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What is ATOMIC_LONG_LOCK_FREE in C?

Abdul Monum

ATOMIC_LONG_LOCK_FREE is a macro constant defined in the <stdatomic.h> header file in the C11 standard. ATOMIC_LONG_LOCK_FREE is relative to the types long and unsigned long but has an additional atomic property. Reading or writing an atomic type happens as a single unit of work, and therefore atomic types are guaranteed to have no race conditions when accessed by multiple threads.

The macro takes on the value 00, 11, or 22 depending on the lock-free property type:

  • 00 defines that the type is never lock-free
  • 11 defines that the type is sometimes lock-free
  • 22 defines that the type is always lock-free

Lock-free programming

To ensure concurrency, Lock-free programming is an alternative paradigm to mutex locks and binary semaphores. Instead of using locking mechanisms to block threads from accessing data, lock-free programming allows atomic operations on the data using a transactional memory model.

Transactional memory model

The transactional memory model bunches up a series of memory accesses and implements them at once as a transaction, similar to how transactions in databases work.

Lock-free programming is non-blocking, which allows lock-free programs to access shared resources without blocking other threads and avoiding race conditions. The non-blocking property of lock-free programming carries many performance advantages over mutex locks because concurrent access to resources is not blocked by other threads accessing the same data.

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