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    <p>Volatile tells the compiler not to optimize anything that has to do with the volatile variable. </p> <p>There is only one reason to use it: When you interface with hardware. </p> <p>Let's say you have a little piece of hardware that is mapped into RAM somewhere and that has two addresses: a command port and a data port:</p> <pre><code>typedef struct { int command; int data; int isbusy; } MyHardwareGadget; </code></pre> <p>Now you want to send some command:</p> <pre><code>void SendCommand (MyHardwareGadget * gadget, int command, int data) { // wait while the gadget is busy: while (gadget-&gt;isbusy) { // do nothing here. } // set data first: gadget-&gt;data = data; // writing the command starts the action: gadget-&gt;command = command; } </code></pre> <p>Looks easy, but it can fail because the compiler is free to change the order in which data and commands are written. This would cause our little gadget to issue commands with the previous data-value. Also take a look at the wait while busy loop. That one will be optimized out. The compiler will try to be clever, read the value of isbusy just once and then go into an infinite loop. That's not what you want. </p> <p>The way to get around this is to declare the pointer gadget as volatile. This way the compiler is forced to do what you wrote. It can't remove the memory assignments, it can't cache variables in registers and it can't change the order of assignments either:</p> <p>This is the correct version:</p> <pre><code> void SendCommand (volatile MyHardwareGadget * gadget, int command, int data) { // wait while the gadget is busy: while (gadget-&gt;isbusy) { // do nothing here. } // set data first: gadget-&gt;data = data; // writing the command starts the action: gadget-&gt;command = command; } </code></pre>
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    1. POWhy is volatile needed in C?
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    1. USVikas Goel
    1. USNils Pipenbrinck
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    1. COPersonally, I'd prefer the integer size to be explicity e.g. int8/int16/int32 when talking to hardware. Nice answer though ;)
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      1. UStonylo
    2. COyes, you should declare things with a fixed register size, but hey - it's just an example.
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      1. USNils Pipenbrinck
    3. COVolatile is also needed in threaded code when you are playing with data that isn't concurrency protected. And yes there are valid times to be doing that, you can for example write a thread safe circular message queue without needing explicit concurrency protection, but it will need volatiles.
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      1. USGordon Wrigley
 

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