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dd3688e4d1
FreeBSD 10 recently changed their definition of RAND_MAX, to try and cover the fact that their evenly distributed results of rand() really are a smaller range than a full power of 2. As a result, I did some investigation, and learned: 1. POSIX requires random() to be evenly distributed across exactly 31 bits. glibc also guarantees this for rand(), but the two are unrelated, and POSIX only associates RAND_MAX with rand(). Avoiding RAND_MAX altogether thus avoids a build failure on FreeBSD 10. 2. Concatenating random bits from a PRNG will NOT provide uniform coverage over the larger value UNLESS the period of the original PRNG is at least as large as the number of bits being concatenated. Simple example: suppose that RAND_MAX were 1 with a period of 2**1 (which means that the PRNG merely alternates between 0 and 1). Concatenating two successive rand() calls would then invariably result in 01 or 10, which is a rather non-uniform distribution (00 and 11 are impossible) and an even worse period (2**0, since our second attempt will get the same number as our first attempt). But a RAND_MAX of 1 with a period of 2**2 (alternating between 0, 1, 1, 0) provides sane coverage of all four values, if properly tempered. (Back-to-back calls would still only see half the values if we don't do some tempering). We therefore want to guarantee a period of at least 2**64, preferably larger (as a tempering factor); POSIX only makes this guarantee for random() with 256 bytes of info. * src/util/virrandom.c (virRandomBits): Use constants that are accurate for the PRNG we are using, not an unrelated PRNG. (randomState): Ensure the period of our PRNG exceeds our usage. Signed-off-by: Eric Blake <eblake@redhat.com> |
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LibVirt : simple API for virtualization
Libvirt is a C toolkit to interact with the virtualization capabilities
of recent versions of Linux (and other OSes). It is free software
available under the GNU Lesser General Public License. Virtualization of
the Linux Operating System means the ability to run multiple instances of
Operating Systems concurrently on a single hardware system where the basic
resources are driven by a Linux instance. The library aim at providing
long term stable C API initially for the Xen paravirtualization but
should be able to integrate other virtualization mechanisms if needed.
Daniel Veillard <veillard@redhat.com>