Win32 Memory Management API Reference

Overview

This section provides detailed reference information for the Win32 API functions used for managing memory within Windows applications. Effective memory management is crucial for performance, stability, and security.

Understanding how to allocate, deallocate, protect, and map memory regions is fundamental for any Windows developer. The Win32 API offers a rich set of tools to control memory behavior at a granular level.

Key Concepts

  • Virtual Memory: Windows uses a virtual memory system, providing each process with its own private virtual address space.
  • Memory Allocation: Functions to reserve and commit memory pages from the virtual address space.
  • Memory Protection: Mechanisms to control read, write, and execute permissions for memory regions.
  • Memory Mapping: Techniques to map files or device memory into the process address space.
  • Heap Management: Low-level functions for managing the heap within a process.

Core Functions

The following are the most commonly used Win32 API functions for memory management:

  • VirtualAlloc

    Reserves, commits, or changes the state of a region of pages in the virtual address space of the calling process.

  • VirtualAllocEx

    Reserves, commits, or changes the state of a region of pages in the virtual address space of a specified process.

  • VirtualFree

    Releases, decommits, or releases and decommits a specified region of pages.

  • VirtualFreeEx

    Releases, decommits, or releases and decommits a specified region of pages in the virtual address space of a specified process.

  • VirtualProtect

    Changes the protection on the specified region of committed pages of memory in the calling process.

  • VirtualProtectEx

    Changes the protection on the specified region of committed pages of memory in a specified process.

  • VirtualQuery

    Retrieves detailed information about a region of pages within the virtual address space of the calling process.

  • HeapAlloc

    Allocates a specified number of bytes from a specified heap.

  • HeapFree

    Frees a specified allocated buffer in a specified heap and invalidates its handle.

  • CreateHeap

    Creates a new heap that can be used by the calling process.

  • GetProcessHeap

    Retrieves a handle to the process's default heap.

  • Mmap (Conceptual - often implemented via CreateFileMapping and MapViewOfFile)

    Maps files or devices into the process address space.

Function Details

VirtualAlloc

LPVOID VirtualAlloc(
  LPVOID lpAddress,
  SIZE_T dwSize,
  DWORD  flAllocationType,
  DWORD  flProtect
);

Parameters:

  • lpAddress: The starting address of the region to allocate.
  • dwSize: The size of the region, in bytes.
  • flAllocationType: The type of memory allocation (e.g., MEM_COMMIT | MEM_RESERVE).
  • flProtect: The memory protection for the region (e.g., PAGE_READWRITE).

Return Value: If the function succeeds, the return value is the base address of the allocated region. If the function fails, the return value is NULL.

Note: Memory allocated with VirtualAlloc must be freed with VirtualFree.

VirtualFree

BOOL VirtualFree(
  LPVOID lpAddress,
  SIZE_T dwSize,
  DWORD  dwFreeType
);

Parameters:

  • lpAddress: A pointer to the starting address of the region of pages to be freed.
  • dwSize: The size of the region, in bytes.
  • dwFreeType: The type of free operation (e.g., MEM_RELEASE or MEM_DECOMMIT).

Return Value: If the function succeeds, the return value is TRUE. If the function fails, the return value is FALSE.

VirtualProtect

BOOL VirtualProtect(
  LPVOID lpAddress,
  SIZE_T dwSize,
  DWORD  flNewProtect,
  PDWORD lpflOldProtect
);

Parameters:

  • lpAddress: A pointer to the base address of the region of pages to be changed.
  • dwSize: The size, in bytes, of the region of pages to be changed.
  • flNewProtect: The new memory protection for the region.
  • lpflOldProtect: A pointer to a variable that receives the previous access protection value.

Return Value: If the function succeeds, the return value is TRUE. If the function fails, the return value is FALSE.

Important: Changing memory protection can have security implications. Ensure you understand the risks before modifying permissions.

HeapAlloc

LPVOID HeapAlloc(
  HANDLE hHeap,
  DWORD  dwFlags,
  SIZE_T dwBytes
);

Parameters:

  • hHeap: A handle to the heap. The handle is returned by the CreateHeap function.
  • dwFlags: The memory allocation attributes.
  • dwBytes: The number of bytes to allocate from the specified heap.

Return Value: If the function succeeds, the return value is a pointer to the allocated buffer. If the function fails, the return value is NULL.

Tip: Use HeapAlloc and HeapFree for more localized memory management within a specific heap, often for frequently allocated small objects.

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