public static void Free(byte *p)
{
var remaining = (int)((long)p % 4096);
var firstWritablePage = p - remaining;
for (int i = 0; i < remaining; i++)
{
if (firstWritablePage[i] != 0xED)
{
throw new InvalidOperationException("Invalid memory usage, you killed Ed!");
}
}
Win32MemoryProtectMethods.MemoryProtection protect;
var address = firstWritablePage - 4096;
// this will access the memory, which will error if this was already freed
if (Win32MemoryProtectMethods.VirtualProtect(address, (UIntPtr)4096, Win32MemoryProtectMethods.MemoryProtection.READWRITE, out protect) ==
false)
{
throw new Win32Exception(Marshal.GetLastWin32Error(), "Failed to VirtualProtect (ElectricFence) at address=" + new IntPtr(address));
}
var dwSize = *(int *)address;
// decommit, not release, they are not available for reuse again, any
// future access will throw
if (Win32MemoryProtectMethods.VirtualFree(address, (UIntPtr)dwSize, Win32MemoryProtectMethods.FreeType.MEM_DECOMMIT) == false)
{
throw new Win32Exception(Marshal.GetLastWin32Error(), "Failed to VirtualFree (ElectricFence) at address=" + new IntPtr(address));
}
}
public static byte *Allocate(int size)
{
var remaining = size % 4096;
var sizeInPages = (size / 4096) + (remaining == 0 ? 0 : 1);
var allocatedSize = ((sizeInPages + 2) * 4096);
var virtualAlloc = Win32MemoryProtectMethods.VirtualAlloc(null, (UIntPtr)allocatedSize, Win32MemoryProtectMethods.AllocationType.COMMIT,
Win32MemoryProtectMethods.MemoryProtection.READWRITE);
if (virtualAlloc == null)
{
throw new Win32Exception(Marshal.GetLastWin32Error(), "Failed to VirtualAlloc (ElectricFence) size=" + size);
}
*(int *)virtualAlloc = allocatedSize;
Win32MemoryProtectMethods.MemoryProtection protect;
if (Win32MemoryProtectMethods.VirtualProtect(virtualAlloc, (UIntPtr)(4096), Win32MemoryProtectMethods.MemoryProtection.NOACCESS,
out protect) == false)
{
throw new Win32Exception(Marshal.GetLastWin32Error(), "Failed to VirtualProtect (ElectricFence) at address=" + new IntPtr(virtualAlloc));
}
if (Win32MemoryProtectMethods.VirtualProtect(virtualAlloc + (sizeInPages + 1) * 4096, (UIntPtr)(4096), Win32MemoryProtectMethods.MemoryProtection.NOACCESS,
out protect) == false)
{
throw new Win32Exception(Marshal.GetLastWin32Error(), "Failed to VirtualProtect (ElectricFence) at address=" + new IntPtr(virtualAlloc + (sizeInPages + 1) * 4096));
}
var firstWritablePage = virtualAlloc + 4096;
Memory.Set(firstWritablePage, 0xED, 4096 * sizeInPages); // don't assume zero'ed mem
if (remaining == 0)
{
return(firstWritablePage);
}
// give the memory out so its end would be at the 2nd guard page
return(firstWritablePage + (4096 - remaining));
}