(libc.info.gz) Aligned Memory Blocks

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 3.2.2.7 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.
 
  -- 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:
 
           void *
           valloc (size_t size)
           {
             return memalign (getpagesize (), size);
           }
 
       Query Memory Parameters for more information about the
      memory subsystem.
 

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