The **`atomic_flag_test_and_set` function** is defined in the `<stdatomic.h>` header file in C. It takes in a single parameter: a `volatile atomic_flag` object. The function returns the value of the atomic_flag pointed to by the `obj` and then sets the current value to `true`.

> `atomic_flag` is an `atomic boolean` type variable that is guaranteed to be lock-free. It is used to provide synchronization within threads in programs.

The illustration below shows how `atomic_flag_test_and_set` works:

# Declaration

The `atomic_flag_test_and_set`  function is defined as follows:
```C
bool atomic_flag_test_and_set( volatile atomic_flag* obj );
```

# Parameter

The`atomic_flag_test_and_set` function takes in a single value of type `volatile atomic flag*`.  

# Return value

The `atomic_flag_test_and_set` function returns a `boolean` value which indicates the state of the flag. The value can be either `true` or `false`.

> The `atomic_flag_test_and_set` is outdated and has been replaced by the `atomic_flag_test_and_set_explicit` function. Therefore, relevant code for `atomic_flag_test_and_set` function does not exist.


What is atomic_flag_test_and_set in C?

atomic_flag_test_and_set is an atomic boolean function used for thread synchronization, taking a volatile atomic_flag object and returning its state.