Race Conditions

1
Race Conditions

• Errors that occur on a very infrequent basis.
• Happen at random times so very difficult to
  isolate and fix.
• Race conditions caused by programmer assuming
  that one thread will complete operation on data
  before another thread will access data.
• That is, programmer makes assumptions as to the
  order in which threads are executed.

2
Assume global cxClient, cyClient LRESULT
CALLBACK WndProc(..) .. case
WM_SIZE cxClient LOWORD(lParam) cyClient
HIWORD(lParam) ..
3
In Thread Void thread1(PVOID pvoid) .
TextOut(buf, cxClient, cyClient/4 ..)
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Possible Schedule

• Main thread cxClient LOWORD(lParam)
• Interrupted
• Other thread TextOut(buf, cxClient, cyClient/4,
  )
• Interrupted in middle of function call.
• Main thread cyClient HIWORD(lParam)
• Thread TextOut statement used new cx_Client and
  old value of cyClient. Inconsistent.

5
Solutions

• Synchronization mechanisms been active area of
  research for last 30 years.
• Structure your use of threads so that race
  conditions cannot develop.
• Structure so that primary thread creates all
  windows, all window procedures, and processes all
  messages to window.
• Thread then becomes background cruncher that
  does not handle user input.

6
Race Conditions

• Threads share data and can be preempted at any
  point in their execution.
• Race conditions caused by programmer assuming
  that one thread will complete operation on data
  before another thread will access data.
• That is, programmer makes assumptions as to the
  order in which threads are executed.
• Errors that occur on a very infrequent basis.
• Happen at random times so very difficult to
  isolate and fix.

7
Race Conditions

• //Global Variables
• Int MyBalance 6000
• VOID GetMoney(PVOID pvoid)
• int widthdraw
• widthdraw pvoid-gtamount
• MyBalance - widthdraw
• // load B widthdraw
• // load A MyBalance
• // sub A, B
• // store AC MyBalance

8
LRESULT CALLBACK WndProc(..)

• int Amount 1000
• switch(msg)
• case WM_CREATE
• _beginthread(GetMoney, 0, Amount)
• _beginthread(GetMoney, 0, Amount)

9
Possible Schedule
GetMoney Load B widthdraw //1000 Load A
MyBalance //6000 (interrupted) -------- --------
GetMoney
-------- --------

Load B widthdraw //1000 Load A MyBalance
//6000 (interrupted)
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sub A, B // 5000 store AC MyBalance // 5000
Thread Exits
sub A, B // 5000 store AC MyBalance // 5000
Final Value of MyBalance is 5000
11
Solution Critical Sections

Protect sections where multiple threads access
and modify shared data.
widthdraw pvoid-gtamount MyBalance -
widthdraw
Do not want both threads executing in critical
section.
12
Critical Section Data Structure

• CRITICAL_SECTION cs
• EnterCriticalSection (cs)
• ltcritical codegt
• LeaveCriticalSection (cs)
• Only one thread at a time will progress past
  EnterCriticalSection call.
• Other threads will be blocked from entering until
  LeaveCriticalSection called

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int MyBalance CRITICAL_SECTION cs
VOID GetMoney(PVOID pvoid) int widthdraw
widthdraw pvoid-gtamount
EnterCriticalSection(cs) MyBalance -
widthdraw LeaveCriticalSection(cs)
VOID GetMoney(PVOID pvoid) int widthdraw
widthdraw pvoid-gtamount
EnterCriticalSection(cs) MyBalance -
widthdraw LeaveCriticalSection(cs)
14
Questions

• What two factors cause race conditions?
• Why do threads share data?
• Why are threads useful?
• What data do threads share?
• What data is private to a thread?

15
Networking in Windows

• Each machine on Internet has unique IP Address
• Most convenient to think of dotted notation
• e.g. 216.47.84.122
• Hosts communicate using a given communication
  protocol
• Communication protocol is set of rules that
  determine how communication takes place between
  two network hosts.
• Most common is TCP/IP protocol suite

16
Transmission Control Protocol

• TCP (Transmission Control Protocol) provides
  reliable communication service
• All data sent will arrive eventually (with
  certain caveats).
• Data provided to application in strict sending
  order.
• No duplicate data.
• Referred to as connection-oriented protocol.

17
User Datagram Protocol

• UDP is stateless or connectionless protocol.
• Data sent may not be received.
• Data may be duplicated.
• Data can arrive in any order.

18
Internet Protocol

• TCP and UDP execute on top of IP.
• Internet Protocol
• Addressing
• Routing
• Fragmentation and reassembly of packets.

19
Client/Server Model

• Most often used paradigm
• One host is the server to which clients
  connect.
• Once connection established, both able to send
  and receive.
• Application accesses network through socket.

20
Client Server Model
Subsequent Communications
Network
Client
Server
21
Sockets

• Socket is an endpoint of communication.
• Software construct that allows applications to
  access network.
• In general, computer has one hardware network
  interface card (NIC).
• May be shared by many sockets.
• Socket Name consists of IP address and Port
  number.

22
Winsock

• Windows networking library called winsock.
• Windows Sockets
• When client socket connects to a server socket
  form an association.
• Consists of five elements
• Protocol (must be the same for both client and
  server).
• Client IP address.
• Client Port number.
• Server IP address.
• Server Port number.

23
Network Program Sketch

• Open a socket.
• Name the socket.
• Associate with another socket.
• Send and receive messages between sockets.
• Very similar to message passing between windows.
• Close the socket.

24
Sockets

• SOCKET sock
• sock socket (AF_INET, SOCK_STREAM, 0)
• SOCK_DGRAM for UDP.
• AF_INET Internet family
• SOCK_STREAM Indicates TCP protocol
• Third value ignored for TCP and UDP.

25
Naming a Socket

• Name consists of three attributes
• Protocol
• IP Address
• Port Number
• Generic Socket Address Structure
• struct sockaddr
• u_short as_family
• char sa_data14 //Value depends on address
  family

26
Socket Address

• struct sock_addr is generic. For TCP/IP use
• struct sockaddr_in data structure.
• struct sockaddr_in
• short sin_family // always AF_INET
• u_short sin_port // in network order
• struct sin_addr // IP Address.

27
Port Number

• IP Address not sufficient since may want multiple
  servers per host.
• Differentiated with port number.
• Port 0 1023 reserved for well known services
• HTTP 80
• FTP 23
• Users can assign numbers 1025 5000.

28
IP Address

• struct sin_addr component holds 32 bit IP Address
• Access s_addr field of sin_addr.
• Example (Almost correct)
• struct sockaddr_in sa
• sa.sin_family AF_INET
• sa.sin_port 3000
• sa.sin_addr.s_addr 32.33.14.55

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Host/Network Order

• sa.sin_port htons(3000)
• htons converts short to network byte order.
• sa.sin_addr.s_addr inet_addr( 32.33.14.55)
• inet_addr takes string (dotted notation) and
  converts is to a long in network order.