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Clean up atomics, non-normative changes

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• target
• document history
• 1 Introduction
• 2 Proposed procedure
• 3 Wording changes
  • Aknowledments

integration into IS ISO/IEC 9899:202y

The concurrent integration of atomics, threads and Annex K into the C standard had a lot of difficulties, such as the use of non-normative terminology (processes, address types, regular type, personification of actions, directories), missing integration of threads and atomic synchronization, misleading introductory references to implicated parts of the standard, missing integration between clause 6 and 7, or spurious claims such as

• … but the operations otherwise have no undefined behavior
• The remaining types can be emulated with atomic_flag, though with less than ideal properties.

This paper builds on the following series of papers: n1955, n2064, n2329, and n2389.

In particular, there were the following votes on n2329 during the 2019 London meeting (see n2376).

• Straw poll: Is WG14 happy with the proposed non-normative changes (section 2.1) in N2329? Yes.

• Straw poll: Shall we use the atomic_fetch_OP_explicit functions as a model definition for all the other compound assignment operations as proposed in N2329? Slight preference, Jens to come back with this.

As an answer to that, n2389 combined these two aspects, but then did not find consensus in the 2019 Ithaca meeting because of the atomic_fetch_OP_explicit functions.

This paper here so tries to address the aspects of non-normative changes that had been agreed upon during the London meeting and removes mis-wordings and ambiguities from the description of the atomic_fetch_OP_explicit functions. Many of the identified problems have already been resolved through other paths and C23 now already has much less defects in this domain. So we may now concentrate on issues that are still left.

Additionally, we make a pass on the other material (mostly in non-normative text) that had been included for C11 but where proper integration between the different additions and with the standard terminology has been missed.

All the proposed changes are supposed to be non-normative and are mostly independent from one another. Therefore any of the proposed changes could have been presented with its own paper resulting in perhaps about 10 papers. This approach would block therefore about 5 hours of committee time, which appears far too much.

Instead, I propose that members of WG14 raise objections against particular changes they find in this paper. If these objections cannot be resolved until the end of June 2025, say, they should be raised, discussed and then be voted separately in the Brno session. For the remaining, undisputed, parts I will then propose to have a single vote.

I will ask the convenor to reserve an appropriate time slot in the session, depending on the objections that are still open, then.

New text is underlined green, removed text is stroke-out red. Possible reorganization of the paragraphs is left to the discretion of the editors.

5.2.2.5 Multi-threaded executions and data races

This clause is particularly misleading, since synchronization operations are not limited to the library, and also within the library concerns much more that the indicated subclauses.

5 The library defines atomic operations (7.17) and operations on mutexes (7.29.4) that are specially identified as synchronization operations. There are operations that are specially identified as synchronization operations. If the implementation supports atomic types these are lvalue conversions (6.3.3.1), some operators (6.5) and most generic functions (7.17) that act on atomic objects. If the implementation supports threads (7.29) these are calls to initialization functions (7.24.1 and 7.29.2), memory management functions (7.24.4), operations on mutexes (7.29.3.5, 7.29.3.6 and 7.29.4), and calls to some thread functions (7.29.5.1, 7.29.5.5 and 7.29.5.6). These operations play a special role in making assignments side effects in one thread visible to another. A synchronization operation on one or more memory locations is one of an acquire operation, a release operation, both an acquire and release operation, or a consume operation. A synchronization operation without an associated memory location is a fence and can be either an acquire fence, a release fence, or both an acquire and release fence. In addition, there are relaxed atomic operations, which are not synchronization operations but still are indivisible and strictly ordered, and atomic read-modify-write operations, which have special characteristics. are those operations defined in 6.5 and 7.17 that act on an atomic object by reading its value, by performing an optional operation with that value and by storing back a value into that object.

…

11 Certain library calls operations synchronize with other library calls operations performed by another thread. In particular, an atomic operation A that performs a release operation on an object M synchronizes with an atomic operation B that performs an acquire operation on M and reads a value written by any side effect in the release sequence headed by A.

…

Remove the use of non-standard terminology and spurious quotes, and use direct language instead of double negation.

