Memory model

 

C++

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memory model

 

Defines the semantics of computer memory storage for the purpose of the C++ abstract machine.

The memory available to a C++ program is one or more contiguous sequences of bytes. Each byte in memory has a unique address.

Byte

A byte is the smallest addressable unit of memory. It is defined as a contiguous sequence of bits, large enough to hold the value of any UTF-8 code unit (256 distinct values) and of (since C++14)any member of the basic execution character set (the 96 characters that are required to be single-byte). Similar to C, C++ supports bytes of sizes 8 bits and greater.

The types char, unsigned char, and signed char use one byte for both storage and value representation. The number of bits in a byte is accessible as CHAR_BIT or std::numeric_limits<unsigned char>::digits.

Memory location

A memory location is

• an object of scalar type (arithmetic type, pointer type, enumeration type, or std::nullptr_t)
• or the largest contiguous sequence of bit fields of non-zero length

Note: Various features of the language, such as references and virtual functions, might involve additional memory locations that are not accessible to programs but are managed by the implementation.

struct S {
    char a;     // memory location #1
    int b : 5;  // memory location #2
    int c : 11, // memory location #2 (continued)
          : 0,
        d : 8;  // memory location #3
    struct {
        int ee : 8; // memory location #4
    } e;
} obj; // The object 'obj' consists of 4 separate memory locations

Threads and data races

A thread of execution is a flow of control within a program that begins with the invocation of a top-level function by std::thread::thread, std::async, or other means.

Any thread can potentially access any object in the program (objects with automatic and thread-local storage duration may still be accessed by another thread through a pointer or by reference).

Different threads of execution are always allowed to access (read and modify) different memory locations concurrently, with no interference and no synchronization requirements.

When an evaluation of an expression writes to a memory location and another evaluation reads or modifies the same memory location, the expressions are said to conflict. A program that has two conflicting evaluations has a data race unless

• both evaluations execute on the same thread or in the same signal handler, or
• both conflicting evaluations are atomic operations (see std::atomic), or
• one of the conflicting evaluations happens-before another (see std::memory_order)

If a data race occurs, the behavior of the program is undefined.

(in particular, release of a std::mutex is synchronized-with, and therefore, happens-before acquisition of the same mutex by another thread, which makes it possible to use mutex locks to guard against data races)

int cnt;
auto f = [&]{cnt++;};
std::thread t1{f}, t2{f}, t3{f}; // undefined behavior

std::atomic<int> cnt{0};
auto f = [&]{cnt++;};
std::thread t1{f}, t2{f}, t3{f}; // OK

Memory order

When a thread reads a value from a memory location, it may see the initial value, the value written in the same thread, or the value written in another thread. See std::memory_order for details on the order in which writes made from threads become visible to other threads.

Forward progress

Obstruction freedom

When only one thread that is not blocked in a standard library function executes an atomic function that is lock-free, that execution is guaranteed to complete (all standard library lock-free operations are obstruction-free)

Lock freedom

When one or more lock-free atomic functions run concurrently, at least one of them is guaranteed to complete (all standard library lock-free operations are lock-free -- it is the job of the implementation to ensure they cannot be live-locked indefinitely by other threads, such as by continuously stealing the cache line)

Progress guarantee

In a valid C++ program, every thread eventually does one of the following:

• terminate
• makes a call to an I/O library function
• performs an access through a volatile glvalue
• performs an atomic operation or a synchronization operation

No thread of execution can execute forever without performing any of these observable behaviors.

Note that it means that a program with endless recursion or endless loop (whether implemented as a for-statement or by looping goto or otherwise) has undefined behavior. This allows the compilers to remove all loops that have no observable behavior, without having to prove that they would eventually terminate.

A thread is said to make progress if it performs one of the execution steps above (I/O, volatile, atomic, or synchronization), blocks in a standard library function, or calls an atomic lock-free function that does not complete because of a non-blocked concurrent thread.

Concurrent forward progress If a thread offers concurrent forward progress guarantee, it will make progress (as defined above) in finite amount of time, for as long as it has not terminated, regardless of whether other threads (if any) are making progress. The standard encourages, but doesn't require that the main thread and the threads started by std::thread offer concurrent forward progress guarantee. Parallel forward progress If a thread offers parallel forward progress guarantee, the implementation is not required to ensure that the thread will eventually make progress if it has not yet executed any execution step (I/O, volatile, atomic, or synchronization), but once this thread has executed a step, it provides concurrent forward progress guarantees (this rule describes a thread in a thread pool that executes tasks in arbitrary order) Weakly parallel forward progress If a thread offers weakly parallel forward progress guarantee, it does not guarantee to eventually make progress, regardless of whether other threads make progress or not. Such threads can still be guaranteed to make progress by blocking with forward progress guarantee delegation: if a thread P blocks in this manner on the completion of a set of threads S, then at least one thread in S will offer a forward progress guarantee that is same or stronger than P. Once that thread completes, another thread in S will be similarly strengthened. Once the set is empty, P will unblock. The parallel algorithms from the C++ standard library block with forward progress delegation on the completion of an unspecified set of library-managed threads.

(since C++17)

See also

C documentation for Memory model