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If a libuuid application is unable to access /dev/random or /dev/urandom then uuid generation by uuid_generate falls back to uuid_generate_time. This could happen in chroot or container environments. The function ul_random_get_bytes from lib/randutils.c uses getrandom if it is available. This could either mean that the libuuid application skips good random bytes because the character special files do not exist or the application trusts in good random bytes just because these files are accessible but not necessarily usable, e.g. limit of open file descriptors reached, lack of data, kernel without getrandom, etc. This commit modifies ul_random_get_bytes to return an integer which indicates if random bytes are of good quality (0) or not (1). Callers can decide based on this information if they want to discard the random bytes. Only libuuid checks the return value. I decided to return 1 instead of -1 because -1 feels more like an error, but weak random bytes can be totally fine. Another issue is that getrandom sets errno to specific values only in case of an error, i.e. with return value -1. Set errno to 0 explicitly if getrandom succeeds so we do not enter the fallback routine for ENOSYS by mistake. I do not think that this is likely to happen, but it really depends on possible wrapper function supplied by a C library. Signed-off-by: Samanta Navarro <ferivoz@riseup.net>
246 lines
5.3 KiB
C
246 lines
5.3 KiB
C
/*
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* SPDX-License-Identifier: BSD-3-Clause
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*
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* General purpose random utilities. Based on libuuid code.
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*
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* This code is free software; you can redistribute it and/or modify it under
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* the terms of the Modified BSD License. The complete text of the license is
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* available in the Documentation/licenses/COPYING.BSD-3-Clause file.
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*/
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#include <stdio.h>
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#include <unistd.h>
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#include <fcntl.h>
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#include <stdlib.h>
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#include <string.h>
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#include <sys/time.h>
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#ifdef __linux__
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#include <sys/syscall.h>
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#endif
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#include "c.h"
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#include "randutils.h"
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#include "nls.h"
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#ifdef HAVE_TLS
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#define THREAD_LOCAL static __thread
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#else
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#define THREAD_LOCAL static
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#endif
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#ifdef HAVE_GETRANDOM
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# include <sys/random.h>
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#elif defined (__linux__)
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# if !defined(SYS_getrandom) && defined(__NR_getrandom)
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/* usable kernel-headers, but old glibc-headers */
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# define SYS_getrandom __NR_getrandom
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# endif
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#endif
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#if !defined(HAVE_GETRANDOM) && defined(SYS_getrandom)
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/* libc without function, but we have syscall */
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#define GRND_NONBLOCK 0x01
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#define GRND_RANDOM 0x02
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static int getrandom(void *buf, size_t buflen, unsigned int flags)
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{
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return (syscall(SYS_getrandom, buf, buflen, flags));
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}
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# define HAVE_GETRANDOM
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#endif
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#if defined(__linux__) && defined(__NR_gettid) && defined(HAVE_JRAND48)
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#define DO_JRAND_MIX
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THREAD_LOCAL unsigned short ul_jrand_seed[3];
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#endif
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int rand_get_number(int low_n, int high_n)
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{
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return rand() % (high_n - low_n + 1) + low_n;
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}
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static void crank_random(void)
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{
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int i;
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struct timeval tv;
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unsigned int n_pid, n_uid;
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gettimeofday(&tv, NULL);
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n_pid = getpid();
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n_uid = getuid();
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srand((n_pid << 16) ^ n_uid ^ tv.tv_sec ^ tv.tv_usec);
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#ifdef DO_JRAND_MIX
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ul_jrand_seed[0] = getpid() ^ (tv.tv_sec & 0xFFFF);
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ul_jrand_seed[1] = getppid() ^ (tv.tv_usec & 0xFFFF);
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ul_jrand_seed[2] = (tv.tv_sec ^ tv.tv_usec) >> 16;
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#endif
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/* Crank the random number generator a few times */
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gettimeofday(&tv, NULL);
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for (i = (tv.tv_sec ^ tv.tv_usec) & 0x1F; i > 0; i--)
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rand();
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}
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int random_get_fd(void)
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{
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int i, fd;
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fd = open("/dev/urandom", O_RDONLY | O_CLOEXEC);
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if (fd == -1)
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fd = open("/dev/random", O_RDONLY | O_NONBLOCK | O_CLOEXEC);
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if (fd >= 0) {
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i = fcntl(fd, F_GETFD);
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if (i >= 0)
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fcntl(fd, F_SETFD, i | FD_CLOEXEC);
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}
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crank_random();
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return fd;
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}
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/*
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* Generate a stream of random nbytes into buf.