33 NOTE 16 This effectively disallows compiler reordering enforces the ordering of atomic operations to a single object, even if both operations are “relaxed ” loads. By doing so, it effectively makes the “cache coherence” guarantee provided by most hardware available to C atomic operations.

34 NOTE 17 The value observed by a load of an atomic object depends on the “happens before ” relation, which in turn depends on the values observed by loads of atomic objects. The intended reading is that there exists an association of atomic loads with modifications they observe that, together with suitably chosen modification orders and the “happens before ” relation derived as described previously, satisfy the resulting constraints as imposed here.

As defined here, a data race is not an event that fits into the happens before relation. So we can’t speak of a result of it.

35 The execution of a program contains a data race if it contains two conflicting actions in different threads, at least one of which is not atomic, and neither happens before the other. Any such data race results in undefined behavior. If a program execution contains a data race, the behavior is undefined.

Move to standard terminology and remove claims about “data-race-free” programs which is a term that is not introduced (only data-race-free program executions are).

36 NOTE 18 It can be shown that programs that correctly use simple mutexes operations on mtx_t and memory_order_seq_cst atomic operations to prevent all data races, and use no other synchronization operations, behave as though the operations executed by their constituent threads were simply interleaved, with each value computation of an object being the last value stored in that interleaving. This is normally referred to as “sequential consistency ”. However, this applies only to data-race-free programs, and data-race-free programs cannot observe most program transformations that do not change single-threaded program semantics. In fact, most single-threaded Many program transformations that are valid in the absence of multiple threads continue to be allowed for sequentially consistent programs, since any execution of such a program that behaves differently as a result of such transformations necessarily has undefined behavior even before such a transformation is applied if the transformation were not applied.

What is a “compiler transformation”? What are “atomics in question”?

37 NOTE 19 Compiler Program transformations that introduce assignments to a potentially shared memory location that would not be modified by the abstract machine are generally precluded by this document, since such an assignment can overwrite another assignment by a different thread in cases in which an abstract machine execution would not have encountered a data race. This includes implementations of data member assignment that overwrite adjacent members in separate memory locations. Reordering of atomic loads in cases in which the atomics in question can atomic operands potentially alias is also generally precluded, since this can violate violates the coherence requirements.

Remove some useless blabla, move to standard terminology.

38 NOTE 20 Transformations that introduce a speculative read of a potentially shared memory location possibly will not preserve the semantics of the program as defined in this document, since they potentially introduce a data race. However, they are typically valid in the context of an optimizing compiler that targets a specific machine a specific implementation with well-defined semantics for data races. They would be invalid for a hypothetical machine are invalid for an implementation that is not tolerant of data races or provides hardware data race detection.

…

6.2.6 Representations of types

We collect all information about atomic types and their operation here to have a unified text. Still some individual clauses then need more precision.

6.2.6.1 General

9 Loads and stores of objects with atomic types are done with memory_order_seq_cst semantics. If not specified otherwise, synchronizing operations on atomic objects have memory_order_seq_cst memory consistency.

6.3.3.1 Lvalues, arrays, and function designators

1 …

2 … If the lvalue has qualified type, the value has the unqualified version of the type of the lvalue; additionally, if the lvalue has atomic type, the value has the non-atomic version of the type of the lvalue and the operation synchronizes with memory_order_seq_cst memory consistency; otherwise, the value has the type of the lvalue. …

…

6.5.4.2 Prefix increment and decrement operators

1 …

Semantics

2 … The expression ++E is similar identical to (E+=1), except where only that the value 1 is of the adjustment type (6.5.3.5). See the discussions of additive operators and compound assignment for information on constraints, types, side effects, synchronization and conversions and the effects of operations on pointers.

…

6.5.17.2 Simple assignment

…

2 In simple assignment (=), the value of the right operand is converted to the type of the assignment expression left operand and replaces the value stored in the object designated by the left operand. ¹¹¹⁾ If the left operand has an atomic type, simple assignment synchronizes with memory_order_seq_cst memory order semantics.

¹¹¹⁾ As described in 6.2.6.1, a store to an object with atomic type is done with memory_order_seq_cst semantics.

