question about Multiply high signed

#include <iostream>
using namespace std;
int mulths(int u,int v)
{
    unsigned u0,v0,w0;
    int u1,v1,w1,w2,t;

    u0 = u & 开发者_如何学运维0xFFFF;
    u1 = u >> 16;

    v0 = v & 0xFFFF;
    v1 = v >> 16;

    w0 = u0 * v0;
    t = u1 * v0 + (w0 >> 16);

    w1 = t & 0xFFFF;
    w2 = t >> 16;
    w1 = u0 * v1 + w1;

    return u1 * v1 + w2 + (w1 >> 16);
}

int main()
{
    int u,v;
    cin >> u >> v;
    cout << mulths(u, v) << endl;
    return 0;
}

does it returns product of two number yes or it returns most significant bit? beacuse when i enter 5 and 7 and it return 0

"Multiply high" returns the high word of the result. E.g. if ints are 32 bits then when you multiply two 32 bit ints you get a 64 bit result. So you can think of this as a 32 bit (signed) high word and a 32 bit (unsigned) low word. E.g. 0x01234567 x 0x456789AB = 0x004EF78252247ACD. High word (signed) = 0x004EF782, low word (unsigned) = 0x52247ACD.

For your test the values of 5 and 9 are too small and so the high word will be zero. Try larger values such as the above to see if you get the correct result, e.g.

$ g++ -m32 -Wall mulths.cpp -o mulths
$ ./mulths 
1000
1000
0
$ ./mulths 
100000
100000
2
$ ./mulths 
19088743    # 0x01234567
1164413355  # 0x456789AB
5175170     # 0x004EF782
$