a pointer to the lock and acquire the lock.
ACE_Guard (LOCK &lock)
: lock_ (&lock)
{ lock_->acquire (); }
// Release the lock when the guard goes out of scope,
~ACE_Guard () { lock_->release (); }
// Other methods omitted…
private:
// Pointer to the lock we’re managing.
LOCK *lock_;
};
•ACE_Write_Guard & ACE_Read_Guard acquire write locks & read locks,
respectively
•Instances of the ACE_Guard* classes can be allocated on the run-time stack
to acquire & release locks in method or block scopes that define critical
sections
201
Sidebar: Overview of ACE_GUARD Macros
•ACE provides macros to simply the use of the different ACE_Guard classes
•These macros check for deadlock and failures in lock operations
# define ACE_GUARD(MUTEX,OBJ,LOCK) \
ACE_Guard< MUTEX > OBJ (LOCK); \
if (OBJ.locked () == 0) return;
# define ACE_GUARD_RETURN(MUTEX,OBJ,LOCK,RETURN) \
ACE_Guard< MUTEX > OBJ (LOCK); \
if (OBJ.locked () == 0) return RETURN;
# define ACE_WRITE_GUARD(MUTEX,OBJ,LOCK) \
ACE_Write_Guard< MUTEX > OBJ (LOCK); \
if (OBJ.locked () == 0) return;
# define ACE_WRITE_GUARD_RETURN(MUTEX,OBJ,LOCK,RETURN) \
ACE_Write_Guard< MUTEX > OBJ (LOCK); \
if (OBJ.locked () == 0) return RETURN;
# define ACE_READ_GUARD(MUTEX,OBJ,LOCK) \
ACE_Read_Guard< MUTEX > OBJ (LOCK); \
if (OBJ.locked () == 0) return;
# define ACE_READ_GUARD_RETURN(MUTEX,OBJ,LOCK,RETURN) \
ACE_Read_Guard< MUTEX > OBJ (LOCK); \
if (OBJ.locked () == 0) return RETURN;
202
The ACE Mutex Classes
Motivation
Class Capabilities
•Most operating systems
provide some form of mutex
mechanism that concurrent
applications can use to
serialize access to shared
resources
•ACE uses the Wrapper Facade pattern to
guide the encapsulation of native OS mutex
synchronization mechanisms with the
ACE_Process_Mutex &
ACE_Thread_Mutex classes
•As with most of the other
platform-specific
capabilities, there are subtle
variations in syntax &
semantics between different
OS platforms
•Mutexes also have different
initialization requirements
203
•These classes implement nonrecursive
mutex semantics portably at system scope
& process scope, respectively
•They can be used to serialize thread access
to critical sections across processes or in
one process
•The interface for the these classes is
identical to the ACE_LOCK* pseudo-class
•The ACE_Null_Mutex class implements
all of its methods as ``no-op'' inline
functions
Using ACE_Thread_Mutex (1/3)
#include "ace/Synch.h"
typedef u_long COUNTER;
static COUNTER request_count; // File scope global variable.
// Mutex protecting request_count (constructor initializes).
static ACE_Thread_Mutex m;
204
virtual int handle_data (ACE_SOCK_Stream *) {
while (logging_handler_.log_record () != -1) {
// Try to acquire the lock.
if (m.acquire () == -1) return 0;
++request_count;
// Count # of requests
m.release (); // Release lock.
This code is tedious
}
& error-prone to
m.acquire ();
write & maintain!
int count = request_count;
m.release ();
ACE_DEBUG ((LM_DEBUG, "request_count = %d\n", count));
logging_handler_.close ();
return 0;
}
Using ACE_Thread_Mutex (2/3)
handle_data() method shown using ACE_GUARD macros
virtual int handle_data (ACE_SOCK_Stream *) {
while (logging_handler_.log_record () != -1) {
// Acquire lock in constructor.
ACE_GUARD_RETURN (ACE_Thread_Mutex, guard, m, -1);
++request_count;
// Count # of requests
// Release lock in destructor.
Obtrusive
m must be initialized properly (C++
}
change
does not guarantee any order)
int count;
Source of accidental complexity
{
ACE_GUARD_RETURN (ACE_Thread_Mutex, guard, m, -1);
count = request_count;
}
ACE_DEBUG ((LM_DEBUG, "request_count = %d\n", count));
}
205
Using ACE_Thread_Mutex (3/3)
Ensuring consistent initialization order of global & static members using
ACE_Object_Manager
// ...
while (logging_handler_.log_record () != -1) {
// Acquire lock in constructor.
