Determining 32 vs 64 bit in C

Question

I m looking for a way to reliably determine whether C   code is being compiled in 32 vs 64 bit  We ve come up with what we think is a reasonable solution using macros  but was curious to know if people could think of cases where this might fail or if there is a better way to do this  Please note we are trying to do this in a cross-platform  multiple compiler environment    if   ULONG MAX      UINT MAX     define IS32BIT  else   define IS64BIT  endif   ifdef IS64BIT DoMy64BitOperation    else DoMy32BitOperation    endif   Thanks

User · Answer

You could do this    if   WORDSIZE    64 char  size    64bits    else char  size    32bits    endif

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I m adding this answer as a use case and complete example for the runtime-check described in another answer   This is the approach I ve been taking for conveying to the end-user whether the program was compiled as 64-bit or 32-bit  or other  for that matter    version h   ifndef MY VERSION  define MY VERSION   include  lt string gt   const std  string version    0 09   const std  string arch    std  to string sizeof void     8     -bit      endif   test cc   include  lt iostream gt   include  version h   int main         std  cerr  lt  lt   My App v   lt  lt  version  lt  lt        lt  lt  arch  lt  lt       lt  lt  std  endl      Compile and Test  g   -g test cc   a out My App v0 09  64-bit

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I d place 32-bit and 64-bit sources in different files and then select appropriate source files using the build system

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Your approach was not too far off  but you are only checking whether long and int are of the same size  Theoretically  they could both be 64 bits  in which case your check would fail  assuming both to be 32 bits  Here is a check that actually checks the size of the types themselves  not their relative size    if   UINT MAX     0xffffffffu       define INT IS32BIT  else      define INT IS64BIT  endif  if   ULONG MAX     0xfffffffful       define LONG IS32BIT  else      define LONG IS64BIT  endif   In principle  you can do this for any type for which you have a system defined macro with the maximal value   Note  that the standard requires long long to be at least 64 bits even on 32 bit systems

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Borrowing from Contango s excellent answer above and combining it with  Better Macros  Better Flags  from Fluent C    you can do      Macro for checking bitness  safer macros borrowed from     https   www fluentcpp com 2019 05 28 better-macros-better-flags    define MYPROJ IS BITNESS  X   MYPROJ IS BITNESS PRIVATE DEFINITION   X       Bitness checks borrowed from https   stackoverflow com a 12338526 201787  if  WIN64        GNUC    amp  amp    x86 64          define MYPROJ IS BITNESS PRIVATE DEFINITION 64   1      define MYPROJ IS BITNESS PRIVATE DEFINITION 32   0      define MYPROJ IF 64 BIT ELSE  x64  x86    x64      static assert  sizeof  void       8   Pointer size is unexpected for this bitness      elif  WIN32      GNUC        define MYPROJ IS BITNESS PRIVATE DEFINITION 64   0      define MYPROJ IS BITNESS PRIVATE DEFINITION 32   1      define MYPROJ IF 64 BIT ELSE  x64  x86    x86      static assert  sizeof  void       4   Pointer size is unexpected for this bitness      else      error  Unknown bitness    endif   Then you can use it like    if MYPROJ IS BITNESS  64       DoMy64BitOperation    else     DoMy32BitOperation    endif   Or using the extra macro I added   MYPROJ IF 64 BIT ELSE  DoMy64BitOperation    DoMy32BitOperation

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Unfortunately there is no cross platform macro which defines 32   64 bit across the major compilers   I ve found the most effective way to do this is the following     First I pick my own representation   I prefer ENVIRONMENT64   ENVIRONMENT32   Then I find out what all of the major compilers use for determining if it s a 64 bit environment or not and use that to set my variables        Check windows  if  WIN32     WIN64  if  WIN64  define ENVIRONMENT64  else  define ENVIRONMENT32  endif  endif     Check GCC  if   GNUC    if   x86 64        ppc64    define ENVIRONMENT64  else  define ENVIRONMENT32  endif  endif   Another easier route is to simply set these variables from the compiler command line

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template lt int gt  void DoMyOperationHelper     template lt  gt  void DoMyOperationHelper lt 4 gt            do 32-bits operations    template lt  gt  void DoMyOperationHelper lt 8 gt            do 64-bits operations       helper function just to hide clumsy syntax inline void DoMyOperation     DoMyOperationHelper lt sizeof size t  gt        int main          appropriate function will be selected at compile time    DoMyOperation        return 0

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People already suggested methods that will try to determine if the program is being compiled in 32-bit or 64-bit   And I want to add that you can use the c  11 feature static assert to make sure that the architecture is what you think it is   to relax     So in the place where you define the macros    if       define IS32BIT   static assert sizeof void       4   Error  The Arch is not what I think it is    elif       define IS64BIT   static assert sizeof void       8   Error  The Arch is not what I think it is    else   error  Cannot determine the Arch   endif

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That won t work on Windows for a start  Longs and ints are both 32 bits whether you re compiling for 32 bit or 64 bit windows  I would think checking if the size of a pointer is 8 bytes is probably a more reliable route

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Compiled in 64 bit  is not well defined in C     C   sets only lower limits for sizes such as int  long and void    There is no guarantee that int is 64 bit even when compiled for a 64 bit platform  The model allows for e g  23 bit ints and sizeof int       sizeof char      There are different programming models for 64 bit platforms    Your best bet is a platform specific test  Your second best  portable decision must be more specific in what is 64 bit

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Try this   ifdef  WIN64    64 bit code  elif  WIN32    32 bit code  else    if sizeof void    4             32 bit code    else             64 bit code     endif

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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

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If you can use project configurations in all your environments  that would make defining a 64- and 32-bit symbol easy  So you d have project configurations like this   32-bit Debug 32-bit Release 64-bit Debug 64-bit Release  EDIT  These are generic configurations  not targetted configurations  Call them whatever you want   If you can t do that  I like Jared s idea

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You should be able to use the macros defined in stdint h  In particular INTPTR MAX is exactly the value you need    include  lt cstdint gt   if INTPTR MAX    INT32 MAX      define THIS IS 32 BIT ENVIRONMENT  elif INTPTR MAX    INT64 MAX      define THIS IS 64 BIT ENVIRONMENT  else      error  Environment not 32 or 64-bit    endif   Some  all   versions of Microsoft s compiler don t come with stdint h  Not sure why  since it s a standard file  Here s a version you can use  http   msinttypes googlecode com svn trunk stdint h

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Below code works fine for most current environments      if defined   LP64       defined  WIN64      defined   x86 64     amp  amp       defined   ILP32         defined  M X64     defined   ia64     defined   M IA64     defined   aarch64       defined   powerpc64         define IS64BIT 1   else      define IS32BIT 1  endif

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