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# =================================================================================
# https://www.gnu.org/software/autoconf-archive/ax_check_page_aligned_malloc.html
# =================================================================================
#
# SYNOPSIS
#
# AX_CHECK_PAGE_ALIGNED_MALLOC
#
# DESCRIPTION
#
# 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 probabilistic 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.
#
# LICENSE
#
# Copyright (c) 2008 Scott Pakin <pakin@uiuc.edu>
#
# Copying and distribution of this file, with or without modification, are
# permitted in any medium without royalty provided the copyright notice
# and this notice are preserved. This file is offered as-is, without any
# warranty.
#serial 11
AC_DEFUN([AX_CHECK_PAGE_ALIGNED_MALLOC],
[AC_CACHE_CHECK([if large mallocs guarantee page-alignment],
[ax_cv_func_malloc_aligned],
[AC_RUN_IFELSE([AC_LANG_SOURCE([[
#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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