Re: Stream_IO

[Stephen Leake <Stephen.Leake@gsfc.nasa.gov>]

Tom Moran wrote:
> 
> Is there a way to use Stream_IO to read a file produced by other than
> the program doing the reading, in particular, a file binary file
> produced, say, with C?  The RM indicates the representation is
> implementation defined, and 'read apparently quite ignores rep
> clauses, so I find one compiler reads integer things of 'size 8 or 16
> as 16, and another reads size 16 things as size 32.  I hate to read
> into a String and then do Unchecked_Conversions or 'address overlays,
> but I don't see any other portable approach.  Help!

I went around on this with Tucker Taft last fall, and the answer is: in
general, you have to do unchecked conversion, and you have to live with
the compiler differences.

To make this (slightly) easier to swallow, consider trying to read a 12
bit integer from a stream. Since streams are defined in terms of memory
words, if you are using an 8 or 16 bit word architecture, you will have
to do some conversion to get 12 bits.

The fact that some compilers read more than 8 bits for 8 bit integers
from 8 bit streams is a consequence of the rules about base integer
types; if the compiler chooses a 32 bit integer base type for an 8 bit
integer type, it MUST read and write 32 bits from/to streams. This is an
area that would benefit from uniformity guidelines, but it is not a
simple issue, since it trades stream uniformity for arithmetic
efficiency.

-- 
- Stephe