Sizes of Char and Stream_Element is always equal?

General
1

Hi!

? Interfaces.C.char’Size always equal to Ada.Stream.Stream_element’Size ?

If they are not the same, what are the most common sizes?

Thanks!

2

RM 13.13 says Stream_Element'Size is implementation defined. On all systems GCC 13/GNAT currently supports:

Ada.Streams line 42

type Stream_Element is mod 2 ** Standard'Storage_Unit;

Code search “Storage_Unit : constant”

Storage_Unit : constant := 8;

C’s char is a fuzzier concept, with no cross-platform size definition. GCC has several compile-time macros and flags that can alter the definition of char, for example -fsigned-char. As far as I can tell, none of those switches change the size to anything other than 8 bits.

GNAT defines char as:
Interfaces.C line 105

type char is new Character;

RM 3.5.2 Character Types limits Character to the 256 Latin-1 characters, but does not guarantee that the machine representation fits inside 8 bits.

However, GNAT uses the same internal representation as GCC’s C char for Character, so in practice it’s always 8 bits.

Other compilers/toolchains may do different things, maybe someone with a PTC Ada license can check the behavior there.

3

Hi!

Very Very Very thanks!

BestWishes,
GrateFull, Dani.

4

The absolutely correct answer is no.

The reasonable answer is yes.

Back in the late 1970’s when Ada was created, H/W had not yet standardized on the byte. DEC machines around that time had 36-bit words, for example. The ARM was therefore written to be H/W independent, and this has been maintained, so things like Storage_Unit and Stream_Element are implementation-defined. But now the byte is pretty much a standard, so unless you’re using unusual H/W, these things are all bytes.

The definition of things in Interfaces.C depend on the C compiler that the Ada compiler writer is interfacing to, but again, it’s rare for char to not be a byte.

1 Like
5

Hi!

Very Very Very Thanks!

BestWishes,
Gratefull, Dani.

6

there were also 7-bit bytes back then. Wasn’t there some other weird <8-bit and >8-bit (but <16) bytes too?

7

There are still some DSPs without byte addressing, where the smallest addressable memory datum is a “word”, for example 32 or 48 bits in size. A C compiler for such a machine is likely to use that size (32 or 48 bits) for char – I know of examples. An Ada compiler would likely make the same choice for Storage_Unit, but could have a smaller Stream_Element, at some cost in the size and speed of Streams code.