ACE_GUARD_RETURN (ACE_Recursive_Thread_Mutex, guard,
ACE_Static_Object_Lock::instance (),
-1);
++request_count;
// Count # of requests
// Release lock in destructor.
}
// ...
ACE_Static_Object_Lock gets preinitialized before
main() runs
206
The ACE Readers/Writer Classes
Motivation
Class Capabilities
•Readers/writer locks allow efficient
concurrent access to resources
whose contents are searched much
more often than they are changed
•ACE encapsulates the native
readers/writer lock mechanisms with
the ACE_RW_Thread_Mutex &
ACE_RW_Process_Mutex classes.
•Operating systems support
readers/writer semantics in their
file-locking APIs
•These classes apply the Wrapper
Facade pattern to implement the
semantics of process- and systemscoped readers/writer locks portably
•Involving the file system in
synchronization activities is
unnecessarily inefficient,
however, & can block under
unpredictable situations
•Moreover, file-locking
mechanisms work only at the
system-scope level, rather than at
process scope
207
•The interface for these classes is
identical to the signatures of
ACE_LOCK* pseudo-class
•The ACE readers/writer
implementation gives preference to
writers, i.e., if there are multiple
readers and a single writer waiting on
the lock, the writer will acquire it first
Using ACE_RW_Thread_Mutex (1/3)
Implementing atomic operations using reader/writer locks
class Atomic_Op
{
public:
// Initialize <count_> to <count>.
Atomic_Op (long count = 0)
: count_ (count) {}
Note the use of the
Scoped Locking
idiom
// Atomically pre-increment <count_>.
long operator++ () {
// Use the <acquire_write> method to acquire a write lock.
ACE_WRITE_GUARD_RETURN (ACE_RW_Thread_Mutex, guard, lock_,
-1);
return ++count_;
}
ACE provides the ACE_Atomic_Op<> template that’s based on this approach.
208
Using ACE_RW_Thread_Mutex (2/3)
// Atomically return <count_>.
operator long () {
// Use the <acquire_read> method to acquire a read lock.
ACE_READ_GUARD_RETURN (ACE_RW_Thread_Mutex, guard, lock_,
0);
Multiple threads can
return count_;
be reading this value
}
concurrently
// ... Other arithmetic operators omitted.
private:
// Readers/writer lock.
ACE_RW_Thread_Mutex lock_;
// Value of the <Atomic_Op> count.
long count_;
};
209
Using ACE_RW_Thread_Mutex (3/3)
Overcoming the need for obtrusive changes using class Atomic_Op
typedef Atomic_Op COUNTER;
static COUNTER request_count; // File scope global variable.
virtual int handle_data (ACE_SOCK_Stream *) {
while (logging_handler_.log_record () != -1)
// Keep track of number of requests.
++request_count; // Actually calls <Atomic_Op::operator++>.
ACE_DEBUG ((LM_DEBUG, "request_count = %d\n",
// Actually calls <Atomic_Op::operator long>.
(long) request_count));
}
210
The ACE Semaphore Classes
Motivation
Class Capabilities
•Semaphores are a powerful
mechanism used to lock and/or
synchronize access to shared
resources in concurrent
applications
•The ACE_Thread_Semaphore &
ACE_Process_Semaphore
classes portably encapsulate
process-scoped & system-scoped
semaphores, respectively, in
accordance with the Wrapper
Façade pattern
•A semaphore contains a count
that indicates the status of a
shared resource
•Application designers assign
the meaning of the semaphore's
count, as well as its initial value
•Semaphores can therefore be
used to mediate access to a
pool of resources
211
•These class interfaces are largely
the same as the ACE_LOCK*
pseudo-class
•The ACE_Null_Semaphore class
implements all of its methods as
``no-op'' inline functions
Using ACE_Thread_Semaphore (1/8)
class Message_Queue
Message queue implementation using
{
ACE_Thread_Semaphore
public:
// Default high and low water marks.
enum {
DEFAULT_LWM = 0,
// 0 is the low water mark.
DEFAULT_HWM = 16 * 1024 // 16 K is the high water mark.
};
// Initialize.
Message_Queue (size_t = DEFAULT_HWM, size_t = DEFAULT_LWM);
// Destroy.
