CODEWITHSUNDEEP
c++multithreadingsynchronizationlocking
Majnu
Majnu

Reputation: 210

Test and Test-and-set in c++

I am trying to implement Test and Test-and-set in c++.

as per C++ atomic_flag query state we cannot check the flag state, so whats the workaround for that

loop will be like this

std::atomic_flag lock_stream = ATOMIC_FLAG_INIT;
void ttas_lock()
{
    while(lock_Stream);
    while(lock_stream.test_and_set());
    return
}

error: could not convert ‘((ttas*)this)->ttas::lock_stream’ from ‘std::atomic_flag’ to ‘bool’ while(lock_stream)

Upvotes: 5

Views: 10669

Answers (2)

ronalchn
ronalchn

Reputation: 12335

If you want to test lock_stream without setting it, you should use std::atomic<bool>.

Therefore, declare it as:

std::atomic<bool> lock_stream = ATOMIC_FLAG_INIT;

Instead of test and set, you just use exchange:

void ttas_lock()
{
    while(lock_stream);
    while(lock_stream.exchange(true)); // this is equivalent to test and set
    return;
}

According to wikipedia, if test and set fails, you should do the test again, which results in:

void ttas_lock()
{
    do {
        while (lock_stream) continue;
    } while (lock_stream.exchange(true)); // actual atomic locking
    return;
}

Upvotes: 2

Paul Evans
Paul Evans

Reputation: 27567

Your function:

void ttas_lock()
{
    while(lock_Stream);
    if(!lock_stream.test_and_set()) return;
}

will always return after lock_Stream is false. Not what you want.

Upvotes: 0

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