The warnings C4311 and C4312 generated by the analyzer when using the /Wp64 switch

C4311 and C4312 are ones of the most frequent warnings generated by the compiler for 32-bit code not adapted for migration to 64-bit platforms. They are generated only when you use the /Wp64 key (detecting 64-bit portability issues) intended to prepare migration of applications to 64-bit systems. Note that the /Wp64 parameter is announced deprecated beginning with Visual C++ 9.0 since the time "to prepare for 64-bit code" has passed.
The C4311 and C4312 warnings tell you about an attempt to put a pointer into a 32-bit variable or vice versa. In a 64-bit system, these conversions are incorrect. If the code is compiled on the 64-bit platform, the pointer's value (64 bits) will be truncated if it is assigned to a variable of the int type (32 bits). This is an example of code that causes the C4311 and C4312 warnings:

int *p;
   int a = (int)p; //C4311
   p = (int *)a; //C4312

To correct the code you should use memsize-types that can store a pointer, for instance size_t, ptrdiff_t, intptr_t, LONG_PTR, etc. This is an example of correct code:

int *p;
   INT_PTR b = (INT_PTR)p; //OK

See the "Development of 64-bit C/C++ applications" course for detailed recommendations on creating safe 64-bit code.
If a program being developed has a short life-cycle and you do not plan to port it to the 64-bit platform, you may eliminate these warnings by disabling the /Wp64 option in the compiler settings.
Note that the /Wp64 switch carries out a rather superficial analysis and detects only crudest errors. To perform a complete analysis of your code, we recommend you to use the specialized static code analyzer Viva64 included into the PVS-Studio package. See the article "Comparing capabilities of PVS-Studio and Visual Studio 2010 in detecting defects in 64-bit programs" for comparison of diagnostic abilities of Visual C++ and Viva64.

References

1. Andrey Karpov. 64 bits, Wp64, Visual Studio 2008, Viva64 and all the rest...
2. Discussion at forum Windows Tech. The warning of the type casting from "HANDLE" to "long"
3. MSDN Library. Compiler Warning (level 1) C4311
4. MSDN Library. Compiler Warning (level 1) C4312