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Function GetReadableSize

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Verifies that the caller has specified access to the specified range of memory.

 

Unit

ELowLevel

 

Syntax

 

Code (Delphi)

function GetReadableSize(

const AAddress: Pointer;

const ASize: Cardinal;

const AAttributes: Cardinal = 0

): Cardinal;

 

Parameters

AAddress

A pointer to the first byte of the memory block.

 

ASize

The size of the memory block, in bytes. If this parameter is zero, the return value is zero.

 

AAttributes

Set of PAGE_XYZ flags, which indicate access to check.

 

Optional. Default is to check for any read access (0 means PAGE_READONLY or PAGE_READWRITE or PAGE_EXECUTE_READ or PAGE_EXECUTE_READWRITE or PAGE_WRITECOPY or PAGE_EXECUTE_WRITECOPY).

 

Return value

If the caller has the specified read access to all bytes in the specified memory range, the return value is ASize.

 

If the caller does not have the specified read access to all bytes in the specified memory range, the return value is less than ASize. It may be 0.

 

Remarks

This function is typically used when working with pointers, where you cannot determine the memory management behavior.

 

Threads in a process are expected to cooperate in such a way that one will not free memory that the other needs. Use of this function does not negate the need to do this. If this is not done, the application may fail in an unpredictable manner.

 

Dereferencing potentially invalid pointers can disable stack expansion in other threads. A thread exhausting its stack, when stack expansion has been disabled, results in the immediate termination of the parent process, with no pop-up error window or diagnostic information.

 

If the calling process has read access to some, but not all, of the bytes in the specified memory range, the return value is in range 0..ASize - 1.

 

In a preemptive multitasking environment, it is possible for some other thread to change the process's access to the memory being tested. Even when the function indicates that the process has read access to the specified memory, you should use structured exception handling when attempting to access the memory. Use of structured exception handling enables the system to notify the process if an access violation exception occurs, giving the process an opportunity to handle the exception.

 

Note

By passing appropriate flags, this function can be used to verify write access to memory.

 

Note

This function is based on VirtualQuery function. Thus, it doesn't have some of the bad effects of IsBadReadPtr function.

 

However, other caveats still appy.

 

Warning

Use this routine with caution. It's heuristic. Even when the function indicates that the process has read access to the specified memory, you should use structured exception handling when attempting to access the memory - because other thread may release this memory between checking and actual access.

 

See also




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Last edited: 2018-11-26
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