malloc() and free() in the memory modelThis issue has been automatically converted from the original issue lists and some formatting may not have been preserved.
Authors: WG 14, Batty
Date: 2011-10-14
Reference document: N1584
Submitted against: C11 / C17
Status: Fixed
Fixed in: C17
Converted from: n2396.htm
The synchronisation afforded to malloc and free is missing some vital ordering, and as the definition stands it makes little sense and creates cycles in happens before. C++11 includes a total order over the allocation and deallocation calls, which fixes the problem and seems to give a sensible semantics. From 18.6.1.4p1 in N3291:
Calls to these functions that allocate or deallocate a particular unit of storage shall occur in a single total order, and each such deallocation call shall happen before the next allocation (if any) in this order.
Unfortunately, there is a typo here. Happens before edges are not transitively closed in to the happens before relation, but the edges here are meant to be. Instead the sentence above should create a synchronises with edge. In light of this, we suggest removing the last two sentences from 7.22.3p2 and replacing them with:
Calls to these functions that allocate or deallocate a particular region of memory shall occur in a single total order, and each such deallocation call shall synchronise with the next allocation (if any) in this order.
See above.
Comment from WG14 on 2017-11-03:
Oct 2011 meeting
- The consensus was that this is an oversight, and should be changed along the lines that are recommended.
Feb 2012 meeting
Replace last two sentences in 7.22.3 paragraph 2 with:
Calls to these functions that allocate or deallocate a particular region of memory shall occur in a single total order, and each such deallocation call shall synchronize with the next allocation (if any) in this order.