Submitter: Martin Sebor
Submission Date: March 22, 2014
Subject: aligned_alloc underspecified


The aligned_alloc function specifies the following constraints on its arguments, alignment and size:

The value of alignment shall be a valid alignment supported by the implementation and the value of size shall be an integral multiple of alignment.

Therefore, the behavior of the function is undefined when either constraint is violated.

According to section 6.2.8, paragraph 1, the greatest alignment a conforming implementation is required to support (known as fundamental alignment) is _Alignof(max_align_t). Furthermore, according to paragraph 2 of the same section, whether alignments greater than the fundamental alignment (known as extended alignments) are supported and in what contexts is implementation-defined.

The standard specifies no mechanism by which programs could determine whether an extended alignment is supported by an implementation, or whether the aligned_alloc function is among the contexts where an extended alignment is supported.

As a result, there is no way for strictly conforming programs to use the aligned_alloc function with an alignment argument greater than the result of _Alignof(max_align_t). Since the malloc function returns objects that meet the same alignment requirement, this restriction makes aligned_alloc useless in portable programs.

This restriction is unnecessary since it's possible, and in fact nearly trivial given access to the internal details of the memory allocator, to implement an efficient aligned_function that fails when its arguments don't meet the specified requirements.

As a data point, the POSIX Advanced Realtime function posix_memalign, as well as the historical BSD memalign function, are both required to return a null pointer when either of their arguments don't meet the specified requirements (in addition to setting errno to EINVAL.

Suggested Technical Corrigendum

The proposed corrigendum below changes the standard to require aligned_alloc to fail by returning a null pointer when either of its constraints is violated.

In section, modify paragraph 2 as indicated below:

The aligned_alloc function allocates space for an object whose alignment is specified by alignment, whose size is specified by size, and whose value is indeterminate. TIf the value of alignment shall be is not a valid alignment supported by the implementation andor the value of size shall beis not an integral multiple of alignment the function shall fail by returning a null pointer.

In addition, in section J.2 Undefined behavior, remove the following bullet:

— The alignment requested of the aligned_alloc function is not valid or not supported by the implementation, or the size requested is not an integral multiple of the alignment (

If the proposal above isn't acceptable, then an alternative solution to consider that would allow aligned_alloc to be used even in strictly conforming programs is to add a new function to determine whether a given alignment is supported by an implementation. For example:

_Bool alignment_is_valid (size_t alignment);


The alignment_is_valid function returns non-zero if the value specified by alignment is a valid alignment argument to the aligned_alloc function, and zero otherwise.