test_and_set_bit How It Works?

Can someone kindly answer my questions under the accepted answer?

I was reading: https://www.kernel.org/doc/htmldocs/kernel-api/API-test-and-set-bit.html but didn't understand that much.

• If the bit was already 1 does it stay 1 or 0?

• If it's 0 and I'm not wrong then it changes to 1?

• What is "bit" here, an input can be bool which is byte not bit...

• By return do they mean that after changing the value we change in back to its original condition? if so then this is useless...

• In C++ when I try to call it I get:

  Use of undeclared identifier 'test_and_set_bit'

Sorry but that page wasn't clear at all.

Usage:

typedef struct lock {
  bool is_locked;
} lock_t;

void init(lock_t* l) {
  l->is_locked = 0;
}

void lock(lock_t* l) {
  while (test_and_set_bit
    (l->is_locked));
}

void unlock(lock_t* l) {
  l->is_locked = 0;
}