Re: Differences between one-dimensional arrays in Java and C

Tim Rentsch wrote: 

Now here's an interesting question.  I believe it's possible to
construct a 2D array (with both dimensions non-constant) by calling
malloc only once.  For example:

> [allocate space for elements, padding, and pointers] 

Since the memory returned by malloc is suitably aligned for all types,
both 'elements' and 'array' are suitably aligned for objects of their
respective types.  Note that 'extra_space' is calculated so that
'element_space + extra_space' is a multiple of 'pointer_size'.

Each of the two areas of memory (at 'elements' and 'array') is used
only to store/retrieve objects of their respective types.

According to my best understanding the code above should work in
standard C.  But I put it as a question - does anyone have an argument
to the contrary?


    Looks all right.  A type's alignment requirement must be
a divisor of its size (otherwise arrays wouldn't work), so the
`extra_space' will be enough padding to get the pointers to
align properly.  In fact, `extra_space' may be more than is
needed; if you really want to be parsimonious you can use a
dodge that someone posted here a year or two ago:

	#include <stddef.h>
	#define alignof(T) offsetof(struct {char c; T t;}, t)

    Manually-inserted padding can also be used to write portable
versions of "the struct hack" for C90 implementations (C99
introduced an easier notation).

--
Eric Sosman
esosman#acm-dot-org.invalid
.

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