Pointer arithmetic for void pointer in C

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 bothoperands shall have arithmetic type,or one operand shall be a pointer toan object type and the other shallhave 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 comprisesan empty set of values;it is an incomplete type that cannotbe 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 alignmentrequirements as a pointer to acharacter type.

However,

The same representation and alignmentrequirements are meant to implyinterchangeability as arguments tofunctions, return values fromfunctions, 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*.

Pointer arithmetic is not allowed on void* pointers.

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