void * arithmetic

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It is a GCC extension.

In GNU C, addition and subtraction operations are supported on pointers to void and on pointers to functions. This is done by treating the size of a void or of a function as 1.

If you add the -pedantic flag it will produce the warning:

warning: wrong type argument to increment

If you want to abide to the standard, cast the pointer to a char*:

k = 1 + (char*)k;

The standard specifies one cannot perform addition (k+1) on void*, because:

  1. Pointer arithmetic is done by treating k as the pointer to the first element (#0) of an array of void (C99 §6.5.6/7), and k+1 will return element #1 in this “array” (§6.5.6/8).

  2. For this to make sense, we need to consider an array of void. The relevant info for void is (§6.2.5/19)

    The void type comprises an empty set of values; it is an incomplete type that cannot be completed.

  3. However, the definition of array requires the element type cannot be incomplete (§6.2.5/20, footnote 36)

    Since object types do not include incomplete types, an array of incomplete type cannot be constructed.

Hence k+1 cannot be a valid expression.

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