Defect Report #445

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Submitter: Joseph Myers
Submission Date: 2013-07-23
Source: WG 14
Reference Document: N1731
Version: 1.1
Date: April 2014
Subject: Issues with alignment in C11, part 2


There are various deficiencies in the C11 text about alignment requirements.

Issue 2: Contexts in which alignments are supported

6.2.8#2 defines "fundamental alignment": "A fundamental alignment is represented by an alignment less than or equal to the greatest alignment supported by the implementation in all contexts, which is equal to _Alignof (max_align_t)."

6.2.8#3 defines "extended alignment": "An extended alignment is represented by an alignment greater than _Alignof (max_align_t). It is implementation-defined whether any extended alignments are supported and the contexts in which they are supported. A type having an extended alignment requirement is an over-aligned type."

6.2.8#4 defines "valid alignment", saying "Alignments are represented as values of the type size_t. Valid alignments include only those values returned by an _Alignof expression for fundamental types, plus an additional implementation-defined set of values, which may be empty. Every valid alignment value shall be a nonnegative integral power of two.".

max_align_t is specified in 7.19#2 as "an object type whose alignment is as great as is supported by the implementation in all contexts".

The memory management functions in 7.22.3 are defined to return a pointer "suitably aligned so that it may be assigned to a pointer to any type of object with a fundamental alignment requirement and then used to access such an object or an array of such objects in the space allocated". In the case of aligned_alloc, there may be a stricter requirement given by the alignment passed to the function, but the alignment passed to the function can't result in memory any less-aligned than a fundamental alignment requirement. The alignment requirement still applies even if the size is too small for any object requiring the given alignment (see the response to C90 DR#075).

There are various problems with the above:

The following principles seem natural for any fix for this issue:

Oct 2013 meeting

Committee Discussion

Apr 2014 meeting

Committee Discussion

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