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