Malloc page than realloc to smaller does not work ? Bastien <blockquote type="cite">Le 25 nov. 2011 14:38, "Eric Blake" <<a href="mailto:eblake@redhat.com">eblake@redhat.com</a>> a écrit : [adding bug-gnulib; replies can drop libvirt] On 11/25/2011 05:51 AM, Paolo Bonzini wrote: >> Indeed; Linux has posix_memalign, and mingw never runs the io helper >> (although it does compile it, hence the #if). If gnulib would give >> us posix_memalign on mingw, we could nuke this #if altogether. > > That's pretty difficult (unless you also add a posix_memalign_free) > because at the time posix_memalign returns you have lost the base > pointer for free(). Providing a posix_memalign_free defeats the purpose - POSIX requires that plain free() will cover the memory returned by posix_memalign. The list of platforms missing posix_memalign is a bit daunting: MacOS X 10.5, FreeBSD 6.0, NetBSD 3.0, OpenBSD 3.8, Minix 3.1.8, AIX 5.1, HP-UX 11, IRIX 6.5, OSF/1 5.1, Solaris 10, Cygwin 1.5.x, mingw, MSVC 9, Interix 3.5, BeOS. but what would be interesting to know is how many of those platforms return page-aligned pointers for any malloc() request of a page or more of memory. That is, we may be able to coerce malloc into aligned results by over-allocating and over-aligning the user's request, if the system malloc() has at least one mode of returning page-aligned memory. Another alternative is to override free() at the same time as providing posix_memalign(). The gnulib malloca module provides a freea() function that can distinguish whether it is paired with an alloca() (nop) or mmalloca() (call the real free()), by over-allocating and probing for a magic header. Similar concepts could be used in writing an rpl_free() that determines whether it was allocated by posix_memalign(): if we guarantee that posix_memalign always returns something aligned to at least a page, then rpl_free() can make a quick check for whether the pointer passed in is page-aligned; if so, do a hash lookup to see if it was a page reserved by our posix_memalign (if so, we know the real address to free); if not or the hash lookup fails, it came from malloc (in which case, regular free should work). Of course, unlike freea() that probes for a magic header, we can't safely probe a header by crossing page boundaries, since that would fault if we didn't allocate the previous page at the same time. -- Eric Blake <a href="mailto:eblake@redhat.com">eblake@redhat.com</a> <a href="tel:%2B1-919-301-3266" value="+19193013266">+1-919-301-3266</a> Libvirt virtualization library <a href="http://libvirt.org" target="_blank">http://libvirt.org</a> </blockquote>