Jens Gustedt's Blog

June 1, 2010

How many bits has a byte?

Filed under: C99, integers, POSIX — Jens Gustedt @ 09:51

I recently stumbled about this seemingly silly question when trying to write a C macro that depends on the width of a type.

So everybody knows the short answer, 8, as is also expressed in the commonly used French word for byte: `octet’. But surprisingly for me the long answer is more complicated than that: it depends on the historical context, the norms that you apply, and even then you have to dig a lot of text to come to it.

C has a definition of the type char, and the language specification basically uses the terms char and  byte interchangeably.

Historically, in times there have been platforms with chars (and thus bytes) that had a width different from 8, in particular some early computers coded printable characters with only 6 bits and had a word size of 36. And later other constructors found it more convenient to have words of 16 bits to be the least addressable unit. C90 didn’t wanted to exclude such platforms and just stated

The number of bits in a char is defined in the macro CHAR_BIT
CHAR_BIT can be any value but must be at least 8

and even C99 still just states:

A byte contains CHAR_BIT bits, and the values of type unsigned char range from 0 to (2^CHAR_BIT) – 1.

But then, on the page for the include file stdint.h it states

The typedef name int N _t designates a signed integer type with width N, no  padding  bits,  and  a  two’s-complement representation. Thus, int8_t denotes a signed integer type with a width of exactly 8 bits.

So far so good, if there is an int8_t we can deduce that sizeof(int8_t) must be 1 and CHAR_BIT must be 8. But then the POSIX standard says

The following types are required:
int8_t
int16_t
int32_t
uint8_t
uint16_t
uint32_t

Which forces CHAR_BIT to be 8, and basically also implies that at least for small width types the representation must be two’s-complement on any POSIX compatible platform.

More reading:

Some forum discussion
The POSIX specification of stdint.h limits.h

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