Module ptr

Manually manage memory through raw pointers.

See also the pointer primitive types.

Safety

Many functions in this module take raw pointers as arguments and read from or write to them. For this to be safe, these pointers must be valid for the given access. Whether a pointer is valid depends on the operation it is used for (read or write), and the extent of the memory that is accessed (i.e., how many bytes are read/written) – it makes no sense to ask “is this pointer valid”; one has to ask “is this pointer valid for a given access”. Most functions use *mut T and *const T to access only a single value, in which case the documentation omits the size and implicitly assumes it to be size_of::<T>() bytes.

The precise rules for validity are not determined yet. The guarantees that are provided at this point are very minimal:

• For operations of size zero, every pointer is valid, including the null pointer. The following points are only concerned with non-zero-sized accesses.
• A null pointer is never valid.
• For a pointer to be valid, it is necessary, but not always sufficient, that the pointer be dereferenceable: the memory range of the given size starting at the pointer must all be within the bounds of a single allocated object. Note that in Rust, every (stack-allocated) variable is considered a separate allocated object.
• All accesses performed by functions in this module are non-atomic in the sense of atomic operations used to synchronize between threads. This means it is undefined behavior to perform two concurrent accesses to the same location from different threads unless both accesses only read from memory. Notice that this explicitly includes read_volatile and write_volatile: Volatile accesses cannot be used for inter-thread synchronization.
• The result of casting a reference to a pointer is valid for as long as the underlying object is live and no reference (just raw pointers) is used to access the same memory. That is, reference and pointer accesses cannot be interleaved.

These axioms, along with careful use of offset for pointer arithmetic, are enough to correctly implement many useful things in unsafe code. Stronger guarantees will be provided eventually, as the aliasing rules are being determined. For more information, see the book as well as the section in the reference devoted to undefined behavior.

We say that a pointer is “dangling” if it is not valid for any non-zero-sized accesses. This means out-of-bounds pointers, pointers to freed me