【iOS之App数据存储】
1、to obtain access to locations outside of your app’s container directory, you must request appropriate entitlements.
为了能够访问app目录之外的目录，你必须申请合适的权限。
2、an app is generally prohibited from accessing or creating files in directories outside of its home directory.
app通常被禁止在主目录外访问或创建目录。
【iOS之App中四个最重要的目录】
1、/Documents/
2、/Library/Application Support
3、/Library/Caches
4、/tmp
On iOS 5.0 and later, the system may delete the Caches directory on rare occasions when the system is very low on disk space. This will never occur while an app is running.
在iOS5.0以后的版本中，系统在低磁盘空间的情况下会删除Caches目录，当本程序运行时，本程序的Caches目录是不会被删除的。
