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You can't assume char is one octet. It is one byte by definition.

A byte is CHAR_BIT bits, where CHAR_BIT >= 8. (It's exactly 8 on most implementations; DSPs are the most common exception).

short and int are both required to be at least 16 bits wide. It's possible for int to be 1 byte (sizeof (int) == 1), but only if CHAR_BIT >= 16.



A clarification: You can certainly assume that char is 8 bits if you don't mind losing portability to a small minority of systems.

If I'm being pedantic, I might add something like

    #if CHAR_BIT != 8
    #error "This code assumes 8-bit char"
    #endif
But realistically, if I'm using headers defined by either POSIX or Windows, that's probably enough of a guarantee. (Though I'd still use CHAR_BIT rather than 8 to refer to the number of bits in a byte.)


posix indeed guarantees CHAR_BIT == 8.




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