VirtualProtect

The VirtualProtect function changes the access protection on a region of committed pages in the virtual address space of the calling process.

To change the access protection of any process, use the VirtualProtectEx function.

BOOL VirtualProtect(
  LPVOID lpAddress,       // region of committed pages
  SIZE_T dwSize,          // size of the region
  DWORD flNewProtect,     // desired access protection
  PDWORD lpflOldProtect   // old protection
);

Parameters

lpAddress

[in] Pointer to the base address of the region of pages whose access protection attributes are to be changed.

All pages in the specified region must be within the same reserved region allocated when calling the VirtualAlloc or VirtualAllocEx function using MEM_RESERVE. The pages cannot span adjacent reserved regions that were allocated by separate calls to VirtualAlloc or VirtualAllocEx using MEM_RESERVE.

dwSize

[in] Specifies the size, in bytes, of the region whose access protection attributes are to be changed. The region of affected pages includes all pages containing one or more bytes in the range from the lpAddress parameter to (lpAddress+dwSize). This means that a 2-byte range straddling a page boundary causes the protection attributes of both pages to be changed.

flNewProtect

[in] Specifies the new access protection. You can specify any one of the following value, along with the PAGE_GUARD and PAGE_NOCACHE values, as necessary.

Value	Meaning
PAGE_READONLY	Enables read access to the committed region of pages. An attempt to write to the committed region results in an access violation. If the system differentiates between read-only access and execute access, an attempt to execute code in the committed region results in an access violation.
PAGE_READWRITE	Enables both read and write access to the committed region of pages.
PAGE_WRITECOPY	Windows NT/2000: Gives copy-on-write access to the committed region of pages.
PAGE_EXECUTE	Enables execute access to the committed region of pages. An attempt to read or write to the committed region results in an access violation.
PAGE_EXECUTE_READ	Enables execute and read access to the committed region of pages. An attempt to write to the committed region results in an access violation.
PAGE_EXECUTE_READWRITE	Enables execute, read, and write access to the committed region of pages.
PAGE_EXECUTE_WRITECOPY	Enables execute, read, and write access to the committed region of pages. The pages are shared read-on-write and copy-on-write.
PAGE_GUARD	Windows NT/2000: Pages in the region become guard pages. Any attempt to access a guard page causes the system to raise a STATUS_GUARD_PAGE exception and turn off the guard page status. Guard pages thus act as a one-shot access alarm. PAGE_GUARD is a page protection modifier. An application uses it with one of the other page protection modifiers, with one exception: it cannot be used with PAGE_NOACCESS. When an access attempt leads the system to turn off guard page status, the underlying page protection takes over. If a guard page exception occurs during a system service, the service typically returns a failure status indicator. Windows 95/98: To simulate this behavior, use PAGE_NOACCESS.
PAGE_NOACCESS	Disables all access to the committed region of pages. An attempt to read from, write to, or execute in the committed region results in an access violation exception, called a general protection (GP) fault.
PAGE_NOCACHE	Allows no caching of the committed regions of pages. The hardware attributes for the physical memory should be specified as "no cache." This is not recommended for general usage. It is useful for device drivers; for example, mapping a video frame buffer with no caching. This value is a page protection modifier; it is only valid when used with one of the page protections other than PAGE_NOACCESS.

lpflOldProtect

[out] Pointer to a variable that receives the previous access protection value of the first page in the specified region of pages. If this parameter is NULL or does not point to a valid variable, the function fails.

Return Values

If the function succeeds, the return value is nonzero.

If the function fails, the return value is zero. To get extended error information, call GetLastError.

Remarks

You can set the access protection value on committed pages only. If the state of any page in the specified region is not committed, the function fails and returns without modifying the access protection of any pages in the specified region.

The VirtualProtect function changes the access protection of memory in the calling process, and the VirtualProtectEx function changes the access protection of memory in a specified process.

Windows NT/2000: The PAGE_GUARD protection modifier establishes guard pages. Guard pages act as one-shot access alarms. For more information, see Creating Guard Pages.

Windows 95/98: You cannot use VirtualProtect on any memory region located in the shared virtual address space (from 0x80000000 through 0xBFFFFFFF).

Requirements

  Windows NT/2000: Requires Windows NT 3.1 or later.
  Windows 95/98: Requires Windows 95 or later.
  Header: Declared in Winbase.h; include Windows.h.
  Library: Use Kernel32.lib.

See Also

Memory Management Overview, Memory Management Functions, VirtualAlloc, VirtualProtectEx