Pointer arithmetic for void pointer in C

Question

When a pointer to a particular type  say int  char  float      is incremented  its value is increased by the size of that data type  If a void pointer which points to data of size x is incremented  how does it get to point x bytes ahead  How does the compiler know to add x to value of the pointer

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You have to cast it to another type of pointer before doing pointer arithmetic

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Void pointers can point to any memory chunk  Hence the compiler does not know how many bytes to increment decrement when we attempt pointer arithmetic on a void pointer  Therefore void pointers must be first typecast to a known type before they can be involved in any pointer arithmetic   void  p   malloc sizeof char  10   p      compiler does how many where to pint the pointer after this increment operation  char   c    char   p  c        compiler will increment the c by 1  since size of char is 1 byte

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cast it to a char pointer an increment your pointer forward x bytes ahead

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Final conclusion  arithmetic on a void  is illegal in both C and C     GCC allows it as an extension  see Arithmetic on void- and Function-Pointers  note that this section is part of the  C Extensions  chapter of the manual    Clang and ICC likely allow void  arithmetic for the purposes of compatibility with GCC   Other compilers  such as MSVC  disallow arithmetic on void   and GCC disallows it if the -pedantic-errors flag is specified  or if the -Werror-pointer-arith flag is specified  this flag is useful if your code base must also compile with MSVC    The C Standard Speaks  Quotes are taken from the n1256 draft   The standard s description of the addition operation states      6 5 6-2  For addition  either both   operands shall have arithmetic type    or one operand shall be a pointer to   an object type and the other shall   have integer type    So  the question here is whether void  is a pointer to an  object type   or equivalently  whether void is an  object type    The definition for  object type  is      6 2 5 1  Types are partitioned into object types  types that fully describe objects    function types  types that describe functions   and incomplete types  types that describe objects but lack information needed to determine their sizes     And the standard defines void as      6 2 5-19  The void type comprises   an empty set of values    it is an incomplete type that cannot   be completed    Since void is an incomplete type  it is not an object type   Therefore it is not a valid operand to an addition operation   Therefore you cannot perform pointer arithmetic on a void pointer   Notes  Originally  it was thought that void  arithmetic was permitted  because of these sections of the C standard      6 2 5-27  A pointer to void shall have the  same representation and alignment   requirements as a pointer to a   character type    However      The same representation and alignment   requirements are meant to imply   interchangeability as arguments to   functions  return values from   functions  and members of unions    So this means that printf   s   x  has the same meaning whether x has type char  or void   but it does not mean that you can do arithmetic on a void    Editor s note  This answer has been edited to reflect the final conclusion

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Pointer arithmetic is not allowed on void  pointers

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The C standard does not allow void pointer arithmetic  However  GNU C is allowed by considering the size of void is 1   C11 standard   6 2 5  Paragraph - 19     The void type comprises an empty set of values  it is an incomplete   object type that cannot be completed    Following program is working fine in GCC compiler    include lt stdio h gt   int main         int arr 2     1  2       void  ptr    amp arr      ptr   ptr   sizeof int       printf   d n     int   ptr       return 0      May be other compilers generate an error

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Compiler knows by type cast  Given a void  x    x 1 adds one byte to x  pointer goes to byte x 1  int  x 1 adds sizeof int  bytes  pointer goes to byte x   sizeof int   float  x 1 addres sizeof float  bytes   etc    Althought the first item is not portable and is against the Galateo of C C    it is nevertheless C-language-correct  meaning it will compile to something on most compilers possibly necessitating an appropriate flag  like -Wpointer-arith

User · Answer

You can t do pointer arithmetic on void   types  for exactly this reason

[c] Pointer arithmetic for void pointer in C

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