Is assigning a pointer in C program considered atomic on x86-64

Bear in mind that atomicity alone is not enough for communicating between threads. Nothing prevents the compiler and CPU from reordering previous/subsequent load and store instructions with that "atomic" store. In old days people used volatile to prevent that reordering but that was never intended for use with threads and doesn't provide means to specify less or more restrictive memory order (see "Relationship with volatile" in there).

You should use C11 atomics because they guarantee both atomicity and memory order.

For almost all architectures, pointer load and store are atomic. A once notable exception was 8086/80286 where pointers could be seg:offset; there was an l[des]s instruction which could make an atomic load; but no corresponding atomic store.

The integrity of the pointer is only a small concern; your bigger issue revolves around synchronization: the pointer was at value Y, you set it to X; how will you know when nobody is using the (old) Y value? A somewhat related problem is that you may have stored things at X, which the other thread expects to find. Without synchronization, other might see the new pointer value, however what it points to might not be up to date yet.

A plain global char *ptr should not be considered atomic. It might work sometimes, especially with optimization disabled, but you can get the compiler to make safe and efficient optimized asm by using modern language features to tell it you want atomicity.

Use C11 stdatomic.h or GNU C __atomic builtins. And see Why is integer assignment on a naturally aligned variable atomic on x86? - yes the underlying asm operations are atomic "for free", but you need to control the compiler's code-gen to get sane behaviour for multithreading.

See also LWN: Who's afraid of a big bad optimizing compiler? - weird effects of using plain vars include several really bad well-known things, but also more obscure stuff like invented loads, reading a variable more than once if the compiler decides to optimize away a local tmp and load the shared var twice, instead of loading it into a register. Using asm("" ::: "memory") compiler barriers may not be sufficient to defeat that depending on where you put them.

So use proper atomic stores and loads that tell the compiler what you want: You should generally use atomic loads to read them, too.

#include <stdatomic.h>            // C11 way
_Atomic char *c11_shared_var;     // all access to this is atomic, functions needed only if you want weaker ordering

void foo(){
   atomic_store_explicit(&c11_shared_var, newval, memory_order_relaxed);
}

char *plain_shared_var;       // GNU C
// This is a plain C var.  Only specific accesses to it are atomic; be careful!

void foo() {
   __atomic_store_n(&plain_shared_var, newval, __ATOMIC_RELAXED);
}

Using __atomic_store_n on a plain var is the functionality that C++20 atomic_ref exposes. If multiple threads access a variable for the entire time that it needs to exist, you might as well just use C11 stdatomic because every access needs to be atomic (not optimized into a register or whatever). When you want to let the compiler load once and reuse that value, do char *tmp = c11_shared_var; (or atomic_load_explicit if you only want acquire instead of seq_cst; cheaper on a few non-x86 ISAs).

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Besides lack of tearing (atomicity of asm load or store), the other key parts of _Atomic foo * are:

• The compiler will assume that other threads may have changed memory contents (like volatile effectively implies), otherwise the assumption of no data-race UB will let the compiler hoist loads out of loops. Without this, dead-store elimination might only do one store at the end of a loop, not updating the value multiple times.

  The read side of the problem is usually what bites people in practice, see Multithreading program stuck in optimized mode but runs normally in -O0 - e.g. while(!flag){} becomes if(!flag) infinite_loop; with optimization enabled.

• Ordering wrt. other code. e.g. you can use memory_order_release to make sure that other threads that see the pointer update also see all changes to the pointed-to data. (On x86 that's as simple as compile-time ordering, no extra barriers needed for acquire/release, only for seq_cst. Avoid seq_cst if you can; mfence or locked operations are slow.)

• Guarantee that the store will compile to a single asm instruction. You'd be depending on this. It does happen in practice with sane compilers, although it's conceivable that a compiler might decide to use rep movsb to copy a few contiguous pointers, and that some machine somewhere might have a microcoded implementation that does some stores narrower than 8 bytes.

  (This failure mode is highly unlikely; the Linux kernel relies on volatile load/store compiling to a single instruction with GCC / clang for its hand-rolled intrinsics. But if you just used asm("" ::: "memory") to make sure a store happened on a non-volatile variable, there's a chance.)

Also, something like ptr++ will compile to an atomic RMW operation like lock add qword [mem], 4, rather than separate load and store like volatile would. (See Can num++ be atomic for 'int num'? for more about atomic RMWs). Avoid that if you don't need it, it's slower. e.g. atomic_store_explicit(&ptr, ptr + 1, mo_release); - seq_cst loads are cheap on x86-64 but seq_cst stores aren't.

Also note that memory barriers can't create atomicity (lack of tearing), they can only create ordering wrt other ops.

In practice x86-64 ABIs do have alignof(void*) = 8 so all pointer objects should be naturally aligned (except in a __attribute__((packed)) struct which violates the ABI, so you can use __atomic_store_n on them. It should compile to what you want (plain store, no overhead), and meet the asm requirements to be atomic.

See also When to use volatile with multi threading? - you can roll your own atomics with volatile and asm memory barriers, but don't. The Linux kernel does that, but it's a lot of effort for basically no gain, especially for a user-space program.

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Side note: an often repeated misconception is that volatile or _Atomic are needed to avoid reading stale values from cache. This is not the case.

All machines that run C11 threads across multiple cores have coherent caches, not needing explicit flush instructions in the reader or writer. Just ordinary load or store instructions, like x86 mov. The key is to not let the compiler keep values of shared variable in CPU registers (which are thread-private). It normally can do this optimization because of the assumption of no data-race Undefined Behaviour. Registers are very much not the same thing as L1d CPU cache; managing what's in registers vs. memory is done by the compiler, while hardware keeps cache in sync. See When to use volatile with multi threading? for more details about why coherent caches is sufficient to make volatile work like memory_order_relaxed.

See Multithreading program stuck in optimized mode but runs normally in -O0 for an example.