site_archiver@lists.apple.com Delivered-To: Darwin-dev@lists.apple.com Conversion to lvalue types happens before the operation is completed. -- Terry _______________________________________________ Do not post admin requests to the list. They will be ignored. Darwin-dev mailing list (Darwin-dev@lists.apple.com) Help/Unsubscribe/Update your Subscription: http://lists.apple.com/mailman/options/darwin-dev/site_archiver%40lists.appl... On May 1, 2008, at 1:55 PM, Steve Checkoway wrote: So let's consider the three cases above: 1. info.denom / 1e9 * info.numer has units of mt * ns / 1e9 = mt * s. 2. 1e9 * info.numer / info.denom has units of 1e9 * ns / mt = as / mt (that is attoseconds per mach time or 10^-18 seconds per mach time) 3. 1e-9 * info.numer / info.denom has units of 1e-9 * ns / mt = s/mt. Your original (i.e., number 3) was correct. I'm not sure exactly what Terry was talking about when he was saying you were making mistakes. The following code is a complete working example. The numbers I get are pretty close to being the number of seconds asleep. The order of the operators in C is such that the multiply happens before the divide. If that happens, and your time base happens to be close (the closer to exactly 1GHz, the more likely this is to be an issue), then you've effectively reduce the value below the resolution of the significance of the digits in the mantissa. When that happens, rounding gets to be very important, and tiny rounding issues get magnified. If I had been assured this wasn't homework, I would have pointed this out. Moving the 1e-9 multiply to the next line ensures the order of the divide and the "multiply" (divide by 1e9) avoid losing significant digits. Amusingly, I've seen this problem before internally; usually it's someone trying to second-guess the clock; I say amusingly because all our clock timers are done on the basis of real intervals, not lbolt values, so in something like a sleep(6);, we don't have the +/- 1 second rounding error you might see on other platforms. This email sent to site_archiver@lists.apple.com