ax_check_page_aligned_malloc

Some operating systems (generally, BSD Unix variants) lack a posix_memalign function, a memalign function, and a working (meaning, the memory can be freed) valloc function. To make up for it, the malloc function promises to return page-aligned addresses if more than one page's worth of memory is allocated. AX_CHECK_PAGE_ALIGNED_MALLOC checks for this condition and defines HAVE_PAGE_ALIGNED_MALLOC if the condition holds.

As an aside, note that valloc'd memory cannot safely be freed on all operating systems. (Again, some flavors of BSD are the troublemakers.) It's best to avoid using valloc in favor of posix_memalign, memalign, or an aligned malloc as detected by AX_CHECK_PAGE_ALIGNED_MALLOC.

Caveat: AX_CHECK_PAGE_ALIGNED_MALLOC takes a probabalistic approach. If 100 calls to malloc all return page-aligned addresses, it assumes that all calls will behave likewise. It is therefore possible -- albeit extremely unlikely -- that AX_CHECK_PAGE_ALIGNED_MALLOC can return a false positive.

AC_DEFUN([AX_CHECK_PAGE_ALIGNED_MALLOC],
[AC_CACHE_CHECK([if large mallocs guarantee page-alignment],
  [ax_cv_func_malloc_aligned],
  [AC_TRY_RUN([
#include <stdio.h>
#include <stdlib.h>
#if HAVE_UNISTD_H
# include <unistd.h>
#endif

int main()
{
  int pagesize = getpagesize();
  int i;

  for (i=0; i<100; i++)
    if ((unsigned long)malloc(pagesize+1) & (pagesize-1))
      exit (1);
  exit (0);
}
              ],
     [ax_cv_func_malloc_aligned=yes],
     [ax_cv_func_malloc_aligned=no],
     [ax_cv_func_malloc_aligned=no])
  ])
if test "$ax_cv_func_malloc_aligned" = yes ; then
  AC_DEFINE([HAVE_PAGE_ALIGNED_MALLOC], [1],
    [Define if `malloc'ing more than one page always returns a page-aligned address.])
fi
])

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