libvirt/src/util/virrandom.c

/*
 * Copyright (C) 2012 Red Hat, Inc.
 *
 * This library is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public
 * License as published by the Free Software Foundation; either
 * version 2.1 of the License, or (at your option) any later version.
 *
 * This library is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with this library; if not, write to the Free Software
 * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307  USA
 *
 * Authors:
 *     Daniel P. Berrange <berrange@redhat.com>
 */

#include <config.h>

#include <stdlib.h>

#include "virrandom.h"
#include "threads.h"
#include "count-one-bits.h"

static char randomState[128];
static struct random_data randomData;
static virMutex randomLock;


int virRandomInitialize(uint32_t seed)
{
    if (virMutexInit(&randomLock) < 0)
        return -1;

    if (initstate_r(seed,
                    randomState,
                    sizeof(randomState),
                    &randomData) < 0)
        return -1;

    return 0;
}

/*
 * virRandomBits:
 * @nbits: Number of bits of randommess required
 *
 * Generate an evenly distributed random number between [0,2^nbits), where
 * @nbits must be in the range (0,64].
 *
 * Return: a random number with @nbits entropy
 */
uint64_t virRandomBits(int nbits)
{
    int bits_per_iter = count_one_bits(RAND_MAX);
    uint64_t ret = 0;
    int32_t bits;

    /* This algorithm requires that RAND_MAX == 2^n-1 for some n;
       gnulib's random_r meets this property. */
    verify(((RAND_MAX + 1U) & RAND_MAX) == 0);

    virMutexLock(&randomLock);

    while (nbits > bits_per_iter) {
        random_r(&randomData, &bits);
        ret = (ret << bits_per_iter) | (bits & RAND_MAX);
        nbits -= bits_per_iter;
    }

    random_r(&randomData, &bits);
    ret = (ret << nbits) | (bits & ((1 << nbits) - 1));

    virMutexUnlock(&randomLock);
    return ret;
}