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Autoconf Macro: ax_check_page_aligned_malloc
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<h1>
ax_check_page_aligned_malloc
</h1>
<h2>
Synopsis
</h2>
<p class="indent" style="white-space:nowrap;">
<code>AX_CHECK_PAGE_ALIGNED_MALLOC</code>
</p>
<h2>
Description
</h2>
<div class="indent">
<p>
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.
</p>
<p>
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.
</p>
<p>
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.
</p>
</div>
<h2>
Author
</h2>
<p class="indent">
Scott Pakin <pakin@uiuc.edu>
</p>
<h2>
Last Modified
</h2>
<p class="indent">
2005-01-22
</p>
<h2>
M4 Source Code
</h2>
<div class="indent">
<pre class="m4source">
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
])
</pre>
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