Re: Is code a safe thread?
Nephi Immortal wrote:
I customize operator new to create my own definition.
m_out_of_memory is a global variable of class Test. m_out_of_memory
can be set to true or false inside operator new function before Test()
is called to construct three data members: a, b, c.
If m_out_of_memory is to be true, then Test() is not called and all
data members cannot be initialized.
My one question is ? is thread safe? What happen if two main
functions are running on two threads at the same time while accessing
m_out_of_memory?
First thread will report out of memory before second thread will skip
to initialize data members. Can you please confirm that it never
happened as long as thread safety is taken care?
Well, it would be impossible to take care of thread safety with the
interface design presented below. The content of the shared static variable
is meant to indicate the status returned by the _previous_ allocation
attempt. Hence, the meaning of the variable would differ from thread to
thread (as they go through different allocation attempts and there is no
trans-thread meaning of "last"). No synchronization would change that. The
only way to deal with it, would be to stop any other threads when it tries
to allocate memory until the first thread doing so has checked the status of
_its_ last allocation attempt. Such synchronization is not possible with the
interface you present.
Or?is m_out_of_memory stored in separate memory on each thread and is
never shared?
Which data are shared depends on the particulars of the thread model. E.g.,
in linux you could use multiple processes or multiple threads. In the later
case, static data would be shared.
I choose this method to avoid using exception.
Aha. And what's wrong with exceptions here?
class Test
{
public:
Test() : a( 1 ), b( 2 ), c( 3 ) {
}
~Test() {
}
bool Is_Sufficient_Memory() const {
return ( m_out_of_memory == false ) ? true : false;
}
void* operator new( size_t size ) {
void* p = malloc( size );
m_out_of_memory = ( p == 0 ) ? true : false;
return p;
}
void operator delete( void* memory ) {
free( memory );
}
void Set_abc( int _a, int _b, int _c ) {
a = _a; b = _b; c = _c;
}
private:
static bool m_out_of_memory;
int a;
int b;
int c;
};
bool Test::m_out_of_memory = false;
int main()
{
Test* T = new Test();
if( T->Is_Sufficient_Memory()) {
Hm .....
If the allocation via new() above fails, then T is 0 here and dereferencing
the pointer is undefined behavior. And yes, accessing a member function
qualifies as dereferencing even when the member function only accesses
static data.
T->Set_abc( 10, 20, 30 );
delete T;
}
return 0;
}
One more question: why do not use a nothrow allocation function. In that
case, a new expression would indicate an allocation failure not via an
exception but by returning 0. You can test for that. No threading issues
arise.
Best,
Kai-Uwe Bux