The Two Flavors of Transactional Memory

C++ supports transactional memory in two flavors: synchronized blocks and atomic blocks.

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Synchronized & Atomic Blocks

Up to now, I only wrote about transactions. Now, I will write about synchronized blocks and atomic blocks. Both can be encapsulated in each other; specifically, synchronized blocks are not atomic blocks because they can execute transaction-unsafe. An example would be code like the output to the console which can not be undone. For this reason, synchronized blocks are often called relaxed blocks.

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Synchronized Blocks

Synchronized blocks behave like they are protected by a global lock, i.e. This means that all synchronized blocks follow a total order. In particular: all changes to a synchronized block are available in the next synchronized block. There is a synchronizes-with relation between the synchronized blocks because of the commit of the transaction synchronizes-with the next start of a transaction. Synchronized blocks cannot cause a deadlock because they create a total order. While a classical lock protects a memory area, a global lock of a synchronized block protects the total program. This is the reason the following program is well-defined: