4. Critical Section Object

The OSI_Threads library provides the object Crit_Sec, which offers critical section synchronization services across Win32 or POSIX platforms. The definition of Crit_Sec is in the file crit_sec.h, which must be included after the #define WIN or #define POSIX statement and the inclusion of the thread.h header file. The public elements of the class crit_sec can be seen below.

class Crit_Sec

{

public:

Crit_Sec();

~Crit_Sec();

void Enter();

void Leave();

};

The member function Enter() is called to enter a critical section of code. If another thread has already entered a critical section of code protected by the same object and not yet called Leave(), subsequent threads which try to call Enter() will be blocked. In the example below, a number of threads are created which all access a region of code within their thread routine that is protected by the same critical section.

#define POSIX or #define WIN

#include<thread.h>

#include<crit_sec.h>

Crit_Sec CSEC; //The critical section object is declared as a global variable

class ThreadDemo : public Thread

{

public:

ThreadDemo() : Thread() {}

void EntryRoutine(void)

{

CSEC.Enter() // Enter critical section protected region of code

// Manipulate protected data here

CSEC.Leave() // Leave critical section protected region of code

}

void ExitRoutine(void) {}

};

void main()

{

ThreadDemo Crit_Threads[10]; //Declare threads

for(int x = 0; x < 10; x++)

Crit_Threads[I].Create(); //Initialize threads in running mode

for(int y = 0; y < 10; y++)

Crit_Threads[y].Join(); //Wait for threads to complete execution

}

The critical section here synchronizes access to the region of code between the calls to Enter() and the calls to Leave() that occur in the thread routine. The OSI_Threads critical section object is process local and cannot be used to implement synchronization across different processes.