Question

Simply put in C and variants (unlike that wuss java with its virtual machine) the size of primitive types on different targets can vary greatly, and there is really no guarantee unless you use the fixed width types defined in stdint.h, and even then your implemenation has to support them.

Anyway hypothetically(because on most modern machines a byte is an octet, for networking purposes I assume(ASCII)) does sizeof return the size of a datatype in bytes or in octets?

Answer 1

Answer: sizeof returns the size of the type in bytes.

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Example: sizeof(char) is 100% guaranteed to be 1, but this does not mean, that it’s one octet (8 bits).

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Proved by the standard:

in 6.5.3.4, point 2:

The sizeof operator yields the size (in bytes) of its operand, which may be an
expression or the parenthesized name of a type. The size is determined from the type of
the operand. The result is an integer. If the type of the operand is a variable length array
type, the operand is evaluated; otherwise, the operand is not evaluated and the result is an
integer constant.

…

When applied to an operand that has type char, unsigned char, or signed char,
(or a qualified version thereof) the result is 1. When applied to an operand that has array
type, the result is the total number of bytes in the array) When applied to an operand
that has structure or union type, the result is the total number of bytes in such an object,
including internal and trailing padding.

Also, in Section 3.6, point 3:

A byte is composed of a contiguous sequence of bits, the number of which is implementation-defined