What are the alignment limitations of the standard

问题:

I'm working on some older code that uses ATL's CComBSTR type. I'm changing it so that it will compile using Visual C++ Express Edition, which does not come with ATL. I used only a very small subset of CComBSTR, so doing this is fairly simple.

However, when allocating the BSTR memory block, I need to fill the first four bytes with a 4 byte length prefix. I'm concerned that if I use a new char[size] expression to allocate the memory for the string, that I will cause alignment faults due to the allocated char array not having the correct alignment for the four byte prefix.

Is there anything in the standard that states what alignment requirements the returned values of new have? All I see in C++11 are:

5.3.4/1 [expr.new]
It is implementation-defined whether over-aligned types are supported (3.11).

3.11/6 [basic.align]
The alignment requirement of a complete type can be queried using an alignof expression (5.3.6). Furthermore, the types char, signed char, and unsigned char shall have the weakest alignment requirement. [ Note: This enables the character types to be used as the underlying type for an aligned memory area (7.6.2).—end note ]

I find this slightly confusing -- "weakest alignment requirement" says to me "least strict constraint on alignment", but the note under this seems to indicate the standard means the opposite.

Am I safe using a new char[sizeof(uint32_t) + 2*(length + 1)] buffer as a BSTR like this?

EDIT: I just realized that in this specific case of BSTR, one needs to use SysAllocString in order to allocate the string anyway; but I'm still interested in whether or not it is okay to use new in this way.

回答1:

5.3.4/1 [expr.new]

It is implementation-defined whether over-aligned types are supported (3.11).

One important thing here: over-aligned means more aligned than any built-in type. For example, on 64 bits machine, pointers are generally 8 bytes aligned and thus on those machines over-aligned means having an alignment strictly greater than 8.

Therefore, over-aligned is only of concern when using vector types, such as those required for SSE or AVX instructions or some variants of C/C++ (like Open CL). In day to day programming, the types you craft from the built-in types are never over-aligned.

§3.11 Alignment [basic.align]

3/ An extended alignment is represented by an alignment greater than alignof(std::max_align_t). It is implementation-defined whether any extended alignments are supported and the contexts in which they are supported (7.6.2). A type having an extended alignment requirement is an over-aligned type.

9/ If a request for a specific extended alignment in a specific context is not supported by an implementation, the program is ill-formed. Additionally, a request for runtime allocation of dynamic storage for which the requested alignment cannot be honored shall be treated as an allocation failure.

Furthermore, it is customary for new to return memory aligned to alignof(std::max_align_t). This is because the regular ::operator new is only aware of the size of the object to allocate for, not of its alignment, and therefore need satisfy the strongest alignment requirements possible in the program.

On the other hand, beware of a char array allocated on the stack, there is no guarantee what its alignment would end up being.

回答2:

It is an implementation detail, but MSVC uses the operating system allocators. HeapAlloc() for CRT allocations, CoTaskMemAlloc() for COM type wrappers like _bstr_t. They both align by 8, both in 32-bit and 64-bit code.

You should never allocate memory for BSTRs with the new operator, the COM allocator must be used to ensure that they get deallocated using the proper heap. Important in any interop scenario, which is where BSTR is used, it is a standard Automation type. CoTaskMemAlloc/Free() is required but always use the BSTR helper functions to ensure they get properly initialized. SysAllocString() and SysFreeString(). Use SysAllocStringLen() to deal with strings containing embedded zeros.

回答3:

You should never try to use C++ memory management functions for BSTRs - they should only be allocated using SysAllocString() family functions. This ensures that whoever obtains a BSTR can use SysFreeString() and other functions of the family on the obtained BSTR. If you violate this requirement your program will run into undefined behavior.