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4/8/2010

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

Syntax


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

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 allocated in a single call to the VirtualAllocfunction.

The pages cannot span adjacent regions that were allocated by separate calls to VirtualAlloc.

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 lpAddressparameter to lpAddressplus 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 flags, along with the PAGE_GUARD and PAGE_NOCACHE protection modifier flags, as necessary.

Value	Description
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_GUARD	Pages in the region become guard pages. An 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. The PAGE_GUARD flag is a page protection modifier. An application uses it with one of the other page protection flags, 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.
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 use. It is useful for device drivers; for example, mapping a video frame buffer with no caching. This flag is a page protection modifier and is valid when used with a page protection other than PAGE_NOACCESS.
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.

lpflOldProtect

[out] Long pointer to a variable that the function sets to 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 Value

Nonzero indicates success. Zero indicates failure. To get extended error information, call GetLastError.

Remarks

The following code example shows how to use the VirtualProtectfunction to change the characteristics of pages in virtual memory that were allocated with READ_WRITE access to READ_ONLY.

	Copy Code
DWORD dwOldProtect; DWORD dwSize = (2 * PAGE_SIZE); //To change a set of two pages, //whose size was defined earlier as PAGE_SIZE if (!VirtualProtect (lpPage, // Beginning of the set of pages to change dwSize, // Length, in bytes, of the set of pages //to change PAGE_READONLY, // What to change it to &dwOldProtect // Place to store the old setting )) {// Your error-handling code goes here. }

Copy Code

DWORD dwOldProtect;
DWORD dwSize = (2 * PAGE_SIZE);  //To change a set of two pages, 
//whose size was defined earlier as PAGE_SIZE
if (!VirtualProtect (lpPage, // Beginning of the set of pages to
change
					 dwSize, // Length, in bytes, of the set of
pages 
								//to change
					 PAGE_READONLY, // What to change it to
					 &dwOldProtect  // Place to store the old
setting
					 ))
{// Your error-handling code goes here. 
}

Requirements

Header	winbase.h
Library	coredll.lib
Windows Embedded CE	Windows CE 1.0 and later
Windows Mobile	Windows Mobile Version 5.0 and later

Syntax

Parameters

Return Value

Remarks

Requirements

See Also

Reference

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