The trouble with volatile [LWN.net]

The trouble with volatile

Posted May 14, 2007 15:07 UTC (Mon) by BenHutchings (subscriber, #37955)
In reply to: The trouble with volatile by mikov
Parent article: The trouble with volatile

"People might argue that it is useful with signals or longjmp(), but the need for volatile in such scenarios is always an indication of a serious problem in the code."

These are the two cases defined by the standard where volatile *must* be used for certain variables. Memory-mapped I/O is the third case it was intended for, but as that's inherently unportable the standard doesn't explicitly mention it. (I think the rationale does.)

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The trouble with volatile

Posted May 14, 2007 17:30 UTC (Mon) by mikov (subscriber, #33179) [Link]

You are right. IIRC, it is required for local variables with setjmp(). I forgot the other case. What was it ? (Hmm, I have my C99 standard somewhere here..)

In any case, it doesn't mean that it actually is a good idea. Requiring volatile for setjmp() is a terrible, fragile, ugly hack. It's been some time since I thought about this, but off the top of my head:

This requirement presumes that the compiler doesn't know anything about setjmp()/longjmp(). What if the function invoking setjmp() gets auto-inlined ? Obviously the programmer can't be required to use volatile for all variables in all functions up the call tree. What is to prevent the auto-inlining ?

I don't view the standard as a mantra. After all, it did standardize "atoi" ... There is no excuse for that. There are things in the C99 that strike me as absolutely horrible.