~Message_Queue ();
// Checks if queue is full/empty.
int is_full () const;
int is_empty () const;
212
Using ACE_Thread_Semaphore (2/8)
// Interface for enqueueing and dequeueing ACE_Message_Blocks.
int enqueue_tail (ACE_Message_Block *, ACE_Time_Value * = 0);
int dequeue_head (ACE_Message_Block *&, ACE_Time_Value * = 0);
Note use of Thread-Safe Interface pattern
private:
// Implementations that enqueue/dequeue ACE_Message_Blocks.
int enqueue_tail_i (ACE_Message_Block *, ACE_Time_Value * = 0);
int dequeue_head_i (ACE_Message_Block *&, ACE_Time_Value * = 0);
// Implement the checks for boundary conditions.
int is_empty_i () const;
int is_full_i () const;
// Lowest number before unblocking occurs.
int low_water_mark_;
// Greatest number of bytes before blocking.
int high_water_mark_;
213
Thread-Safe Interface Pattern
Context
•Components in multi-threaded
applications that contain intracomponent method calls
Problem
•Thread-safe components should be
designed to avoid unnecessary
locking
•Thread-safe components should be
designed to avoid “self-deadlock”
Solution
•Apply the Thread-safe Interface
design pattern (P2) to minimize
locking overhead & ensure that intracomponent method calls do not incur
‘self-deadlock’ by trying to reacquire a
lock that is held by the component
214 already
This pattern structures all components
that process intra-component method
invocations according two design
conventions:
•Interface methods check
•All interface methods, such
as C++ public methods,
should only acquire/release
component lock(s), thereby
performing synchronization
checks at the ‘border’ of the
component.
•Implementation methods trust
•Implementation methods,
such as C++ private and
protected methods, should
only perform work when
called by interface methods.
Using ACE_Thread_Semaphore (3/8)
// Current number of bytes in the queue.
int cur_bytes_;
// Current number of messages in the queue.
int cur_count_;
// Number of threads waiting to dequeue a message.
size_t dequeue_waiters_;
// Number of threads waiting to enqueue a message.
size_t enqueue_waiters_;
// C++ wrapper facades to coordinate concurrent access.
mutable ACE_Thread_Mutex lock_;
ACE_Thread_Semaphore notempty_;
ACE_Thread_Semaphore notfull_;
Using ACE_Thread_Semaphore
// Remaining details of queue implementation omitted....
};
215
Using ACE_Thread_Semaphore (4/8)
Message_Queue::Message_Queue (size_t hwm, size_t lwm)
: low_water_mark_ (lwm),
high_water_mark (hwm),
cur_bytes_ (0),
cur_count_ (0),
dequeue_waiters_ (0),
enqueue_waiters_ (0),
notempty_ (0),
notfull_ (1)
{ /* Remaining constructor implementation omitted ... */ }
int Message_Queue::is_empty () const {
ACE_GUARD_RETURN (ACE_Thread_Mutex, guard, lock_, -1);
return is_empty_i ();
}
int Message_Queue::is_full () const {
ACE_GUARD_RETURN (ACE_Thread_Mutex, guard, lock_, -1);
return is_full_i ();
}
216
Using ACE_Thread_Semaphore (5/8)
int Message_Queue::is_empty_i () const {
return cur_bytes_ <= 0 && cur_count_ <= 0;
}
int Message_Queue::is_full_i () const {
return cur_bytes_ >= high_water_mark_;
}
int Message_Queue::enqueue_tail (ACE_Message_Block *new_mblk,
ACE_Time_Value *timeout)
{
ACE_GUARD_RETURN (ACE_Thread_Mutex, guard, lock_, -1);
int result = 0;
217
// Wait until the queue is no longer full.
while (is_full_i () && result != -1) {
++enqueue_waiters_;
guard.release ();
result = notfull_.acquire (timeout);
guard.acquire ();
}
Using ACE_Thread_Semaphore (6/8)
if (result == -1) {
if (enqueue_waiters_ > 0)
--enqueue_waiters_;
if (errno == ETIME)
errno = EWOULDBLOCK;
return -1;
}
// Enqueue the message at the tail of the queue.
int queued_messages = enqueue_tail_i (new_mblk);
// Tell any blocked threads that the queue has a new item!
if (dequeue_waiters_ > 0) {
--dequeue_waiters_;
notempty_.release ();
}
return queued_messages; // guard's destructor releases lock_.
