If the volatile keyword is not used for a variable, will it be considered shared across threads?

Without the volatile keyword, a variable does not guarantee visibility of changes between threads. This means that one thread may change the value of a variable, but another thread may not see this change immediately or may not see it at all due to caching and compiler optimizations.

volatile provides:

• Visibility of variable changes to all threads immediately after writing.
• Prevention of reordering operations on this variable.

If a variable is not declared as volatile and synchronization is not used, it is not considered shared in terms of visibility, although physically it is the same. To correctly work with shared variables in a multithreaded environment, you need to use volatile or other synchronization mechanisms.