Converting a pointer into an integer

Question

I am trying to adapt an existing code to a 64 bit machine  The main problem is that in one function  the previous coder uses a void  argument that is converted into suitable type in the function itself  A short example   void function MESSAGE ID id  void  param        if id    FOO            int real param    int param                           Of course  on a 64 bit machine  I get the error   error  cast from  void   to  int  loses precision   I would like to correct this so that it still works on a 32 bit machine and as cleanly as possible  Any idea

User · Answer

size t  and  ptrdiff t  are required to match your architecture  whatever it is    Therefore  I think rather than using  int   you should be able to use  size t   which on a 64 bit system should be a 64 bit type   This discussion unsigned int vs size t goes into a bit more detail

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Use uintptr t as your integer type

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I think the  meaning  of void  in this case is a generic handle  It is not a pointer to a value  it is the value itself   This just happens to be how void  is used by C and C   programmers    If it is holding an integer value  it had better be within integer range   Here is easy rendering to integer   int x    char  p -  char  0    It should only give a warning

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include  lt stdint h gt  Use uintptr t standard type defined in the included standard header file

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Use intptr t and uintptr t   To ensure it is defined in a portable way  you can use code like this    if defined   BORLANDC        typedef unsigned char uint8 t      typedef   int64 int64 t      typedef unsigned long uintptr t   elif defined  MSC VER      typedef unsigned char uint8 t      typedef   int64 int64 t   else      include  lt stdint h gt   endif   Just place that in some  h file and include wherever you need it   Alternatively  you can download Microsoft   s version of the stdint h file from here or use a portable one from here

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I d say this is the modern C   way    include  lt cstdint gt  void  p  auto i   reinterpret cast lt std  uintptr t gt  p     EDIT   The correct type to the the Integer  so the right way to store a pointer as an integer is to use the uintptr t or intptr t types   See also in cppreference integer types for C99    these types are defined in  lt stdint h gt  for C99 and in the namespace std for C  11 in  lt cstdint gt   see integer types for C      C  11  and onwards  Version   include  lt cstdint gt  std  uintptr t i    C  03 Version  extern  C     include  lt stdint h gt     uintptr t i    C99 Version   include  lt stdint h gt  uintptr t i    The correct casting operator  In C there is only one cast and using the C cast in C   is frowned upon  so don t use it in C     In C   there is different casts  reinterpret cast is the correct cast for this conversion  See also here    C  11 Version  auto i   reinterpret cast lt std  uintptr t gt  p     C  03 Version  uintptr t i   reinterpret cast lt uintptr t gt  p     C Version  uintptr t i    uintptr t p     C Version   Related Questions   What is uintptr t data type

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I came across this question while studying the source code of SQLite    In the sqliteInt h  there is a paragraph of code defined a macro convert between integer and pointer  The author made a very good statement first pointing out it should be a compiler dependent problem and then implemented the solution to account for most of the popular compilers out there     if defined   PTRDIFF TYPE        This case should work for GCC      define SQLITE INT TO PTR X     void     PTRDIFF TYPE    X     define SQLITE PTR TO INT X     int    PTRDIFF TYPE    X    elif  defined   GNUC             Works for compilers other than LLVM      define SQLITE INT TO PTR X     void   amp   char  0  X     define SQLITE PTR TO INT X     int    char  X - char  0    elif defined HAVE STDINT H       Use this case if we have ANSI headers      define SQLITE INT TO PTR X     void   intptr t  X     define SQLITE PTR TO INT X     int  intptr t  X    else                             Generates a warning - but it always works          define SQLITE INT TO PTR X     void   X     define SQLITE PTR TO INT X     int  X    endif   And here is a quote of the comment for more details          The following macros are used to cast pointers to integers and    integers to pointers   The way you do this varies from one compiler    to the next  so we have developed the following set of  if statements    to generate appropriate macros for a wide range of compilers        The correct  ANSI  way to do this is to use the intptr t type     Unfortunately  that typedef is not available on all compilers  or    if it is available  it requires an  include of specific headers    that vary from one machine to the next        Ticket  3860   The llvm-gcc-4 2 compiler from Apple chokes on    the   void   amp   char  0  X   construct   But MSVC chokes on   void   X       So we have to define the macros in different ways depending on the    compiler       Credit goes to the committers

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Since uintptr t is not guaranteed to be there in C   C  11  if this is a one way conversion you can consider uintmax t  always defined in  lt cstdint gt    auto real param   reinterpret cast lt uintmax t gt  param     To play safe  one could add anywhere in the code an assertion   static assert sizeof  uintmax t   gt   sizeof  void                     No suitable integer type for conversion from pointer type

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Several answers have pointed at uintptr t and  include  lt stdint h gt  as  the  solution   That is  I suggest  part of the answer  but not the whole answer   You also need to look at where the function is called with the message ID of FOO   Consider this code and compilation     cat kk c  include  lt stdio h gt  static void function int n  void  p        unsigned long z     unsigned long   p      printf   d -  lu n   n  z      int main void        function 1  2       return 0       rmk kk         gcc -m64 -g -O -std c99 -pedantic -Wall -Wshadow -Wpointer-arith               -Wcast-qual -Wstrict-prototypes -Wmissing-prototypes               -D FILE OFFSET BITS 64 -D LARGEFILE SOURCE kk c -o kk  kk c  In function  main   kk c 10  warning  passing argument 2 of  func  makes pointer from integer without a cast     You will observe that there is a problem at the calling location  in main        converting an integer to a pointer without a cast   You are going to need to analyze your function   in all its usages to see how values are passed to it   The code inside my function   would work if the calls were written   unsigned long i   0x2341  function 1   amp i     Since yours are probably written differently  you need to review the points where the function is called to ensure that it makes sense to use the value as shown   Don t forget  you may be finding a latent bug   Also  if you are going to format the value of the void   parameter  as converted   look carefully at the  lt inttypes h gt  header  instead of stdint h     inttypes h provides the services of stdint h  which is unusual  but the C99 standard says  t he header  lt inttypes h gt  includes the header  lt stdint h gt  and extends it with additional facilities provided by hosted implementations  and use the PRIxxx macros in your format strings   Also  my comments are strictly applicable to C rather than C    but your code is in the subset of C   that is portable between C and C     The chances are fair to good that my comments apply

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The best thing to do is to avoid converting from pointer type to non-pointer types  However  this is clearly not possible in your case   As everyone said  the uintptr t is what you should use   This link has good info about converting to 64-bit code   There is also a good discussion of this on comp std c

[c++] Converting a pointer into an integer

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