Unfortunately, in a cross platform, cross compiler environment, there is no single reliable method to do this purely at compile time.
- Both _WIN32 and _WIN64 can sometimes both be undefined, if the project settings are flawed or corrupted (particularly on Visual Studio 2008 SP1).
- A project labelled "Win32" could be set to 64-bit, due to a project configuration error.
- On Visual Studio 2008 SP1, sometimes the intellisense does not grey out the correct parts of the code, according to the current #define. This makes it difficult to see exactly which #define is being used at compile time.
Therefore, the only reliable method is to combine 3 simple checks:
- 1) Compile time setting, and;
- 2) Runtime check, and;
- 3) Robust compile time checking.
Simple check 1/3: Compile time setting
Choose any method to set the required #define variable. I suggest the method from @JaredPar:
// Check windows
#if _WIN32 || _WIN64
#if _WIN64
#define ENV64BIT
#else
#define ENV32BIT
#endif
#endif
// Check GCC
#if __GNUC__
#if __x86_64__ || __ppc64__
#define ENV64BIT
#else
#define ENV32BIT
#endif
#endif
Simple check 2/3: Runtime check
In main(), double check to see if sizeof() makes sense:
#if defined(ENV64BIT)
if (sizeof(void*) != 8)
{
wprintf(L"ENV64BIT: Error: pointer should be 8 bytes. Exiting.");
exit(0);
}
wprintf(L"Diagnostics: we are running in 64-bit mode.\n");
#elif defined (ENV32BIT)
if (sizeof(void*) != 4)
{
wprintf(L"ENV32BIT: Error: pointer should be 4 bytes. Exiting.");
exit(0);
}
wprintf(L"Diagnostics: we are running in 32-bit mode.\n");
#else
#error "Must define either ENV32BIT or ENV64BIT".
#endif
Simple check 3/3: Robust compile time checking
The general rule is "every #define must end in a #else which generates an error".
#if defined(ENV64BIT)
// 64-bit code here.
#elif defined (ENV32BIT)
// 32-bit code here.
#else
// INCREASE ROBUSTNESS. ALWAYS THROW AN ERROR ON THE ELSE.
// - What if I made a typo and checked for ENV6BIT instead of ENV64BIT?
// - What if both ENV64BIT and ENV32BIT are not defined?
// - What if project is corrupted, and _WIN64 and _WIN32 are not defined?
// - What if I didn't include the required header file?
// - What if I checked for _WIN32 first instead of second?
// (in Windows, both are defined in 64-bit, so this will break codebase)
// - What if the code has just been ported to a different OS?
// - What if there is an unknown unknown, not mentioned in this list so far?
// I'm only human, and the mistakes above would break the *entire* codebase.
#error "Must define either ENV32BIT or ENV64BIT"
#endif
Update 2017-01-17
Comment from @AI.G:
4 years later (don't know if it was possible before) you can convert
the run-time check to compile-time one using static assert:
static_assert(sizeof(void*) == 4);. Now it's all done at compile time
:)
Appendix A
Incidentially, the rules above can be adapted to make your entire codebase more reliable:
- Every if() statement ends in an "else" which generates a warning or error.
- Every switch() statement ends in a "default:" which generates a warning or error.
The reason why this works well is that it forces you to think of every single case in advance, and not rely on (sometimes flawed) logic in the "else" part to execute the correct code.
I used this technique (among many others) to write a 30,000 line project that worked flawlessly from the day it was first deployed into production (that was 12 months ago).
