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copied!<p>Re-entrant code has no state in a single point. You can call the code while something is executing in the code. If the code uses global state, one call can conceivably overwrite the global state, breaking the computation in the other call.</p> <p>Thread safe code is code with no race conditions or other concurrency issues. A race condition is where the order in which two threads do something affects the computation. A typical concurrency issue is where a change to a shared data structure can be partially completed and left in an inconsistent state. In order to avoid this, you have to use concurrency control mechanisms such as semaphores of mutexes to ensure that nothing else can access the data structure until the operation is completed.</p> <p>For example, a piece of code can be non re-entrant but thread-safe if it is guarded externally by a mutex but still has a global data structure where the state must be consistent for the entire duration of the call. In this case, the same thread could initiate a call-back into the procedure while still protected by an external coarse-grained mutex. If the call-back occured from within the non re-entrant procedure the call could leave the data structure in a state that could break the computation from the caller's point of view.</p> <p>A piece of code can be re-entrant but non thread-safe if it can make a non-atomic change to a shared (and sharable) data structure that could be interrupted in the middle of the update leaving the data structure in an incosistent state. In this case another thread accessing the data structure could be affected by the half-changed data structure and either crash or perform an operation that corrupts the data.</p>

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