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    <p>It is legal -- as in the OS won't stop you from doing it. </p> <p>But is it smart? No, you should have some type of synchronization.</p> <p>There wouldn't be "mangled bits on the wire". They will come out either as ones or zeros. But there's nothing to say that all your bits will be written out before another process tries to read them. And there are NO guarantees on how fast they'll be written vs how fast they'll be read.</p> <p>You should always assume there is absolutely NO relationship between the actions of 2 processes (or threads for that matter).</p> <p>Hardware level bus locking does not happen unless you get it right. It can be harder then expected to make your compiler / library / os / cpu get it right. Synchronization primitives are written to makes sure it happens right.</p> <p>Locking will make it safe, and it's not that hard to do. So just do it.</p> <hr> <p>@unknown - The question has changed somewhat since my answer was posted. However, the behavior you describe is defiantly platform (hardware, os, library and compiler) dependent. </p> <p>Without giving the compiler specific instructions, you are actually not guaranteed to have 32 bits written out in one shot. Imagine a situation where the 32 bit word is not aligned on a word boundary. This unaligned access is acceptable on x86, and in the case of the x68, the access is turned into a series of aligned accesses by the cpu. </p> <p>An interrupt can occurs between those operations. If a context switch happens in the middle, some of the bits are written, some aren't. Bang, You're Dead. </p> <p>Also, lets think about 16 bit cpus or 64 bit cpus. Both of which are still popular and don't necessarily work the way you think.</p> <p>So, actually you can have a situation where "some other cpu-core picks up a word sized value 1/2 written to". You write you code as if this type of thing is expected to happen if you are not using synchronization.</p> <p>Now, there are ways to preform your writes to make sure that you get a whole word written out. Those methods fall under the category of synchronization, and creating synchronization primitives is the type of thing that's best left to the library, compiler, os, and hardware designers. Especially if you are interested in portability (which you should be, even if you never port your code)</p>
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    1. COActually this is *wrong*. You say "... all your bits written...NO garuntee's ... ". That's incorrect. There is a garuntee, as the poster remarked, "writes .. shmid" (shmid is a machiene word sized value). And he goes on later to say, " only 32 bits are being written and read.". He is garuntee'd to have syncronized read's and write's to those 32-bit quantities, the CPU will **NOT** read/write like 5 bit's or something and have some other CPU-CORE pick up a word-sized value 1/2 written too. Think about it, that would mean the CORE's are reading/writing by the bit, which is not the case.
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    2. CO@unknown - my response has been updated
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    3. CO@RandomNickName42. As James has pointed out. on x86 hardware you do not need to have your data properly aligned. If you don't then the store of the value need not be atomic and can result in "half written" values. I think "Bang!, You're dead" summed it up nicely :o)
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