atomic_flag_test_and_set, atomic_flag_test_and_set_explicit

< c‎ | atomic
Defined in header <stdatomic.h>
_Bool atomic_flag_test_and_set( volatile atomic_flag* obj );
(1) (since C11)
_Bool atomic_flag_test_and_set_explicit( volatile atomic_flag* obj, memory_order order );
(2) (since C11)

Atomically changes the state of a atomic_flag pointed to by obj to set (true) and returns the previous value. The first version orders memory accesses according to memory_order_seq_cst, the second version orders memory accesses according to order.

The argument is pointer to a volatile atomic flag to accept addresses of both non-volatile and volatile (e.g. memory-mapped I/O) atomic flags.


obj - pointer to the atomic flag object to modify
order - the memory synchronization ordering for this operation: all values are permitted

Return value

The previous value held by the atomic flag pointed to by obj.


  • C11 standard (ISO/IEC 9899:2011):
  • The atomic_flag_test_and_set functions (p: 285-286)

See also

sets an atomic_flag to false
C++ documentation for atomic_flag_test_and_set, atomic_flag_test_and_set_explicit