}
218
Using ACE_Thread_Semaphore (7/8)
int Message_Queue::dequeue_head (ACE_Message_Block *&first_item,
ACE_Time_Value *timeout)
{
ACE_GUARD_RETURN (ACE_Thread_Mutex, guard, lock_, -1);
int result = 0;
// Wait until the queue is no longer empty.
while (is_empty_i () && result != -1) {
++dequeue_waiters_;
guard.release ();
result = notempty_.acquire (timeout);
guard.acquire ();
}
219
if (result == -1) {
if (dequeue_waiters_ > 0)
--dequeue_waiters_;
if (errno == ETIME)
errno = EWOULDBLOCK;
return -1;
}
Using ACE_Thread_Semaphore (8/8)
// Remove the first message from the queue.
int queued_messages = dequeue_head_i (first_item);
// Only signal if we've fallen below the low water mark.
if (cur_bytes_ <= low_water_mark_ && enqueue_waiters_ > 0) {
enqueue_waiters_--;
notfull_.release ();
}
return queued_messages; // <guard> destructor releases <lock_>
}
220
ACE Condition Variable Classes (1/2)
Motivation
•Condition variables allow threads to coordinate & schedule their
processing efficiently
•Condition variables are more appropriate than mutexes or semaphores
when complex condition expressions or scheduling behaviors are needed
•e.g., condition variables are often used to implement synchronized
message queues that provide “producer/consumer” communication to
pass messages between threads
Producer <<put>>
Thread
Request Queue
<<get>>
put()
get()
uses
uses 2
221
ACE_Thread_Condition
ACE_Thread_Mutex
wait()
signal()
broadcast()
acquire()
release()
Consumer
Thread
ACE Condition Variable Classes (2/2)
Class Capabilities
•The ACE_Condition_Thread_Mutex uses the Wrapper
Façade pattern to guide its encapsulation of process-scoped
condition variable semantics
•The ACE_Null_Condition is a zero-cost class whose
interface conforms to the ACE_Condition_Thread_Mutex
222
Using ACE_Condition_Thread_Mutex (1/3)
ACE_Recursive_Thread_Mutex class definition
Class ACE_Recursive_Thread_Mutex {
public:
ACE_Recursive_Thread_Mutex (const char *name, void *arg);
int acquire ();
int release ();
// ...
private:
Demonstrating use of
int nesting_level_;
ACE condition variable
ACE_thread_t owner_id_;
wrapper facade
ACE_Thread_Mutex lock_;
ACE_Condition_Thread_Mutex lock_available_;
};
ACE also has an ACE_Condition_Recursive_Thread_Mutex class!
223
Using ACE_Condition_Thread_Mutex (2/3)
ACE_Recursive_Thread_Mutex::ACE_Recursive_Thread_Mutex
(const char *name, void *arg)
: nesting_level_ (0),
owner_id_ (0),
lock_ (name, arg),
// Initialize the condition variable.
lock_available_ (lock_, name, arg) { }
int ACE_Recursive_Thread_Mutex::release ()
{
// Automatically acquire mutex.
ACE_GUARD_RETURN (ACE_Thread_Mutex, guard, lock_, -1);
Demonstrating use of
condition variables
nesting_level_--;
if (nesting_level_ == 0) {
lock_available_.signal (); // Inform waiters the lock is free.
owner_id_ = 0;
}
return 0; // Destructor of <guard> releases the <lock_>.
}
224
Using ACE_Condition_Thread_Mutex (3/3)
1 int ACE_Recursive_Thread_Mutex::acquire ()
2 {
3
ACE_thread_t t_id = ACE_OS::thr_self ();
4
5
ACE_GUARD_RETURN (ACE_Thread_Mutex, guard, lock_, -1);
6
7
if (nesting_level_ == 0) {
8
owner_id_ = t_id;
9
nesting_level_ = 1;
10
}
11
else if (t_id == owner_id_)
12
nesting_level_++;
13
else {
Demonstrating use of
14
while (nesting_level_ > 0)
condition variables
15
lock_available_.wait ();
16
17
owner_id_ = t_id;
18
nesting_level_ = 1;
19
}
20
return 0;
21 }
225
Additional Information
•Patterns & frameworks for concurrent & networked objects
•www.posa.uci.edu
•ACE & TAO open-source middleware
•www.cs.wustl.edu/~schmidt/ACE.html
•www.cs.wustl.edu/~schmidt/TAO.html
•ACE research papers
•www.cs.wustl.edu/~schmidt/ACE-papers.html
•Extended ACE & TAO tutorials
•UCLA extension, July 2004
•www.cs.wustl.edu/~schmidt/UCLA.html
•ACE books
•www.cs.wustl.edu/~schmidt/ACE/
226

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