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Mutex Test and Set

t im (45)
I read the code about Mutex test below:
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static int flag=0;

void lock(){
  while(TestAndSet(&flag,1)==1);
  //flag=1;
}
void unlock(){
  flag=0;
}

And TestAndSet:
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int TestAndSet(int *ptr, int new) {
    int old = *ptr;
    *ptr = new;
    return old;
}

I am confused about why the TestAndSet can avoid two threads get the mutex.
Supposing thread A executes TestAndSet first and arrives at 'int old = *ptr'; Meanwhile, thread B executes the 'int old = *ptr' before thread A executes '*ptr = new'. In that case, the two threads get the same return value and can both get mutex.
So, what's wrong with my thought?
Last edited on
doug4 (1538)
This doesn't seem to be valid C++. I don't believe the keyword 'new' can be used as a variable name.

I wouldn't trust this information.
t im (45)
Sorry for the mistake.
I just wonder how I can implement a TestAndSet function.
Thanks for your help.
Peter87 (11235)
Yeah, and infinite loops (without side-effects) are UB...

But I guess it's just pseudocode. TestAndSet would have to be an atomic operation for this to work.

Wikipedia wrote:
the function must execute atomically. That is, no other process must be able to interrupt the function mid-execution, thereby seeing a state that only exists while the function executes. That requires hardware support; it cannot be implemented as shown.
https://en.wikipedia.org/wiki/Test-and-set#Software_implementation_of_test-and-set
Last edited on
t im (45)
Thanks for your reply which makes me figure it out.
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