what does os_atomic_or2o actually do?

Question

When I dive into source code of GCD (Grand Central Dispatch), I found some low-level C functions used in dispatch_block_cancel showed below

dispatch_block_cancel(dispatch_block_t db)
{
    dispatch_block_private_data_t dbpd = _dispatch_block_get_data(db);
    if (unlikely(!dbpd)) {
        DISPATCH_CLIENT_CRASH(0, "Invalid block object passed to "
                "dispatch_block_cancel()");
    }
    (void)os_atomic_or2o(dbpd, dbpd_atomic_flags, DBF_CANCELED, relaxed);
}

The os_atomic_or2o macro seems to set DBF_CANCELED in dbpd which is the private data of the block. But after use gcc -E expand the macro, I can't find any setting behavior.

(void)({ 
__typeof__(__c11_atomic_load(((__typeof__(*((&(dbpd)->dbpd_atomic_flags))) _Atomic *)((&(dbpd)->dbpd_atomic_flags))), memory_order_relaxed)) _v = (((1u))), 
_r = __c11_atomic_fetch_or(((__typeof__(*((&(dbpd)->dbpd_atomic_flags))) _Atomic *)((&(dbpd)->dbpd_atomic_flags))), _v, memory_order_relaxed);
(__typeof__(_r))(_r | _v); 
});

_v is a variable inited by type of &(dbpd)->dbpd_atomic_flags, value of 1u.

_r is a variable inited by | operation between _v and &(dbpd)->dbpd_atomic_flags.

And finally do an | operation and end. Where is the assigning code ...

Answer 1

Find the secret behind the macro os_atomic_or2o, the assigning code is __c11_atomic_fetch_or which is __atomic_fetch_or and in GCC built_in functions referrence, this function will assign new value to the first param.