Multithreading

1
Multithreading

• Allows application to split itself into multiple
  threads of execution (threads of execution).
• OS support for creating threads, terminating
  threads, and preemptively switches control among
  threads.
• A thread is simply a function.
• Can call other functions
• The same function can be used to create multiple
  threads.
• Once created, all threads execute concurrently.

2
Multithreading

• Threads are part of same process and share all
  process resources
• Memory
• Open Files
• Global Variables
• Static Variables (of same function)
• Each thread has its own
• processor (and math coprocessor) state
• stack

3
Thread Context

• Thread context (e.g., processor state and stack)
  saved and restored during thread switches.
• Multithreading can be extremely difficult (if not
  done correctly).
• All threads shared global variables and static
  variables of thread function.
• Can be preempted at any time.

4
Thread Syntax

• Declare functions that will become threads
• VOID ThreadFunc(Void pvoid)
• function code
• Create thread
• _beginthread (ThreadFunc, 0, NULL)

5
Example

• LRESULT WinAPI WinMain(.)
• // Declare window class, create window(s)
  etc.
• VOID ThreadProc(PVOID pvoid)
• .
• LRESULT CALLBACK WndProc (hwnd, msg, wParam,
  lParam)
• switch (msg)
• case WM_CREATE
• _beginthread (ThreadFunc, 0, NULL)

6
LRESULT CALLBACK WndProc (hwnd, msg, wParam,
lParam)

• switch (msg)
• case WM_CREATE
• _beginthread (ThreadFunc, 0, NULL)

ThreadFunc
Main Thread
CPU
7
LRESULT CALLBACK WndProc (hwnd, msg, wParam,
lParam)

• switch (msg)
• case WM_CREATE
• _beginthread (ThreadFunc, 0, NULL)
• _beginthread (ThreadFunc, 0, NULL)

ThreadFunc
Main Thread
ThreadFunc
CPU
8
// GLOBAL VARIABLES. Accessible to all
threads. include ltwindow.hgt include
ltprocess.hgt int cxClient, cyClient HWND
my_hwnd
int WINAPI WinMain() ..
9
VOID ThreadFunc(Void pvoid) static int
Doober 0 /Shared by all threads created
with this func/ HDC hdc /
Automatic variable, private to all threads /
while(1) hdc GetDC(my_hwnd)
DrawText(hdc, Message, ..)
ReleaseDC(hdc) Doober 2

10
LRESULT CALLBACK WndProc (hwnd, msg, wParam,
lParam)

• switch (msg)
• case WM_CREATE
• my_hwnd hwnd // All threads now have
  handle to parent window.
• _beginthread (ThreadFunc, 0, NULL)

ThreadFunc
Main Thread
Share cxClient, cyClient, my_hwnd
CPU
11
LRESULT CALLBACK WndProc (hwnd, msg, wParam,
lParam)

• switch (msg)
• case WM_CREATE
• _beginthread (ThreadFunc, 0, NULL)
• _beginthread (ThreadFunc, 0, NULL)

ThreadFunc
Main Thread
ThreadFunc
CPU
12
Shared Among all Threads
my_hwnd, cxClient, cyClient
ThreadFunc
ThreadFunc
Main Thread
Shared between thread functions.
Doober
Private copy for each ThreadFunc function.
HDC hdc
HDC hdc
13
Thread Context

• Thread context (e.g., processor state and stack)
  saved and restored during thread switches.
• Multithreading can be extremely difficult (if not
  done correctly).
• All threads shared global variables and static
  variables.
• Can be preempted at any time.

14
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.

15
Assume global cxClient, cyClient LRESULT
CALLBACK WndProc(..) .. case
WM_SIZE cxClient LOWORD(lParam) cyClient
HIWORD(lParam) ..
16
In Thread Void thread1(PVOID pvoid) .
TextOut(buf, cxClient, cyClient/4 ..)
17
Possible Schedule

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

18
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.