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### 1524. Allocation functions are missing happens-before requirements and guarantees

Section: 17.7.3.5 [new.delete.dataraces] Status: C++11 Submitter: Hans Boehm Opened: 2011-02-26 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses US-34

Technical details:

When the same unit of storage is allocated and deallocated repeatedly, operations on it can't be allowed to race between the allocator and the user program. But I don't see any mention of happens-before in the descriptions of allocation and deallocation functions.

Proposed resolution (not wording yet):

- The call to an allocation function returning a pointer `P` must happen-before the matching deallocation call with `P` as a parameter. Otherwise the behavior is undefined. I don't know whether receiving `P` with `memory_order_consume` fits this requirement. `memory_order_relaxed` does not.

- If some memory is passed to a deallocation function, the implementation must ensure that the deallocation call happens-before any allocation call that returns the same memory address.

[2011-02-26: Hans comments and drafts wording]

The second requirement already exists, almost verbatim, as 17.7.3.5 [new.delete.dataraces] p. 1. I think this is where the statement belongs. However, this paragraph requires work to correctly address the first part of the issue.

[Adopted at Madrid, 2011-03]

Proposed resolution:

Change 17.7.3.5 [new.delete.dataraces] p. 1 as follows:

> 1 The library versions of `operator new` and `operator delete`, user replacement versions of global `operator new` and `operator delete`, and the C standard library functions `calloc`, `malloc`, `realloc`, and `free` shall not introduce data races (6.9.2 [intro.multithread]) as a result of concurrent calls from different threads. For purposes of determining the existence of data races, the library versions of `operator new`, user replacement versions of global `operator new`, and the C standard library functions `calloc` and `malloc` shall behave as though they accessed and modified only the storage referenced by the return value. The library versions of `operator delete`, user replacement versions of `operator delete`, and the C standard library function `free` shall behave as though they accessed and modified only the storage referenced by their first argument. The C standard library `realloc` function shall behave as though it accessed and modified only the storage referenced by its first argument and by its return value. 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.