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Overview
The "Synchronization of Dual Threads" problem is a classic demonstration of concurrent thread management. Given two threads, A and B, with the responsibilities of printing "foo" and "bar" respectively, the task is to ensure a synchronized output of "foobar" repetitively for a given 'n' times. This problem emphasizes the significance of thread synchronization in producing consistent and predictable results in concurrent systems.
Pseudocode
Initialize a semaphore or mutex called fooLock with a value of 1 Initialize a semaphore or mutex called barLock with a value of 0 Function printFoo(): for i from 1 to n do: Acquire fooLock Print "foo" Release barLock Function printBar(): for i from 1 to n do: Acquire barLock Print "bar" Release fooLock Main: Start Thread A -> Target: printFoo Start Thread B -> Target: printBar
Explanation
- We start by defining a
FooBar
class that has methodsfoo
andbar
to print "foo" and "bar" respectively. - The synchronization is achieved using a mutex
mtx
and two condition variablesfooPrinted
andbarPrinted
. - In the
foo
method, we ensure that "foo" is printed first. Once printed, it setsfooDone
to true and signals thebarPrinted
condition variable to allow thebar
method to print "bar". - The
bar
method waits forfooDone
to be true. Once "bar" is printed, it resetsfooDone
to false and signals thefooPrinted
condition variable to allow the next iteration. - In the
main
function, we create two threads,threadFoo
andthreadBar
, to execute thefoo
andbar
methods respectively. - The
pthread_join
function ensures the main thread waits for both threads to finish their execution.
Algorithm Walkthrough
Let’s emphasize synchronization aspects in the walkthrough to help understand how the program ensures proper ordering of "foo" and "bar" printing.
Initial Setup
- Create
foobar
: An instance ofFooBar
withn = 5
. - Initialize Synchronization Primitives:
mtx
,fooPrinted
, andbarPrinted
. - Set Flag:
fooDone
is set tofalse
.
Starting Threads
- Start
threadFoo
: Initiatesfoobar.foo()
, which contains the logic to print "foo". - Start
threadBar
: Initiatesfoobar.bar()
, which contains the logic to print "bar".
Walkthrough of Loop Iterations with Synchronization Focus:
Iteration 1
-
threadFoo
:- Acquire Lock: Locks
mtx
to enter the critical section. - Check Condition: Since
fooDone
isfalse
, it proceeds without waiting. - Print "foo": Outputs "foo" to the console.
- Update Flag: Sets
fooDone
totrue
. - Signal
threadBar
: SignalsbarPrinted
to wake upthreadBar
(if it's waiting). - Release Lock: Unlocks
mtx
.
- Acquire Lock: Locks
-
threadBar
- Acquire Lock: Locks
mtx
to enter the critical section. - Wait for "foo": Waits on
barPrinted
untilfooDone
istrue
(ensures "foo" is printed before "bar"). - Print "bar": Outputs "bar" to the console (completing "foobar").
- Update Flag: Sets
fooDone
tofalse
. - Signal
threadFoo
: SignalsfooPrinted
to wake upthreadFoo
for the next iteration. - Release Lock: Unlocks
mtx
.
- Acquire Lock: Locks
Iterations 2 to 5
- Repeat the Steps: The same synchronization-focused steps as Iteration 1 are repeated. This ensures that "foo" is always printed before "bar" and "foobar" is printed 5 times in total.
Joining Threads
- Wait for
threadFoo
:pthread_join(thread1, NULL)
ensures the main thread waits untilthreadFoo
has finished executing. - Wait for
threadBar
:pthread_join(thread2, NULL)
ensures the main thread waits untilthreadBar
has finished executing.
Final Result
- By the end of the program, "foobar" has been printed 5 times. The synchronization constructs (mutex and condition variables) have ensured that "foo" is always printed before "bar", maintaining the proper order despite the concurrent execution of threads. The program showcases a critical aspect of multithreading – ensuring coordinated execution and maintaining the integrity of shared resources (in this case, the order of "foo" and "bar" printing).
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