#ifdef for 32-bit platform

问题:

In an application I maintain, we've encountered a problem with file descriptor limitations affecting the stdlib. This problem only affects the 32-bit version of the standard lib.

I have devised a fix for my code and would like to implement it, but only when compiling for 32-bit executable. What pre-processor symbol can I #ifdef for to determine whether the code is being compiled for a 32 or 64-bit target?

EDIT

Sorry, didn't mention, the code is cross-platform, linux, windows, solaris and a few other unix flavors, mostly using GCC for compilation. Any de-facto standards I can use cross-platform?

EDIT 2

I've found some definitions "__ILP23" and "__LP64" that seem like they may work... a discussion here explains the background on the unix platform. Anyone had any experience with using these defines? Is this going to be usable?

回答1:

I'm not sure if there is a universal #if def that is appropriate. The C++ standard almost certainly does not define one. There are certainly platform spcefic ones though.

For example, Windows

#if _WIN64 
// 64 bit build
#else
// 32 bit build
#endif

EDIT OP mentioned this is a cross compile between Windows and Non-Windows using GCC and other compilers

There is no universal macro that can be used for all platforms and compilers. A little bit of preprocessor magic though can do the trick. Assuming you're only working on x86 and amd64 chips the following should do the trick. It can easily be expanded for other platforms though

#if _WIN64 || __amd64__
#define PORTABLE_64_BIT
#else
#define PORTABLE_32_BIT
#endif

回答2:

I recommend bookmarking the predef SourceForge. There's no one answer, but it can certainly help you get started.

EDIT: For GCC-only code, you can use __i386__ to check for 32-bit x86 chips, and I suggest trying __X86_64__ or something similar to check for 64-bit x86 chips. (Note: It has come to my attention that the previous answer involving __ia86__ is actually a different chip, not a 64-bit x86 chip. This just shows my lack of hardware experience. For those more knowledgeable about hardware than I, consule the SourceForge page on predefined macros that I link to above. It's much more accurate than I am.) There are some other ones that would work, but those two should be fairly universal amongs GCC versions.

回答3:

Have a look at that:

i386 macros
AMD64 macros

回答4:

You could check a well known type for it's size e.g. sizeof(int*) == 4 for a 32 bit platform.

As sizeof is known at compiletime I believe a

if(sizeof(int*) == 4)
{
  ...
}

should do the trick

Edit: the comments are right, you need to use a regular if, #if won't work.

If you are using C++ You could create templated code and let the compiler choose the specialization for you based on the sizeof() call. If you build for a 32 bit platform the compiler would only instantiate the code for the 32 bit platform. If you build for a 654 bit platform the compiler would only instantiate the code for the 64 bit platform.

回答5:

What I would probably end up doing, is within a Makefile, determine if you are on a 32 bit platform or 64 bit using uname. Then, add to your CFLAGS, -DX32, or -DX64. That you could just #ifdef X64.

But this is just a unixy solution. I'm not sure what I would do on windows.

回答6:

At least 32-bit Solaris has a limit of 256 file pointers because the structure stores the file descriptor in an unsigned char field. This is retained for backwards compatibility with some almost impossibly old versions of SunOS. Other platforms - I'm tempted to say most other platforms - do not share that limitation. On the other hand, it is relatively unusual for an ordinary user program to need that many files open concurrently; it more often indicates a bug (not closing the files when finished with them) than not. Having said that, though, it can be a problem for things like database servers which need to have lots of data files open at the same time.

---

One comment says:

That's almost it. We don't have a large number of files open, but the server handles a large number of connections from clients. Socket handles and file descriptors seem to come from the same place. When we have a lot of connections, 'fopen' fails because the system-level call returns and fd > 255.

'Socket handles' are file descriptors at the system call level, so they come from the same place as regular file descriptors for files.

If you have to work around this, then you need to wrap your current socket opening code so that if it gets an file descriptor in the range 0..255, then it calls 'dup2()' to create a file descriptor in the range that stdio won't use - and then close the original file descriptor. The only snag with this is that you have to keep track of which file descriptors are available, because dup2 will merrily close the target file descriptor if it is currently open.

Of course, I'm assuming your socket code reads file descriptors and not file pointers. If that's the case, you have bigger problems - too many things want to use the same resources and they can't all use them at the same time.

回答7:

I would test it indirectly, via the maximum pointer value constant:

#include <stdint.h>

#if UINTPTR_MAX == 0xffFFffFF
// 32-bit platform
#elif UINTPTR_MAX == 0xffFFffFFffFFffFF
// 64-bit platform
#else
#error Unknown platform - does not look either like 32-bit or 64-bit
#endif

This way you don't rely on any platform-specific define for architecture, but on the direct consequence of having a specific architecture - the pointer size.

回答8:

Depends on your OS and compiler, those are implementation decisions.

回答9:

I use a construction like this for Windows:

#if defined(_WIN64)
   //64-bit code
#elif  defined(_M_IX86)
   //32-bit code
#else
#error "Unknown platform"
#endif

Versus:

#if defined(_WIN64)
  // 64 bit code
#else
  // 32-bit code
#endif

In the former solution, because of the #error the compiler will be able to tell you where you need to add code for a new platform. This aids maintainability should you ever encounter a platform that is neither 64-bit nor 32-bit. Yes, the _M_IX86 is not exactly synonymous with 32-bit, but I think the only 32-bit platform most of us are supporting is in fact x86. So as a practical measure it suffices.

In the later solution you'll have to manually figure out where you need code for your new platform using grep or something like that. This is tedious and error prone.

It occurs to me that the following construction would also be acceptable, although I have not tested it in production nor I have really thought about it very much.

#if defined(_WIN64)
   //64-bit code
#elif  defined(_WIN32)
   //32-bit code
#else
#error "Unknown platform"
#endif

回答10:

I believe the define is _WIN64

回答11:

There is no such symbol defined by the C++ standard - your specific platform (which you haven't specified in your question) may provide one.