uint64_t writes in 32 bit machine

Question

When 2 process communicating via shared memory on 32 bit solaris i386 Arch

1. Is it guaranteed that for the value of uint64_t datatype, (value < 2^32) is written in single memory location and (value > 2^32 ) is written in 2 memory location?
2. Is the 32 bit memory read is atomic.?

Answer 1

A 64-bit value is always written into 64 bits of memory!¹ The write is almost certainly not atomic (unless the architecture explicitly guarantees this).

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_{1. Except, of course, when it's not written to memory at all (i.e. when there's no register spill. But that's beside the point.}