Forget casts. Use memcpy. float xhalf = 0.5f*x; uint32_t i; assert(sizeof(x) == sizeof(i)); std::memcpy(&i, &x, sizeof(i)); i = 0x5f375a86 – (i>>1); std::memcpy(&x, &i, sizeof(i)); x = x*(1.5f – xhalf*x*x); return x; The original code tries to initialize the int32_t by first accessing the float object through an int32_t pointer, which is where the rules are … Read more
To re-iterate, type-punning through unions is perfectly fine in C (but not in C++). In contrast, using pointer casts to do so violates C99 strict aliasing and is problematic because different types may have different alignment requirements and you could raise a SIGBUS if you do it wrong. With unions, this is never a problem. … Read more
First of all, you assume that sizeof(long) == sizeof(int) == sizeof(float). This is not always true, and totally unspecified (platform dependent). Actually, this is true on my Windows using clang-cl and wrong on my Linux using the same 64-bit machine. Different compilers on the same OS/machine can give different results. A static assert is at … Read more
On .NET a float is represented using an IEEE binary32 single precision floating number stored using 32 bits. Apparently the code constructs this number by assembling the bits into an int and then casts it to a float using unsafe. The cast is what in C++ terms is called a reinterpret_cast where no conversion is … Read more
First off, let’s examine why you get the aliasing violation warnings. Aliasing rules simply say that you can only access an object through its own type, its signed / unsigned variant type, or through a character type (char, signed char, unsigned char). C says violating aliasing rules invokes undefined behavior (so don’t!). In this line … Read more
The behavior of type punning with union changed from C89 to C99. The behavior in C99 is the same as C11. As Wug noted in his answer, type punning is allowed in C99 / C11. An unspecified value that could be a trap is read when the union members are of different size. The footnote … Read more
To re-iterate, type-punning through unions is perfectly fine in C (but not in C++). In contrast, using pointer casts to do so violates C99 strict aliasing and is problematic because different types may have different alignment requirements and you could raise a SIGBUS if you do it wrong. With unions, this is never a problem. … Read more
I don’t know how to encode the float number using integer format. There is a function for that: f32::to_bits which returns an u32. There is also the function for the other direction: f32::from_bits which takes an u32 as argument. These functions are preferred over mem::transmute as the latter is unsafe and tricky to use. With … Read more
The purpose of unions is rather obvious, but for some reason people miss it quite often. The purpose of union is to save memory by using the same memory region for storing different objects at different times. That’s it. It is like a room in a hotel. Different people live in it for non-overlapping periods … Read more
A typical situation where you encounter strict aliasing problems is when overlaying a struct (like a device/network msg) onto a buffer of the word size of your system (like a pointer to uint32_ts or uint16_ts). When you overlay a struct onto such a buffer, or a buffer onto such a struct through pointer casting you … Read more