In C is “i+=1;” atomic?

Question

In C, is i+=1; atomic?

Answer 1

Not usually.

If i is volatile, then it would depend on your CPU architecure and compiler - if adding two integers in main memory is atomic on your CPU, then that C statement might be atomic with a volatile int i.

Answer 2

No, the C standard doesn't guarantee atomicity, and in practice, the operation won't be atomic. You have to use a library (eg the Windows API) or compiler builtin functions (GCC, MSVC).

Answer 3

The C standard does not define whether it is atomic or not.

In practice, you never write code which fails if a given operation is atomic, but you might well write code which fails if it isn't. So assume it isn't.

Answer 4

The C / C++ language itself makes no claim of atomicity or lack thereof. You need to rely on intrinsics or library functions to ensure atomic behavior.

Answer 5

Whether the expression is atomic or not depends only on the machine code that the compiler generates, and the CPU architectre that it will run on. Unless the addition can be achieved in one machine instruction, its unlikely to be atomic.

If you are using Windows then you can use the InterlockedIncrement() API function to do a guaranteed atomic increment. There are similar functions for decrement, etc.

Answer 6

It really depends on your target and the mnemonic set of your uC/processor. If i is a variable held in a register then it is possible to have it atomic.