Question

The codebase at work contains some code that looks roughly like this:

#define DATA_LENGTH 64

u_int32 SmartKey::SerialNumber()
{
    unsigned char data[DATA_LENGTH];
    // ... initialized data buffer
    return *(u_int32*)data;
}

This code works correctly, but GCC gives the following warning:

warning: dereferencing pointer ‘serialNumber’ does break strict-aliasing rules

Can someone explain this warning? Is this code potentially dangerous? How can it be improved?

Update
With thanks to James McNellis’ answer I came up with the following utility function:

template<class T, class Data>
T BinaryCast(const Data & inData)
{
    T ret;
    std::copy(&inData[0], &inData[0] + sizeof(ret), reinterpret_cast<char*>(&ret));
    return ret;
}

u_int32 SmartKey::SerialNumber()
{
    unsigned char data[DATA_LENGTH];
    // ... initialized data buffer
    return BinaryCast<u_int32>(data);
}

Feel free to suggest improvements!

Answer 1

The warning is because you are violating the strict aliasing rule.

One way to do it correctly would be to copy the bytes from the data buffer into a u_int32 object and return that object:

unsigned char data[DATA_LENGTH];
// ... initialized data buffer

u_int32 i;
assert(sizeof (i) <= DATA_LENGTH);
std::copy(&data[0], &data[0] + sizeof (i), reinterpret_cast<char*>(&i));
return i;

This solution works because in C++ it is permitted to access any type of object as an array of char.

(std::copy() is in <algorithm>)