Question

How can one portably perform pointer arithmetic with single byte precision?

Keep in mind that:

• char is not 1 byte on all platforms
• sizeof(void) == 1 is only available as an extension in GCC
• While some platforms may have pointer deref pointer alignment restrictions, arithmetic may still require a finer granularity than the size of the smallest fundamental POD type

Answer 1

Your assumption is flawed – sizeof(char) is defined to be 1 everywhere.

From the C99 standard (TC3), in section 6.5.3.4 (“The sizeof operator”):

(paragraph 2)

The sizeof operator yields the size
(in bytes) of its operand, which may
be an expression or the
parenthesized name of a type.

(paragraph 3)

When applied to an operand that has
type char, unsigned char, or signed
char, (or a qualified version
thereof) the result is 1.

When these are taken together, it becomes clear that in C, whatever size a char is, that size is a “byte” (even if that’s more than 8 bits, on some given platform).

A char is therefore the smallest addressable type. If you need to address in units smaller than a char, your only choice is to read a char at a time and use bitwise operators to mask out the parts of the char that you want.