…

6.7.2.4 Atomic type specifiers

add an example at the end of the clause

5 EXAMPLE This disambiguation of the grammar is necessary in case a qualifier or specifier is followed by an opening parenthesis.

typedef double toto;

void ic(int const tutu);  // valid prototype, void g(int tutu)
void hc(int const(tutu)); // valid prototype, void g(int tutu)
void gc(int const(toto)); // valid prototype, void g(int(*)(double))

void ia(int _Atomic tutu);  // valid prototype, void g(int tutu)
void ha(int _Atomic(tutu)); // invalid prototype, tutu not a type for _Atomic()
void ga(int _Atomic(toto)); // invalid prototype, two type names in parameter declaration

Atomics <stdatomic.h>

7.17.1 Introduction

Atomics are not only relevant for threads but also for communication with signal handlers.

1 The header <stdatomic.h> defines several macros and declares several types and functions for performing atomic operations on data shared with signal handlers and between threads.³⁰²⁾

…

This paragraph about the synopses is currently a weird mixture of adding precision to the synopses and adding semantic properties to certain operations.

6 In the following synopses:

• An A refers to an atomic type.

• A C refers to its corresponding non-atomic type.

Remove the useless defintion on M (no specific operation is defined for pointer types).

• An M refers to the type of the other argument for arithmetic operations. For atomic integer types, M is C. For atomic pointer types, M is ptrdiff_t.

The following item attempts to add a semantic property to the non-_explicit operations. This addition is superfluous because it is already governed by a general rule and so normatively it is not necessary. Nevertheless, it might be good to point this property out, so we transform this into a note. Also donate a plural “s” because there is one generic function that has two memory_order parameters.

Note to the editors: maybe this note should be shifted to the end of the clause and joined with the note that is already placed, there.

• The functions not ending in _explicit have the same semantics as the corresponding _explicit function with memory_order_seq_cst for the memory_order arguments.

NOTE: Most of the operations are provided by a generic function with a name with the suffix _explicit and one that omits that suffix. For the first, one or two trailing parameters of type memory_order specify the operation ordering. For the second, the prototype omits these trailing parameters and the operation ordering is as if additional arguments of value memory_order_seq_cst were provided to the first kind of function in every function call.

…

Replace some unredacted text from the original proposal by standardese.

8 NOTE Many operations are volatile-qualified. The “volatile as device register” semantics have not changed in the standard. This qualification means that volatility is preserved when applying these operations to volatile objects. Most of these generic functions have volatile-qualified parameters to allow their application to volatile-qualified objects.

…

7.17.2.2 The atomic_init generic function

…

Be clear that atomic_init does not synchronize and avoid repetition.

3 Although this function initializes an atomic object, it does not avoid data races it is not a synchronizing operation; concurrent access to the object being initialized, even via an atomic operation, constitutes a data race.

…

7.17.3 Order and consistency

7.17.3.1 General

It is not necessary clear to the occasional reader what a “stronger memory_order” specifications (see 7.17.7.5) would be. Therefore, add a new note after p12 to provide words for a relation between different memory consistency models.

p12′ NOTE 2′ The memory orderings of memory_order impose different ordering constraints on certain operations. memory_order_relaxed, memory_order_consume, memory_order_acquire, memory_order_acq_rel and memory_order_seq_cst form an inclusive chain of such constraints, from weakest to strongest. memory_order_release imposes constraints that are incompatible with memory_order_consume and memory_order_acquire, and that are stronger than memory_order_relaxed and weaker than memory_order_acq_rel.

7.17.5 Lock-free property

7.17.5.1 General

1 The atomic lock-free macros indicate the lock-free property of integer and address atomic pointer types. A value of 0 indicates that the type is never lock-free; a value of 1 indicates that the type is sometimes lock-free; a value of 2 indicates that the type is always lock-free.

2 NOTE In addition to the synchronization properties between threads, the lock-free property of a type warrants that operations are perceived indivisible and strictly ordered in the presence of signals, see 5.2.2.4.

