Allocating more than 4GB memory in a 64 bit system

问题:

I'm running this code, compiled on 64 bits vc++ 2005, on Windows Server 2008 R2 with 32GB. There is an access violation inside the for loop.

#include <iostream>
using namespace std;


int main(int argc, char* argv[])
{   
    double *x = new double[536870912];

    cout << "memory allocated" << endl;

    for(long int i = 0; i < 536870912; i++)
    {   
        cout << i << endl;
        x[i] = 0;
    }

    delete [] x;
    return 0;
}

So if there is no exception in new double[536870912], why am I getting an access violation when doing an assignment over a particular array position?

Another point worth mentioning is that this program was succesfully tested on another computer.

回答1:

It is probably one of the following problem:

• long int is 32-bits: that mean your maximum value is 2147483647, and sizeof(double)*536870912 >= 2147483647. (I don't really know if that has sense. It probably depend on how the compiller work)
• Your allocation is failing.

I suggest you to test the following code:

#include<conio.h>
#include <iostream>
using namespace std;

#define MYTYPE unsigned long long


int main(int argc, char* argv[])
{   
    // Test compiling mode
    if (sizeof(void*) == 8) cout << "Compiling 64-bits" << endl;
    else cout << "Compiling 32-bits" << endl;

    // Test the size of mytype
    cout << "Sizeof:" << sizeof(MYTYPE) << endl;
    MYTYPE len;

    // Get the number of <<doubles>> to allocate
    cout << "How many doubles do you want?" << endl;
    cin >> len;
    double *x = new (std::nothrow) double[len];
    // Test allocation
    if (NULL==x)
    {
        cout << "unable to allocate" << endl;
        return 0;
    }
    cout << "memory allocated" << endl;

    // Set all values to 0
    for(MYTYPE i = 0; i < len; i++)
    {   
        if (i%100000==0) cout << i << endl;
        x[i] = 0;
    }

    // Wait before release, to test memory usage
    cout << "Press <Enter> key to continue...";
    getch();

    // Free memory.
    delete [] x;

}

Editing: Using this code, i just achieved allocate a single block of 9GB.