Are assignment = and subtraction assignment -= atomic operations in C?

Question

int b = 1000;
b -= 20;

Is any of the above an atomic operation? What is an atomic operation in C?

Answer 1

It depends on the implementation. By the standard, nothing is atomic in C. If you need atomic ops you can look at your compiler's builtins.

Answer 2

It is architecture/implementation dependent.

If you want atomic operations, I think sig_atomic_t type is standardized by C99, but not sure.

From the GNU LibC docs:

In practice, you can assume that int and other integer types no longer than int are atomic. You can also assume that pointer types are atomic; that is very convenient. Both of these are true on all of the machines that the GNU C library supports, and on all POSIX systems we know of.