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In C, is i+=1; atomic?
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In practice, you can assume that int is atomic. You can also assume that pointer types are atomic; that is very convenient. Both of these assumptions are true on all of the machines that the GNU C Library supports and on all POSIX systems we know of.
On objects without an atomic type, standard never defines ++ as an atomic operation.
The increment-memory machine instruction on an X86 is atomic only if you use it with a LOCK prefix. x++ in C and C++ doesn't have atomic behavior.
Atomics as part of the C language are an optional feature that is available since C11. Their purpose is to ensure race-free access to variables that are shared between different threads. Without atomic qualification, the state of a shared variable would be undefined if two threads access it concurrently.
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.
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No.
The only operation guaranteed by the C language standard to be atomic is assigning or retrieving a value to/from a variable of type sig_atomic_t, defined in <signal.h>.
(C99, chapter 7.14 Signal handling.)
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