MEMCOMMIT

MEMCOMMIT (often written as MEM_COMMIT) is a memory allocation flag in Microsoft Windows native and Win32 APIs used to mark virtual memory pages as committed. When pages are committed, the operating system allocates physical storage (RAM or pagefile backing) and maps it to the process's virtual address space; committed pages are zero-initialized and consume commit charge against the system limit (physical memory plus configured paging file capacity).

In practice MEM_COMMIT is passed to functions such as VirtualAlloc, VirtualAllocEx and NtAllocateVirtualMemory to request immediate allocation of physical backing for a range of virtual addresses. It is commonly used together with MEM_RESERVE (to reserve address space and then commit it) but can be specified alone; the system will select an appropriate address range if none is provided. Protection attributes (for example PAGE_READWRITE) are supplied alongside MEM_COMMIT to define access permissions for the committed pages.

Distinguishing reserve and commit is central to usage: reserving only obtains a block of virtual address space without consuming physical resources, while committing allocates the actual storage. Committed memory can be released or decommitted by calling VirtualFree (using MEM_DECOMMIT or MEM_RELEASE as appropriate). Allocation can fail if the requested commit would exceed the system commit limit, if alignment or allocation-granularity constraints are violated, or for other resource or permission reasons. Proper management of commit and reserve operations helps prevent excessive resource consumption and reduces the risk of out-of-memory conditions.