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* Use /dev/urandom if possible, and if not,
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* use glibc pseudo-random functions.
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*/
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#define UL_RAND_READ_ATTEMPTS 8
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#define UL_RAND_READ_DELAY 125000 /* microseconds */
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/*
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* Write @nbytes random bytes into @buf.
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*
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* Returns 0 for good quality of random bytes or 1 for weak quality.
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*/
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int ul_random_get_bytes(void *buf, size_t nbytes)
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{
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unsigned char *cp = (unsigned char *)buf;
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size_t i, n = nbytes;
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int lose_counter = 0;
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#ifdef HAVE_GETRANDOM
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while (n > 0) {
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int x;
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errno = 0;
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x = getrandom(cp, n, GRND_NONBLOCK);
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if (x > 0) { /* success */
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n -= x;
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cp += x;
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lose_counter = 0;
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errno = 0;
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} else if (errno == ENOSYS) { /* kernel without getrandom() */
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break;
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} else if (errno == EAGAIN && lose_counter < UL_RAND_READ_ATTEMPTS) {
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xusleep(UL_RAND_READ_DELAY); /* no entropy, wait and try again */
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lose_counter++;
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} else
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break;
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}
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if (errno == ENOSYS)
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#endif
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/*
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* We've been built against headers that support getrandom, but the
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* running kernel does not. Fallback to reading from /dev/{u,}random
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* as before
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*/
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{
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int fd = random_get_fd();
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lose_counter = 0;
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if (fd >= 0) {
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while (n > 0) {
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ssize_t x = read(fd, cp, n);
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if (x <= 0) {
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if (lose_counter++ > UL_RAND_READ_ATTEMPTS)
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break;
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xusleep(UL_RAND_READ_DELAY);
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continue;
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}
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n -= x;
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cp += x;
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lose_counter = 0;
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}
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close(fd);
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}
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}
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/*
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* We do this all the time, but this is the only source of
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* randomness if /dev/random/urandom is out to lunch.
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*/
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crank_random();
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for (cp = buf, i = 0; i < nbytes; i++)
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*cp++ ^= (rand() >> 7) & 0xFF;
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#ifdef DO_JRAND_MIX
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{
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unsigned short tmp_seed[3];
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memcpy(tmp_seed, ul_jrand_seed, sizeof(tmp_seed));
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ul_jrand_seed[2] = ul_jrand_seed[2] ^ syscall(__NR_gettid);
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for (cp = buf, i = 0; i < nbytes; i++)
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*cp++ ^= (jrand48(tmp_seed) >> 7) & 0xFF;
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memcpy(ul_jrand_seed, tmp_seed,
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sizeof(ul_jrand_seed)-sizeof(unsigned short));
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}
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#endif
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return n != 0;
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}
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/*
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* Tell source of randomness.
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*/
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const char *random_tell_source(void)
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{
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#ifdef HAVE_GETRANDOM
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return _("getrandom() function");
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#else
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size_t i;
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static const char *random_sources[] = {
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"/dev/urandom",
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"/dev/random"
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};
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for (i = 0; i < ARRAY_SIZE(random_sources); i++) {
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if (!access(random_sources[i], R_OK))
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return random_sources[i];
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}
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#endif
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return _("libc pseudo-random functions");
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}
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#ifdef TEST_PROGRAM_RANDUTILS
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#include <inttypes.h>
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int main(int argc, char *argv[])
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{
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size_t i, n;
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int64_t *vp, v;
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char *buf;
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size_t bufsz;
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n = argc == 1 ? 16 : atoi(argv[1]);
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printf("Multiple random calls:\n");
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for (i = 0; i < n; i++) {
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ul_random_get_bytes(&v, sizeof(v));
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printf("#%02zu: %25"PRIu64"\n", i, v);
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}
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printf("One random call:\n");
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bufsz = n * sizeof(*vp);
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buf = malloc(bufsz);
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if (!buf)
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err(EXIT_FAILURE, "failed to allocate buffer");
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ul_random_get_bytes(buf, bufsz);
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for (i = 0; i < n; i++) {
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vp = (int64_t *) (buf + (i * sizeof(*vp)));
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printf("#%02zu: %25"PRIu64"\n", i, *vp);
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}
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return EXIT_SUCCESS;
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}
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#endif /* TEST_PROGRAM_RANDUTILS */
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