Re: HELP ! need to insert value in array !!

[ok@goanna.cs.rmit.edu.au (Richard A. O'Keefe)]

dewar@merv.cs.nyu.edu (Robert Dewar) writes:

>What would one expect on a PDP10 with 5 characters per word and an unused
>bit left over, or does the ANSI C standard require 36 bits/character on 
>this machine?

UCHAR_MAX must be at least 255.  All objects in C must have a size that is
a multiple of the size of a byte, and since we'd expect 36-bit int and
float on a PDP-11, that means the size of a "byte" has to be a factor of
36 that is greater than or equal to 8.  That means the possible sizes for
a char in a PDP-10 implementation of C are
	 9 bits	(4 per word)
	18 bits (2 per word)
	36 bits (1 per word)
I've actually used the Snyder C compiler for the PDP-10, but I can't
remember what it did.  I'm pretty sure it wasn't 9 bits per char.

Note that 5 ASCII characters per word was a software convention; for C
programs to inter-operate with other code, you'd need to convert, but then
for things like opening files you'd need to convert to _six bit_ characters.

>suppose that type T is a struct with a double field and an int field.
>The size of this struct would be 12 bytes, but its alignment requirement
>is 8, so it would not be possible to laout the array b without alignment
>gaps???

The point is that the size of an _isolated_ variable of type T must be
exactly the same as the size of an _array element_ of type T.  Yes, there
would be padding in your type T, but it would be in _all_ variables of
that type, not just ones in an array.  (You could think of every object in
C being an element of a one-element array, and with respect to pointer
arithmetic, that model would not mislead you.)

-- 
Four policemen playing jazz on an up escalator in the railway station.
Richard A. O'Keefe; http://www.cs.rmit.edu.au/%7Eok; RMIT Comp.Sci.