Re: xor

["Randy Brukardt" <randy@rrsoftware.com>]

"Michael Moeller" <mic2@t-online.de> wrote in message 
news:Pine.GSO.4.64.1203272029240.2819@kodiak1...
...
> I have to read in what in C is called an unsigned byte, which in fact is
> a stream of bytes without any interpretation as a symbol. Just a number
> from 0 to 255.
>
> In addition it would be nice to call something like 'fstat' to get the 
> size of file in advance instead of checking every byte for EOF.

Its not (perfectly) safe to read the size of the file first, because someone 
or something might change the file after you make this check and before you 
start reading it. It's more of a problem for long-running programs (like a 
web server), but it is a good habit to get into (otherwise you are running a 
risk of introducing a vulnerability). When it comes to I/O, the old Firesign 
Theater joke really does apply: "Everything you know is wrong!". Sooner or 
later, your I/O *will* fail for some unforseen reason, and your code ought 
to be resilient enough to deal with it. Again, this is more critical in 
programs with a longer life; but it is a lot like washing your hands 
frequently -- it *usually* doesn't matter, but you'll be glad you did it 
when it *does* matter.

Luckily, Ada makes it easy. For most of the I/O packages (Sequential_IO, 
Direct_IO, Text_IO, and language-defined stream attributes), End_Error is 
raised when you reach the end of the file. So all you do is handle End_Error 
in some appropriate location (usually a block that surrounds your reading 
loop) and otherwise you don't have to worry about the end of the file. (I'm 
sure someone here will be happy to show an example.)

If you are using Stream_IO directly, it's a bit messier; you would have to 
check that the number of elements read matches the number requested. Any 
other number means you've reached the end of the file (or socket, or 
whatever).

                                      Randy.