VirtualProtect

The VirtualProtect function changes the access protection on a region of committed pages in the virtual address space of the calling process. This function differs from VirtualProtectEx, which changes the access protection of any process.

BOOL VirtualProtect(

LPVOID lpAddress,	// address of region of committed pages
DWORD dwSize,	// size of the region
DWORD flNewProtect,	// desired access protection
PDWORD lpflOldProtect	// address of variable to get old protection
);

Parameters

lpAddress

Points to the base address of the region of pages whose access protection attributes are to be changed.

All pages in the specified region must have been allocated in a single call to the VirtualAlloc or VirtualAllocEx function. The pages cannot span adjacent regions that were allocated by separate calls to VirtualAlloc or VirtualAllocEx.

dwSize

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

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	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	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	Pages in the region become guard pages. Any attempt to access a guard page causes the operating 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 operating 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 usage. It is useful for device drivers; for example, mapping a video frame buffer with no caching. This flag is a page protection modifier, only valid when used with one of the page protections other than PAGE_NOACCESS.

lpflOldProtect

Points 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 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.

The PAGE_GUARD protection modifier flag establishes guard pages. Guard pages act as one-shot access alarms. See Guard Pages for further discussion of guard pages.

See Also

VirtualAlloc, VirtualProtectEx

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