How to implement a atomic operation?

Question

The gcc-built-in atomic operation: http://gcc.gnu.org/onlinedocs/gcc/_005f_005fatomic-Builtins.html

I need an atomic operation：An add opration with CAS. It's a little like the gcc built-in function __atomic_compare_exchange_n, but different:

1. the function prototype just like this : bool atomic_compare_add(int &ptr, int &expected, int val)

2. the function execute atomicly : if (ptr != expected) { ptr = ptr + val; return true; } else { return false; }

The function I want is different from __atomic_compare_exchange_n and __atomic_add_fetch, __atomic_compare_exchange_n means if (*ptr == *expected) { *ptr = desire; return true; } else { return false; } and __atomic_add_fetch means *ptr = *ptr + val; return *ptr;.

How to implement that operation on Linux gcc/g++?

Answer 1

Thanks everyone, I've got one solution:

bool atomic_compare_add(int *ptr, int not_expected, int val) {
    int old_val;
    do {
        old_val = __atomic_load_n(ptr, __ATOMIC_CONSUME);
        if (old_val == not_expected) {
            return false;
        }
    } while (!__atomic_compare_exchange_n(ptr, &old_val, old_val + val, true, __ATOMIC_ACQ_REL, __ATOMIC_RELAXED));
    return true;
}