Memory API
The Memory API provides functions for managing memory in Windows applications. These functions allow you to allocate, free, map, and query memory regions. Understanding and utilizing these functions is crucial for efficient and robust memory management, especially in low-level programming and performance-critical applications.
Overview
Memory management is a fundamental aspect of any operating system. The Win32 Memory API offers a set of powerful tools to interact with the system's memory resources. This includes:
- Virtual Memory Management: Functions that deal with the virtual address space of processes.
- Physical Memory Access: While less common for typical applications, some functions allow limited access to physical memory.
- Memory Mapping: Creating mappings between files or devices and memory regions.
Key Functions
Core Memory Allocation Functions
VirtualAlloc
Allocates a region of pages within the virtual address space of the calling process.
LPVOID VirtualAlloc(
LPVOID lpAddress,
SIZE_T dwSize,
DWORD flAllocationType,
DWORD flProtect
);Parameters
| Name | Type | Description |
|---|---|---|
lpAddress |
LPVOID |
The starting address of the region to allocate. If this parameter is NULL, the system determines where to allocate the region. |
dwSize |
SIZE_T |
The size, in bytes, of the region of memory to allocate. |
flAllocationType |
DWORD |
The type of memory allocation. Can be MEM_COMMIT, MEM_RESERVE, or a combination. |
flProtect |
DWORD |
The memory protection for the region of pages to be allocated. Can be PAGE_READWRITE, PAGE_EXECUTE_READWRITE, etc. |
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. To get extended error information, call GetLastError.
VirtualAllocEx
Allocates, commits, or changes the state of a region of pages in the virtual address space of a specified process.
LPVOID VirtualAllocEx(
HANDLE hProcess,
LPVOID lpAddress,
SIZE_T dwSize,
DWORD flAllocationType,
DWORD flProtect
);Parameters
| Name | Type | Description |
|---|---|---|
hProcess |
HANDLE |
A handle to the process in which the memory is to be allocated. |
lpAddress |
LPVOID |
The starting address of the region to allocate. |
dwSize |
SIZE_T |
The size of the region to allocate. |
flAllocationType |
DWORD |
The type of memory allocation. |
flProtect |
DWORD |
The memory protection for the region. |
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.
VirtualFree
Deallocates, uncommits, or releases a range of pages within the virtual address space of the calling process.
BOOL VirtualFree(
LPVOID lpAddress,
SIZE_T dwSize,
DWORD dwFreeType
);Parameters
| Name | Type | Description |
|---|---|---|
lpAddress |
LPVOID |
A pointer to the starting address of the region of pages to be freed. |
dwSize |
SIZE_T |
The size, in bytes, of the region of pages to be freed. |
dwFreeType |
DWORD |
The type of free operation. Can be MEM_DECOMMIT or MEM_RELEASE. |
Return Value
If the function succeeds, the return value is TRUE. If the function fails, the return value is FALSE.
VirtualFreeEx
Deallocates, uncommits, or releases a region of pages in the virtual address space of a specified process.
BOOL VirtualFreeEx(
HANDLE hProcess,
LPVOID lpAddress,
SIZE_T dwSize,
DWORD dwFreeType
);Parameters
| Name | Type | Description |
|---|---|---|
hProcess |
HANDLE |
A handle to the process whose address space is to be freed. |
lpAddress |
LPVOID |
A pointer to the starting address of the region to be freed. |
dwSize |
SIZE_T |
The size, in bytes, of the region to be freed. |
dwFreeType |
DWORD |
The type of free operation. |
Return Value
If the function succeeds, the return value is TRUE. If the function fails, the return value is FALSE.
VirtualProtect
Changes the protection on the commit of a region of pages in the virtual address space of the calling process.
BOOL VirtualProtect(
LPVOID lpAddress,
SIZE_T dwSize,
DWORD flNewProtect,
PDWORD lpflOldProtect
);Parameters
| Name | Type | Description |
|---|---|---|
lpAddress |
LPVOID |
A pointer to the base address of the region of pages to be changed. |
dwSize |
SIZE_T |
The size, in bytes, of the region of pages to be changed. |
flNewProtect |
DWORD |
The new protection setting for the region. |
lpflOldProtect |
PDWORD |
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.
VirtualProtectEx
Changes the protection on the commit of a region of pages in the virtual address space of a specified process.
BOOL VirtualProtectEx(
HANDLE hProcess,
LPVOID lpAddress,
SIZE_T dwSize,
DWORD flNewProtect,
PDWORD lpflOldProtect
);Parameters
| Name | Type | Description |
|---|---|---|
hProcess |
HANDLE |
A handle to the process whose memory protection is to be changed. |
lpAddress |
LPVOID |
A pointer to the base address of the region of pages to be changed. |
dwSize |
SIZE_T |
The size, in bytes, of the region of pages to be changed. |
flNewProtect |
DWORD |
The new protection setting for the region. |
lpflOldProtect |
PDWORD |
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.
VirtualQuery
Retrieves detailed information about a region of pages in the virtual address space of the calling process.
SIZE_T VirtualQuery(
LPCVOID lpAddress,
PMEMORY_BASIC_INFORMATION lpBuffer,
SIZE_T dwLength
);Parameters
| Name | Type | Description |
|---|---|---|
lpAddress |
LPCVOID |
A pointer to the starting address of the region of pages to be queried. |
lpBuffer |
PMEMORY_BASIC_INFORMATION |
A pointer to a MEMORY_BASIC_INFORMATION structure that receives information about the specified region. |
dwLength |
SIZE_T |
The size, in bytes, of the buffer pointed to by lpBuffer. |
Return Value
If the function succeeds, the return value is the number of bytes stored in the buffer. If the function fails, the return value is 0.
VirtualQueryEx
Retrieves detailed information about a region of pages in the virtual address space of a specified process.
SIZE_T VirtualQueryEx(
HANDLE hProcess,
LPCVOID lpAddress,
PMEMORY_BASIC_INFORMATION lpBuffer,
SIZE_T dwLength
);Parameters
| Name | Type | Description |
|---|---|---|
hProcess |
HANDLE |
A handle to the process whose virtual address space is to be queried. |
lpAddress |
LPCVOID |
A pointer to the starting address of the region of pages to be queried. |
lpBuffer |
PMEMORY_BASIC_INFORMATION |
A pointer to a MEMORY_BASIC_INFORMATION structure that receives information about the specified region. |
dwLength |
SIZE_T |
The size, in bytes, of the buffer. |
Return Value
If the function succeeds, the return value is the number of bytes stored in the buffer. If the function fails, the return value is 0.
Important Considerations
When working with memory, always be mindful of memory leaks. Ensure that allocated memory is properly freed when it's no longer needed to prevent resource exhaustion. Also, be cautious when changing memory protection flags, as incorrect settings can lead to access violations and application crashes.
Performance Tips
For frequent small allocations, consider using custom memory allocators or object pools to reduce the overhead of system calls. For large contiguous memory blocks, VirtualAlloc is generally efficient.