In computing, a memory barrier, also known as a membar, memory fence or fence instruction, is a kind of barrier instruction that causes a central processing unit (CPU) or compiler to enforce an ordering constraint on memory operations issued before and after the barrier instruction. This typically means that operations issued prior to the barrier are guaranteed to be carried out earlier than operations issued after the barrier. Memory barriers are essential because most fashionable CPUs make use of performance optimizations that can result in out-of-order execution. This reordering of memory operations (masses and stores) normally goes unnoticed within a single thread of execution, but could cause unpredictable habits in concurrent programs and device drivers unless rigorously controlled. The precise nature of an ordering constraint is hardware dependent and outlined by the structure's memory ordering model. Some architectures provide a number of boundaries for implementing different ordering constraints. Memory barriers are typically used when implementing low-degree machine code that operates on memory shared by multiple units. Such code consists of synchronization primitives and lock-free data buildings on multiprocessor systems, and system drivers that talk with computer hardware.

When a program runs on a single-CPU machine, the hardware performs the mandatory bookkeeping to make sure that the program executes as if all memory operations were carried out in the order specified by the programmer (program order), so memory boundaries are not necessary. However, when the memory is shared with multiple gadgets, similar to other CPUs in a multiprocessor system, or memory-mapped peripherals, out-of-order access might affect program conduct. For example, a second CPU might see memory modifications made by the first CPU in a sequence that differs from program order. A program is run through a process which will be multi-threaded (i.e. a software program thread similar to pthreads as opposed to a hardware thread). Completely different processes do not share a memory house so this dialogue doesn't apply to 2 programs, each running in a unique process (hence a special memory house). It applies to 2 or extra (software brainwave audio program) threads running in a single course of (i.e. a single memory house the place multiple software threads share a single memory space).

Multiple software program threads, within a single course of, may run concurrently on a multi-core processor. 1 loops whereas the worth of f is zero, then it prints the value of x. 2 shops the value forty two into x and then shops the value 1 into f. Pseudo-code for the 2 program fragments is proven beneath. The steps of the program correspond to individual processor instructions. In the case of the PowerPC processor, brainwave audio program the eieio instruction ensures, as memory fence, that any load or retailer operations previously initiated by the processor are totally accomplished with respect to the main memory before any subsequent load or retailer operations initiated by the processor access the principle memory. 2's retailer operations are executed out-of-order, it is possible for f to be updated before x, and the print assertion would possibly therefore print "0". 1's load operations could also be executed out-of-order and it is feasible for x to be learn earlier than f is checked, and once more the print statement might therefore print an unexpected value.

For most programs neither of those conditions is acceptable. 2's task to f to ensure that the new value of x is visible to other processors at or previous to the change in the worth of f. 1's access to x to ensure the value of x just isn't learn prior to seeing the change in the value of f. If the processor's retailer operations are executed out-of-order, the hardware module may be triggered before information is prepared in memory. For one more illustrative instance (a non-trivial one which arises in precise follow), see double-checked locking. Multithreaded programs normally use synchronization primitives offered by a excessive-stage programming environment-resembling Java or .Internet-or an utility programming interface (API) such as POSIX Threads or Windows API. Synchronization primitives such as mutexes and semaphores are supplied to synchronize access to assets from parallel threads of execution. These primitives are often applied with the memory barriers required to supply the anticipated memory visibility semantics. In such environments specific use of memory limitations will not be usually needed.

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Pub: 24 Nov 2025 13:42 UTC

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