std::atomic_thread_fence

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atomic_thread_fence
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Defined in header <atomic>
extern "C" void atomic_thread_fence( std::memory_order order ) noexcept;
 (since C++11)

Establishes memory synchronization ordering of non-atomic and relaxed atomic accesses, as instructed by order, without an associated atomic operation.

Fence-atomic synchronization

A release fence F in thread A synchronizes-with atomic acquire operation Y in thread B, if

  • there exists an atomic store X (with any memory order)
  • Y reads the value written by X (or the value would be written by release sequence headed by X if X were a release operation)
  • F is sequenced-before X in thread A

In this case, all non-atomic and relaxed atomic stores that are sequenced-before F in thread A will happen-before all non-atomic and relaxed atomic loads from the same locations made in thread B after Y.

Atomic-fence synchronization

An atomic release operation X in thread A synchronizes-with an acquire fence F in thread B, if

  • there exists an atomic read Y (with any memory order)
  • Y reads the value written by X (or by the release sequence headed by X)
  • Y is sequenced-before F in thread B

In this case, all non-atomic and relaxed atomic stores that are sequenced-before X in thread A will happen-before all non-atomic and relaxed atomic loads from the same locations made in thread B after F.

Fence-fence synchronization

A release fence FA in thread A synchronizes-with an acquire fence FB in thread B, if

  • There exists an atomic object M,
  • There exists an atomic write X (with any memory order) that modifies M in thread A
  • FA is sequenced-before X in thread A
  • There exists an atomic read Y (with any memory order) in thread B
  • Y reads the value written by X (or the value would be written by release sequence headed by X if X were a release operation)
  • Y is sequenced-before FB in thread B

In this case, all non-atomic and relaxed atomic stores that are sequenced-before FA in thread A will happen-before all non-atomic and relaxed atomic loads from the same locations made in thread B after FB

Parameters

order - the memory ordering executed by this fence

Return value

(none)


Notes

atomic_thread_fence imposes stronger synchronization constraints than an atomic store operation with the same std::memory_order. While an atomic store-release operation prevents all preceding writes from moving past the store-release, an atomic_thread_fence with memory_order_release ordering prevents all preceding writes from moving past all subsequent stores.

Fence-fence synchronization can be used to add synchronization to a sequence of several relaxed atomic operations, for example 

//Global
std::string computation(int);
void print( std::string );
 
std::atomic<int> arr[3] = { -1, -1, -1 };
std::string data[1000] //non-atomic data
 
// Thread A, compute 3 values
void ThreadA( int v0, int v1, int v2 )
{
//assert( 0 <= v0, v1, v2 < 1000 );
data[v0] = computation(v0);
data[v1] = computation(v1);
data[v2] = computation(v2);
std::atomic_thread_fence(std::memory_order_release);
std::atomic_store_explicit(&arr[0], v0, std::memory_order_relaxed);
std::atomic_store_explicit(&arr[1], v1, std::memory_order_relaxed);
std::atomic_store_explicit(&arr[2], v2, std::memory_order_relaxed);
}
 
// Thread B, prints between 0 and 3 values already computed.
void ThreadB()
{
int v0 = std::atomic_load_explicit(&arr[0], std::memory_order_relaxed);
int v1 = std::atomic_load_explicit(&arr[1], std::memory_order_relaxed);
int v2 = std::atomic_load_explicit(&arr[2], std::memory_order_relaxed);
std::atomic_thread_fence(std::memory_order_acquire);
// v0, v1, v2 might turn out to be -1, some or all of them.
// otherwise it is safe to read the non-atomic data because of the fences:
if( v0 != -1 ) { print( data[v0] ); }
if( v1 != -1 ) { print( data[v1] ); }
if( v2 != -1 ) { print( data[v2] ); }
}

Examples

Scan an array of mailboxes, and process only the ones intended for us, without unnecessary synchronization. This example uses atomic-fence synchronization.

Run this code
const int num_mailboxes = 32;
std::atomic<int> mailbox_receiver[num_mailboxes];
std::string mailbox_data[num_mailboxes];
 
// The writer threads update non-atomic shared data 
// and then update mailbox_receiver[i] as follows
mailbox_data[i] = ...;
std::atomic_store_explicit(&mailbox_receiver[i], receiver_id, std::memory_order_release);
 
// Reader thread needs to check all mailbox[i], but only needs to sync with one
for (int i = 0; i < num_mailboxes; ++i) {
    if (std::atomic_load_explicit(&mailbox_receiver[i], std::memory_order_relaxed) == my_id) {
        std::atomic_thread_fence(std::memory_order_acquire); // synchronize with just one writer
        do_work( mailbox_data[i] ); // guaranteed to observe everything done in the writer thread before
                    // the atomic_store_explicit()
    }
 }


See also

(C++11)
 defines memory ordering constraints for the given atomic operation
(enum)
(C++11)
 fence between a thread and a signal handler executed in the same thread
(function)
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