Mail Archives: djgpp-workers/2003/06/24/02:47:38
Tim Van Holder wrote:
>
... snip ...
>
> Below is what the glibc 2.3.2 (Red Hat 8.0) has to say; note that it
> does not say what it does if BOUNDARY is NOT a power of 2.
> I could check the glibc sources if necessary.
>
> Allocating Aligned Memory Blocks
> ................................
>
> The address of a block returned by `malloc' or `realloc' in the GNU
> system is always a multiple of eight (or sixteen on 64-bit systems).
> If you need a block whose address is a multiple of a higher power of
> two than that, use `memalign', `posix_memalign', or `valloc'.
> `memalign' is declared in `malloc.h' and `posix_memalign' is declared
> in `stdlib.h'.
>
> With the GNU library, you can use `free' to free the blocks that
> `memalign', `posix_memalign', and `valloc' return. That does not work
> in BSD, however--BSD does not provide any way to free such blocks.
>
> - Function: void * memalign (size_t BOUNDARY, size_t SIZE)
> The `memalign' function allocates a block of SIZE bytes whose
> address is a multiple of BOUNDARY. The BOUNDARY must be a power
> of two! The function `memalign' works by allocating a somewhat
> larger block, and then returning an address within the block that
> is on the specified boundary.
>
> - Function: int posix_memalign (void **MEMPTR, size_t ALIGNMENT,
> size_t SIZE)
> The `posix_memalign' function is similar to the `memalign'
> function in that it returns a buffer of SIZE bytes aligned to a
> multiple of ALIGNMENT. But it adds one requirement to the
> parameter ALIGNMENT: the value must be a power of two multiple of
> `sizeof (void *)'.
>
> If the function succeeds in allocation memory a pointer to the
> allocated memory is returned in `*MEMPTR' and the return value is
> zero. Otherwise the function returns an error value indicating
> the problem.
>
> This function was introduced in POSIX 1003.1d.
Those don't agree with the memalign prototype I was fed, and
frankly I think they are seriously flawed. For reference:
void *memalign(size_t amount, size_t alignment);
which is logically equivalent to malloc, except it places
constraints on the returned pointer and requires alignment to be a
power of 2. If the returned pointer is later realloced the
alignment restrictions may disappear.
--
Chuck F (cbfalconer AT yahoo DOT com) (cbfalconer AT worldnet DOT att DOT net)
Available for consulting/temporary embedded and systems.
<http://cbfalconer.home.att.net> USE worldnet address!
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