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POContiguous VirtualAlloc behaviour on Windows Mobile
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copied!<p>I have been optimising memory performance on a Windows Mobile application and have encountered some differences in behaviour between <strong>VirtualAlloc</strong> <a href="http://msdn.microsoft.com/en-us/library/windows/desktop/aa366887.aspx" rel="nofollow noreferrer">on Win32</a> and <a href="http://msdn.microsoft.com/en-us/library/ee488564.aspx" rel="nofollow noreferrer">Windows CE</a>. </p> <p>Consider the following test:</p> <pre><code>// Allocate 64k of memory BYTE *a = (BYTE*)VirtualAlloc(0, 65536, MEM_RESERVE|MEM_COMMIT, PAGE_READWRITE); // Allocate a second contiguous 64k of memory BYTE *b = (BYTE*)VirtualAlloc(a+65536, 65536, MEM_RESERVE|MEM_COMMIT, PAGE_READWRITE); BYTE *c = a + 65528; // Set a pointer near the end of the first allocation BOOL valid1 = !IsBadWritePtr(c, 8); // Expect TRUE BOOL valid2 = !IsBadWritePtr(c+8, 4088); // Expect TRUE BOOL valid3 = !IsBadWritePtr(c, 4096); // TRUE on Win32, FALSE on WinCE </code></pre> <p>The code "allocates" 4096 of data starting at "c". On Win32 this works. I can find no mention in the <strong>VirtualAlloc</strong> documentation whether it is legal or coincidence but there are many examples of code that I have found via google that expect this behaviour.</p> <p>On Windows CE 5.0/5.2 if I use the memory block at "c", in 99% of cases there are no problems, however on some (not all) Windows Mobile 6 devices, <strong>ReadFile</strong> & <strong>WriteFile</strong> will fail with error 87 (The parameter is incorrect.). I assume <strong>ReadFile</strong> is calling <strong>IsBadWritePtr</strong> or similar and return false due to this. If I perform two <strong>ReadFile</strong> calls then everything works fine. (There may of course be other API calls that will also fail.)</p> <p>I am looking for a way to extend the memory returned by <strong>VirtualAlloc</strong> so that I can make the above work. Reserving a large amount of memory on Windows CE is problematic as each process only gets 32MB and due to other items being loaded it is not possible to reserve a large region of memory without causing other problems. (It is possible to reserve a larger amount of memory in the shared region but this also has other problems.)</p> <p>Is there a way to get <strong>VirtualAlloc</strong> to enlarge or combine regions without reserving it up front?</p> <p>I suspect it may be problematic given the following examples:</p> <pre><code>HANDLE hHeap1 = HeapCreate(0, 0, 0); // Heap defaults to 192k BYTE * a1 = (BYTE*)HeapAlloc(hHeap1, 0, 64000); // +96 bytes from start of heap BYTE * b1 = (BYTE*)HeapAlloc(hHeap1, 0, 64000); // +16 bytes from end of a1 BYTE * c1 = (BYTE*)HeapAlloc(hHeap1, 0, 64000); // +16 bytes from end of b1 BYTE * d1 = (BYTE*)HeapAlloc(hHeap1, 0, 64000); // +4528 bytes from end of c1 HANDLE hHeap2 = HeapCreate(0, 4*1024*1024, 4*1024*1024); // 4MB Heap BYTE * a2 = (BYTE*)HeapAlloc(hHeap2, 0, 64000); // +96 bytes from start of heap BYTE * b2 = (BYTE*)HeapAlloc(hHeap2, 0, 64000); // +16 bytes from end of a2 BYTE * c2 = (BYTE*)HeapAlloc(hHeap2, 0, 64000); // +16 bytes from end of b2 BYTE * d2 = (BYTE*)HeapAlloc(hHeap2, 0, 64000); // +16 bytes from end of c2 </code></pre>

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