I want to get the low 32 bit of a int64 as int32

Question

I have an Int64 value, but I only need the lower 32 bits. Thus I want a quick way to get the Int32 value from the lower 32 bits of the Int64 value.

Thanks

Answer 1

In C/C++ the best way (in my opinion) is to use a union. For example the following function uses an anonymous union to extract the lower-order 32 bits from a 64 bit integer.

uint32_t lower_32_bits(uint64_t value) {
    union { uint64_t value; struct { uint32_t high, low; }; } converter;
    converter.value = value;
    return converter.low;
}

This union trick can be used for all sorts of things, like getting the bits of a floating point value into an integer of the same bitlength, e.g. for doing bitwise operations and other hacks.