kreijack@xxxxxxxxx wrote on 2016/05/26 13:08 +0200:
----Messaggio originale----
Da: "Qu Wenruo" <quwenruo@xxxxxxxxxxxxxx>
Data: 26/05/2016 2.38
A: <kreijack@xxxxxxxxx>
Ogg: Re: [PATCH 1/2] btrfs-progs: utils: Introduce new pseudo random API
Goffredo Baroncelli wrote on 2016/05/25 18:15 +0200:
On 2016-05-25 06:14, Qu Wenruo wrote:
+void rand_seed(u64 seed)
+{
+ int i;
+ /* only use the last 48 bits */
+ for (i = 0; i < 3; i++) {
+ seeds[i] = (unsigned short)(seed ^ (unsigned short)(-1));
+ seed >>= 16;
+ }
+ seed_initlized = 1;
+}
+
+u32 rand_u32(void)
+{
+ struct timeval tv;
+
+ if (seed_initlized)
+ return nrand48(seeds);
+
+ /*
+ * It's possible to use /dev/random, but we don't need that true
+ * random number nor want to wait for entropy,
+ * since we're only using random API to do corruption to test.
+ * Time and pid/ppid based seed would be good enough, and won't
+ * cause sleep for entropy pool.
+ */
+ gettimeofday(&tv, 0);
+ seeds[0] = getpid() ^ (tv.tv_sec & 0xFFFF);
+ seeds[1] = getppid() ^ (tv.tv_usec & 0xFFFF);
+ seeds[2] = (tv.tv_sec ^ tv.tv_usec) >> 16;
+ seed_initlized = 1;
+
+ return (u32)nrand48(seeds);
+}
Just for my curiosity, which is the advantage of rand48() respect to rand()
if we utilize only the
lower 32 bit ? It wouldn't be more simple to use:
Of course rand() is my first candidate.
However a quick check on manpage of rand(3) shows, the up limit of
RAND_MAX, is not always the same.
In my system, RAND_MAX is 2^31 -1, while example code from rand(3)
assumes the up limit is 32767.
In glibc RAND_MAX is always ~2^31: http://www.gnu.
org/software/libc/manual/html_node/ISO-Random.html
Thanks for the reference on RAND_MAX is alwasy 2^31 - 1.
However, that's still not enough, for 2^31 - 1 case, the highest(sign)
bit is always 0.
This doesn't fully use the 32 bits. It's OK for u16 and u8, but not u32
or u64.
On the other hand, jrand48() can return signed result, in range [- 2^31,
2^31).
Which fully uses all the 32bits.
Thanks,
Qu
If it is true for glibc, it should be true also for the others linux libc
(dietlibc[1], musl[2]); I think that assuming in linux RAND_MAX = 2^31-1
should be safe.
BR
Goffredo
[1] https://github.com/ensc/dietlibc/blob/master/include/stdlib.h#L101
[2] https://github.com/cloudius-systems/musl/blob/master/include/stdlib.h#L81
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