zircon/kernel/lib/libc/rand.c

// Copyright 2016 The Fuchsia Authors
// Copyright (c) 2008-2014 Travis Geiselbrecht
//
// Use of this source code is governed by a MIT-style
// license that can be found in the LICENSE file or at
// https://opensource.org/licenses/MIT

#include <rand.h>
#include <sys/types.h>

#include <kernel/atomic.h>

static uint64_t randseed;

void srand(unsigned int seed) { atomic_store_u64_relaxed(&randseed, seed - 1); }

int rand_r(uint64_t* seed) {
  for (;;) {
    uint64_t old_seed = atomic_load_u64_relaxed(seed);
    uint64_t new_seed = 6364136223846793005ULL * old_seed + 1;
    if (atomic_cmpxchg_u64_relaxed(seed, &old_seed, new_seed)) {
      return new_seed >> 33;
    }
  }
}

int rand(void) { return rand_r(&randseed); }