atomic_flag_clear functions

Atomically sets the value pointed to by object to true.

void atomic_flag_clear ( volatile atomic_flag  *object )
void atomic_flag_clear_explicit ( volatile atomic_flag  *object ,
  memory_order order)
void atomic_flag_clear_explicit ( volatile atomic_flag  *object ,
  memory_order order,
  memory_scope scope)


The order argument shall not be memory_order_acquire nor memory_order_acq_rel.


Atomically sets the value pointed to by object to false. Memory is affected according to the value of order.

In these operation definitions:

  • An A refers to one of the atomic types.
  • A C refers to its corresponding non-atomic type.
  • An M refers to the type of the other argument for arithmetic operations. For atomic integer types, M is C.
  • The functions not ending in explicit have the same semantics as the corresponding explicit function with memory_order_seq_cst for the memory_order argument.
  • The functions that do not have memory_scope argument have the same semantics as the corresponding functions with the memory_scope argument set to memory_scope_device.

NOTE: With fine-grained system SVM, sharing happens at the granularity of individual loads and stores anywhere in host memory. Memory consistency is always guaranteed at synchronization points, but to obtain finer control over consistency, the OpenCL atomics functions may be used to ensure that the updates to individual data values made by one unit of execution are visible to other execution units. In particular, when a host thread needs fine control over the consistency of memory that is shared with one or more OpenCL devices, it must use atomic and fence operations that are compatible with the C11 atomic operations.

We can't require C11 atomics since host programs can be implemented in other programming languages and versions of C or C++, but we do require that the host programs use atomics and that those atomics be compatible with those in C11.


All operations on atomic types must be performed using the built-in atomic functions. C11 and C++11 support operators on atomic types. OpenCL C does not support operators with atomic types. Using atomic types with operators should result in a compilation error.

The atomic_bool, atomic_char, atomic_uchar, atomic_short, atomic_ushort, atomic_intmax_t and atomic_uintmax_t types are not supported by OpenCL C.

OpenCL C requires that the built-in atomic functions on atomic types are lock-free.

The _Atomic type specifier and _Atomic type qualifier are not supported by OpenCL C.

The behavior of atomic operations where pointer arguments to the atomic functions refers to an atomic type in the private address space is undefined


OpenCL Specification

Also see

Atomic Functions
