Overview

The VirtualAlloc function reserves, commits, or changes the state of a region of pages in the virtual address space of the calling process.

This function is a fundamental part of memory management in Windows. It allows applications to dynamically allocate memory from the operating system, control its access permissions, and manage its state (reserved, committed, or free).

Syntax

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

Parameters

  • lpAddress [in, optional]
    The starting address of the region of pages to be allocated. If this parameter is NULL, the system determines where to allocate the region.
  • dwSize [in]
    The size, in bytes, of the region of pages to be allocated. If lpAddress is NULL, this parameter is ignored and the function allocates a page-aligned block of memory that is at least the size specified by dwSize.
  • flAllocationType [in]
    The type of memory allocation. This parameter can be one of the following values:
    • MEM_COMMIT: Allocates physical memory for the specified region of pages.
    • MEM_RESERVE: Reserves a range of the process's virtual address space without any (or changing) physical storage for it.
    • MEM_RESET: Indicates that data in the memory range specified by lpAddress and dwSize is no longer of interest. The pages should not be written to the paging file. In the future, the calling process can read the memory pages, but the content of the memory that has been committed beyond the allocation granularity of the process is zero.
    • MEM_RESET_UNDO: Indicates that the data in the memory range specified by lpAddress and dwSize is of interest again.
    • MEM_LARGE_PAGES: Allocates memory using large page support.
    • MEM_PHYSICAL: Allocates physical memory.
    • MEM_TOP_DOWN: Allocates memory from the highest available address.
  • flProtect [in]
    The protection for the region of pages to be allocated. This parameter can be one of the following values:
    • PAGE_EXECUTE
    • PAGE_EXECUTE_READ
    • PAGE_EXECUTE_READWRITE
    • PAGE_EXECUTE_WRITECOPY
    • PAGE_NOACCESS
    • PAGE_READONLY
    • PAGE_READWRITE
    • PAGE_WRITECOPY
    • PAGE_TARGETS_INVALID
    • PAGE_TARGETS_NO_UPDATE
    You can also use bitwise OR to combine page protection flags. For example, PAGE_READWRITE | PAGE_NOCACHE.

Return Value

  • If the function succeeds, the return value is the base address of the allocated region of pages.
  • If the function fails, the return value is NULL. To get extended error information, call GetLastError.

Remarks

VirtualAlloc is the primary function for managing memory pages. It offers fine-grained control over memory allocation, including:

When you commit memory, the system initializes the memory region to all zeros. If you reserve memory and then commit it later, the content of the pages committed beyond the allocation granularity of the process is zero.

Example: Allocating and Writing to Memory


#include <windows.h>
#include <stdio.h>

int main() {
    LPVOID mem_block;
    SIZE_T size = 1024; // Allocate 1KB

    // Allocate and commit memory with read/write access
    mem_block = VirtualAlloc(NULL, size, MEM_COMMIT | MEM_RESERVE, PAGE_READWRITE);

    if (mem_block == NULL) {
        fprintf(stderr, "VirtualAlloc failed: %lu\n", GetLastError());
        return 1;
    }

    printf("Memory allocated at: %p\n", mem_block);

    // Write some data to the allocated memory
    const char* message = "Hello, VirtualAlloc!";
    memcpy(mem_block, message, strlen(message) + 1);

    printf("Data written: %s\n", (char*)mem_block);

    // Free the allocated memory
    if (VirtualFree(mem_block, 0, MEM_RELEASE) == 0) {
        fprintf(stderr, "VirtualFree failed: %lu\n", GetLastError());
        return 1;
    }

    printf("Memory freed.\n");

    return 0;
}

See Also