Recommended practice

Operations that are lock-free should also be address-free. That is, atomic operations on the same memory storage location via two different addresses (e.g when mapped by an implementation-specific feature to different addresses in concurrent program executions) will communicate atomically synchronize (for a memory order other than relaxed) and be indivisible and strictly ordered. The implementation should not depend on any per-process execution specific state. This restriction enables communication via memory mapped into a process more than once and memory shared between two processes.

7.17.6 Atomic integer types

…

Recommended practice

3 The representation of an atomic integer type is not required to have the same size as the corresponding regular type non-atomic version of the direct type but it should have the same size whenever possible, as it eases effort required to port existing code.

…

7.17.7 Operations on atomic types

7.17.7.1 General

1 There are only a few kinds of operations on atomic types, though there are many instances of those kinds. This subclause specifies each general kind. This clause describes several generic functions that operate on all atomic types other than atomic_flag. Also, 7.17.7.6 provides such generic functions for some read-modify-write operations on atomic integer types that are defined in addition to the operators defined in clauses 6.5.3.5, 6.5.4.2 and 6.5.17.

7.17.7.5 The atomic_compare_exchange generic functions

…

Description

The failure argument shall not be memory_order_release nor memory_order_acq_rel. The failure argument shall be no stronger not impose more constraints on the operation than the success argument.

7.17.7.6 The atomic_fetch and modify generic functions

…

In the Synopsis replace M by C.

…

Description

Atomically replaces the value pointed to by object with the result of the computation applied to the value pointed to by object and the given operand. Memory is affected according to the value of order. These operations are atomic read-modify-write operations (5.2.2.5). For signed integer types, arithmetic performs silent wraparound on integer overflow; there are no undefined results. The value stored is the mathematical result of the operation wrapped around to the width of the non-atomic type C.^FNTA)

^FTNA) For the generic functions of this clause this completely defines the behavior.

For address types, the result may be an undefined address, but the operations otherwise have no undefined behavior.

7.17.8 Atomic flag type and operations

17.17.8.1 General

1 The atomic_flag type provides the classic test-and-set functionality. It has an atomic data primitive that has exactly two states, set and clear.

2 …

3 NOTE Hence, as per 7.17.5, the operations should also be address-free. No other type requires lock-free operations, so the atomic_flag type is the minimum hardware-implemented type needed to conform to this document that is asynchronous signal safe and that is expected to be compatible with implementation-specific extensions for shared objects between different program executions. The remaining types can be emulated with atomic_flag, though with less than ideal properties.

…

7.30.3.1 General

…

³⁸⁶⁾ This does not mean that these functions are forbidden to read global state that describes the time and calendar settings of the execution, such as the LC_TIME locale or the implementation-defined specification of the local time zone. Only the setting of that state by setlocale or by means of implementation-defined functions can constitute data races.

K .3.5.2 Operations on files

K .3.5.2.1 The tmpfile_s function

…

Recommended practice

(note to the editors: the paragraph number is missing, similar changes should also be applied to the clause of tmpfile)

5′

It should be is possible to open at least TMP_MAX_S temporary files during the lifetime of the program program execution (this limit can be shared with tmpnam_s) and there should be no limit on the number simultaneously open other than this limit and any limit on the number of open files (FOPEN_MAX).

…

K .3.5.2.2 The tmpnam_s function

…

Recommended practice

People don’t create files, concurrent program executions do. Race condition is not an introduced term. Directories are not a concept that we can refer to in this standard.

7 After a program execution obtains a file name using the tmpnam_s function and before the program execution creates a file with that name, the possibility exists that someone else can create a concurrent program execution creates a file with that same name. To avoid this race condition, the tmpfile_s function should be used instead of tmpnam_s when possible. One situation that requires the use of the tmpnam_s function is when the program needs to create a temporary directory rather than a temporary file.

8 Implementations should take care in choosing the patterns used for names returned by tmpnam_s. For example, making a thread ID part of the names avoids the race condition and possible conflict when that multiple programs and threads run simultaneously by the same user concurrently and generate the same temporary file names.

Thanks to Javier Múgica and Joseph Myers for review and text improvements.

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