error cast from void to int loses precision

Question

I have a function with prototype void  myFcn void  arg  which is used as the starting point for a pthread   I need to convert the argument to an int for later use   int x    int arg    The compiler  GCC version 4 2 4  returns the error   file cpp 233  error  cast from  void   to  int  loses precision   What is the proper way to cast this

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You can cast it to an intptr t type  It s an int type guaranteed to be big enough to contain a pointer  Use  include  lt cstdint gt  to define it

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In my case  I was using a 32-bit value that needed to be passed to an OpenGL function as a void   representing an offset into a buffer   You cannot just cast the 32-bit variable to a pointer  because that pointer on a 64-bit machine is twice as long   Half your pointer will be garbage   You need to pass an actual pointer   This will get you a pointer from a 32 bit offset   int32 nOffset     762           random offset char   pOffset    NULL          pointer to hold offset pOffset            nOffset      this will now hold the value of 762 correctly glVertexAttribPointer binding  nStep  glType  glTrueFalse  VertSize    pOffset

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Casting a pointer to void  and back is valid use of reinterpret cast lt    So you could do this   pthread create  amp thread  NULL  myFcn  new int 5       implicit cast to void  from int    Then in myFcn   void  myFcn void  arg        int   data   reinterpret cast lt int  gt  arg       int   x       data      delete data    Note  As sbi points out this would require a change on the OP call to create the thread   What I am trying to emphasis that conversion from int to pointer and back again can be frough with problems as you move from platform to platform  BUT converting a pointer to void  and back again is well supported  everywhere    Thus as a result it may be less error prone to generate a pointer dynamcially and use that  Remembering to delete the pointer after use so that we don t leak

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I would create a structure and pass that as void  to pthread create  struct threadArg       int intData      long longData      etc         threadArg thrArg  thrArg intData   4      pthread create  amp thread  NULL  myFcn   void   threadArg   amp thrArg     void  myFcn void  arg        threadArg  pThrArg    threadArg  arg      int computeSomething   pThrArg- gt intData              Keep in mind that thrArg should exist till the myFcn   uses it

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There s no proper way to cast this to int in general case  C99 standard library provides intptr t and uintptr t typedefs  which are supposed to be used whenever the need to perform such a cast comes about  If your standard library  even if it is not C99  happens to provide these types - use them  If not  check the pointer size on your platform  define these typedefs accordingly yourself and use them

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If you call your thread creation function like this  pthread create  amp thread  NULL  myFcn  reinterpret cast lt void  gt  5      then the void  arriving inside of myFcn has the value of the int you put into it  So you know you can cast it back like this  int myData   reinterpret cast lt int gt  arg     even though the compiler doesn t know you only ever pass myFcn to pthread create in conjunction with an integer    Edit   As was pointed out by Martin  this presumes that sizeof void   gt  sizeof int   If your code has the chance to ever be ported to some platform where this doesn t hold  this won t work

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Safest way     static cast lt int gt  reinterpret cast lt long gt  void   your variable      long guarantees a pointer size on Linux on any machine  Windows has 32 bit long only on 64 bit as well  Therefore  you need to change it to long long instead of long in windows for 64 bits  So reinterpret cast has casted it to long type and then static cast safely casts long to int  if you are ready do truncte the data

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new fd is a int pthread create  amp threads threads in use    NULL  accept request   void    long new fd       inside the method I then have int client  client    long new fd    Hope this helps

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What you may want is  int x   reinterpret cast lt int gt  arg     This allows you to reinterpret the void   as an int

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Well it does this because you are converting a 64 bits pointer to an 32 bits integer so you loose information    You can use a 64 bits integer instead howerver I usually use a function with the right prototype and I cast the function type   eg   void thread func int arg           and I create the thread like this    pthread create  amp tid  NULL   void     void   thread func   void  arg

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Again  all of the answers above missed the point badly  The OP wanted to convert a pointer value to a int value  instead  most the answers  one way or the other  tried to wrongly convert the content of arg points to to a int value  And  most of these will not even work on gcc4   The correct answer is  if one does not mind losing data precision   int x      int    amp arg      This works on GCC4   The best way is  if one can  do not do such casting  instead  if the same memory address has to be shared for pointer and int  e g  for saving RAM   use union  and make sure  if the mem address is treated as an int only if you know it was last set as an int

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There is no  correct  way to store a 64-bit pointer in an 32-bit integer  The problem is not with casting  but with the target type loosing half of the pointer  The 32 remaining bits stored inside int are insufficient to reconstruct a pointer to the thread function  Most answers just try to extract 32 useless bits out of the argument   As Ferruccio said  int must be replaced with intptr t to make the program meaningful

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Instead of using a long cast  you should cast to size t   int val   int   size t arg

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The proper way is to cast it to another pointer type  Converting a void  to an int is non-portable way that may work or may not  If you need to keep the returned address  just keep it as void

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I meet this problem too   ids i     int  arg     error occur here   gt  I change this to below   ids i     uintptr t  arg    Then I can continue compiling  Maybe you can try this too

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A function pointer is incompatible to void   and any other non function pointer

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Don t pass your int as a void   pass a int  to your int  so you can cast the void  to an int  and copy the dereferenced pointer to your int   int x    static cast lt int  gt  arg

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Instead of   int x    int arg    use   int x    long arg    On most platforms pointers and longs are the same size  but ints and pointers often are not the same size on 64bit platforms   If you convert  void   to  long  no precision is lost  then by assigning the  long  to an  int   it properly truncates the number to fit

[c++] error: cast from 'void*' to 'int' loses precision

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