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Re: malloc and alignment
Re: malloc and alignment
17 Jun 2003 11:25:32 +0900
"Stefan Monnier" <monnier+gnu/address@hidden> writes:
> > You could use `memalign'....
> How standard is it ?
Here's the glibc info page about it. The traditional `portable' version
is exactly the sort of over-allocate-and-use-the-middle type thing you
mentioned. I guess the biggest problem is how to _free_ such blocks --
as noted below, glibc can do so, but BSD can't (who knows how old that
info is, though)...
File: libc.info, Node: Aligned Memory Blocks, Next: Malloc Tunable
Parameters, Prev: Efficiency and Malloc, Up: Unconstrained Allocation
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
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,
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
This function was introduced in POSIX 1003.1d.
- Function: void * valloc (size_t SIZE)
Using `valloc' is like using `memalign' and passing the page size
as the value of the second argument. It is implemented like this:
valloc (size_t size)
return memalign (getpagesize (), size);
*Note Query Memory Parameters:: for more information about the
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