Re: [libvirt] [PATCH] random: don't mix RAND_MAX with random_r

On 08/30/2013 03:36 AM, Daniel P. Berrange wrote: > On Thu, Aug 29, 2013 at 05:19:21PM -0600, Eric Blake wrote: >> FreeBSD 10 recently changed their definition of RAND_MAX, to try >> and cover the fact that their evenly distributed results really are >> a smaller range than a full power of 2. As a result, I did some >> investigation, and learned: >> >> +/* The algorithm of virRandomBits relies on gnulib's guarantee that >> + * random_r() matches the POSIX requirements on random() of being >> + * evenly distributed among exactly [0, 2**31) (that is, we always get >> + * exactly 31 bits). While this happens to be the value of RAND_MAX >> + * on glibc, note that POSIX only requires RAND_MAX to be tied to the >> + * weaker rand(), so there are platforms where RAND_MAX is smaller >> + * than the range of random_r(). For the results to be evenly >> + * distributed among up to 64 bits, we also rely on the period of >> + * random_r() to be at least 2**64, which POSIX only guarantees for >> + * random() if you use 256 bytes of state. */ >> +enum { >> + RANDOM_BITS_PER_ITER = 31, >> + RANDOM_BITS_MASK = (1U << RANDOM_BITS_PER_ITER) - 1, >> +}; > > Using an enum feels a bit wierd for this. Seems like these are > simply 2 constants to #define. Using enums instead of #define makes gdb behave nicer - you can do 'p/x RANDOM_BITS_MASK' and actually get a value, instead of having to dig up the source file and look for the #define. > > ACK whether you change the enum or not. Thanks; pushed after tweaking the comment to not trigger a false negative during 'make syntax-check'. -- Eric Blake eblake redhat com +1-919-301-3266 Libvirt virtualization library http://libvirt.org