Mac OS X: Starting up in Safe Mode (Last Modified: November 19, 2011) specifies


but I forgot to shut down the other day and, seemingly successfully, booted into Safe Mode from a restart (OS X 10.6.8), and I'm wondering whether that somehow compromised the Safe Boot functionality (if such a thing is actually possible)?

Thanks.

I don't know if it's of any help, but when one boots into Safe Boot mode in Lion, the words Safe Boot in red appear on the screen.

Thanks, but no help...perhaps because I was a bit unclear.

"Seemingly successfully" meant that the boot went (apparently) according to Hoyle...progress bar, "SAFE BOOT," and all, except that it started at the wrong square-one, so I was wondering whether that particular aspect compromised the entire exercise?

Safe boot from a restart is just fine.

Apple's how-to documents don't list every possible way to do something; just one way that's sure to work. They're striving for clarity and brevity, rather than exhaustive completeness.

For all practical purposes, restart is exactly the same as shutdown/power-on, and Apple's use of the word "shutdown" in this case clearly meant to encompass Restart as one kind of shutdown. (Some early Macintoshes would do a RAM check on power up, but skip the RAM check on a restart. Modern Macs have too much RAM to check quickly, so there's no longer a RAM check either way. I don't know that there was ever any other difference between a "cold start" and a "warm start".)

When Apple decides to do extra checking of this nature on some boots, the test now is based on whether you are doing a Safe Boot and/or did not shutdown/restart cleanly, not on whether you are booting from a full power down.)

Well, there is one other difference when booting from a shutdown: when you shut down your computer, power is removed from all devices and busses. Some peripherals may conceivably treat this differently than the reset signal they get as part of any boot. Any such difference, though, will be due to the loss of power, not to whether there's a Safe Boot in the picture.

Nice explanation; thanks.

My question was based on having heard that (in the context of troubleshooting) logging out/in does not do as much as restarting, which does not do as much as shutting down and starting up, but I've got no real idea of what "as much" may mean literally, just a suspicion that it has something to do with diagnostics and/or cache clearing and/or..., but I guess that's just another myth debunked, not that it won't always be lurking in the back of my head waiting to jump out and holler "Gimme a shot!

Some system software code is cached in RAM and re-used in a restart. There is no problem with doing that, provided the cached code is not corrupted. A boot from a cold start replaces everything in RAM.

So there is at least a bit of difference between shutting down and just restarting.

Thanks for pointing it out.

Seems odd, then, that in Mac OS X 10.6 Help: Starting up in safe mode, they say

The doc to which I linked in my original post doesn't specify a 10 wait.

My point is that the 10 second wait specified in the document I linked to tends to support an interpretation of "be sure your Mac is shut down" to mean something other than "a restart is fine; we're just trying to keep it simple."

Kernel extensions can take a small but noticeable time to load, so the result of loading them is cached (along with some other stuff) on disk in /System/Library/Caches/, to speed up the next restart. One of the things Safe Boot does is discard all your startup caches, including this one. (Font caches are also discarded.) Your kernel extensions are loaded the slow but sure way, this time and on your next normal boot. (Since a Safe Boot also alters which kernel extensions load, it would be pointless to rewrite the kext cache until after a normal boot reloads the normal set of extensions.)

But that cache-flushing is because of the Safe Boot, not because of the power down. Nothing in RAM is preserved across a restart. (Even if it could be preserved, a Safe Boot wouldn't trust RAM, and wouldn't use any left-over garbage in it.) On the flip side, even after a shutdown, a normal boot will load a cached kernel from disk into RAM, and use it.

I'm sure suggestions to "power down and wait 10 seconds" and the like come in the spirit of "as long as you want to forcibly reset the state of the OS, you may as well forcibly reset all your peripherals too."

Which isn't necessarily bad advice. I'm just saying that failing to shut down won't make a Safe Boot "not take", which was the original concern.

Your point wasn't lost on me; I guess the "required" 10 second wait is for a complete power drain, and it does appear to support your position.

I was pointing out, though, that Apple's own position on the issue is inconsistent, and a coupl'a second looks give me the impression that my linked doc is more recent than yours, because the former was updated on Nov. 19, 2011 and mentions Lion, whereas the latter is strictly a Snow Leopard doc.