# atomic_flag_test_and_set, atomic_flag_test_and_set_explicit

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< 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.

### Parameters

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`.

### References

- C17 standard (ISO/IEC 9899:2018):

  - 7.17.8.1 The atomic_flag_test_and_set functions (p: 209)

- C11 standard (ISO/IEC 9899:2011):

  - 7.17.8.1 The atomic_flag_test_and_set functions (p: 285-286)

### See also

atomic_flag_clear atomic_flag_clear_explicit (C11) | sets an atomic_flag to false (function)

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