The last two are identical; "atomic" is the default behavior (note that it is not actually a keyword; it is specified only by the absence of nonatomic -- atomic was added as a keyword in recent versions of llvm/clang).
Assuming that you are @synthesizing the method implementations, atomic vs. non-atomic changes the generated code. If you are writing your own setter/getters, atomic/nonatomic/retain/assign/copy are merely advisory. (Note: @synthesize is now the default behavior in recent versions of LLVM. There is also no need to declare instance variables; they will be synthesized automatically, too, and will have an _ prepended to their name to prevent accidental direct access).
With "atomic", the synthesized setter/getter will ensure that a whole value is always returned from the getter or set by the setter, regardless of setter activity on any other thread. That is, if thread A is in the middle of the getter while thread B calls the setter, an actual viable value -- an autoreleased object, most likely -- will be returned to the caller in A.
In nonatomic, no such guarantees are made. Thus, nonatomic is considerably faster than "atomic".
What "atomic" does not do is make any guarantees about thread safety. If thread A is calling the getter simultaneously with thread B and C calling the setter with different values, thread A may get any one of the three values returned -- the one prior to any setters being called or either of the values passed into the setters in B and C. Likewise, the object may end up with the value from B or C, no way to tell.
Ensuring data integrity -- one of the primary challenges of multi-threaded programming -- is achieved by other means.
Adding to this:
atomicity of a single property also cannot guarantee thread safety when multiple dependent properties are in play.
Consider:
@property(atomic, copy) NSString *firstName;
@property(atomic, copy) NSString *lastName;
@property(readonly, atomic, copy) NSString *fullName;
In this case, thread A could be renaming the object by calling setFirstName: and then calling setLastName:. In the meantime, thread B may call fullName in between thread A's two calls and will receive the new first name coupled with the old last name.
To address this, you need a transactional model. I.e. some other kind of synchronization and/or exclusion that allows one to exclude access to fullName while the dependent properties are being updated.
It's really easy to do from the command line. I had a gist of a script for doing this. It has now been incorporated into the ipa_sign script in https://github.com/RichardBronosky/ota-tools which I use daily. If you have any questions about using these tools, don't hesitate to ask.
The heart of it is this:
CODESIGN_ALLOCATE=`xcrun --find codesign_allocate`; export CODESIGN_ALLOCATE
IPA="/path/to/file.ipa"
PROVISION="/path/to/file.mobileprovision"
CERTIFICATE="Name of certificate: To sign with" # must be in keychain
# unzip the ipa
unzip -q "$IPA"
# remove the signature
rm -rf Payload/*.app/_CodeSignature
# replace the provision
cp "$PROVISION" Payload/*.app/embedded.mobileprovision
# sign with the new certificate (--resource-rules has been deprecated OS X Yosemite (10.10), it can safely be removed)
/usr/bin/codesign -f -s "$CERTIFICATE" Payload/*.app
# zip it back up
zip -qr resigned.ipa Payload
Your new signed app is called resigned.ipa
Best Answer
Ad-Hoc releases have to be provisioned to specific devices. Your account allows you to register up to 100 devices.
In-House is for distribution to any company device.
So in most cases In-House can serve all your needs. But in the event that you wanted someone who was not part of your company to test something or you wanted to restrict distribution to specific devices you could use the ad-hoc provisioning.
Update: Ad-Hoc now allows up to 100 of each type of device. ie. 100 iPhones, 100 iPads, 100 Apple TVs, etc