why 1024*1024*1024*2 != 1024*1024*1024*2 [duplicate]

at the beginning, I write a demo to get 2G memory, but the I got a problem like this, I don't know why, what different of num1, num2, num3?

#include<stdio.h>
int main(){
    unsigned long num1 = 1024*1024*1024*2;
    unsigned long num2 = 1024*1024*1024*2.0;
    unsigned long num3 = 1024*1024*1024;
    num3 *= 2;
    printf("num1:%lu\n num2:%lu\n num3:%lu\n", num1, num2, num3);
    return 0;
}

output:

num1:18446744071562067968
num2:2147483648
num3:2147483648

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转载于:https://stackoverflow.com/questions/53059040/why-1024102410242-1024102410242

The first line, unsigned long num1 = 1024*1024*1024*2;, initializes an unsigned long (num1) with an int. The computation 1024*1024*1024*2 has type int because all the values are int literals. Use the UL suffix on at least one of them to make it unsigned long:

unsigned long num1 = 1024*1024*1024*2UL;

As for why the other two methods work: the second line initializes num2 with a double, which has enough precision to produce the correct value in this case; the third line initializes num3 with an int as well, but 1024*1024*1024 is representable by a 32-bit int (it's 2^30, which is less than 2^31-1). The multiplication by 2 after that is done on an unsigned long, so it works as expected.