Reading a recent junk-thread, I realized the algorithms for rand
and random have changed numerous times since the late 80's.
As such you cannot trust that a seed given 20 years ago, produces the
same sequence today.
I did a rough study of the commits to our versions. Other operating
systems changed their algorithms at different times. I think rand and
random algorithm changed about 5 times (combined). But wait there is
MORE! These functions mix "long" and "int", and rely upon wraparound,
so 32-bit vs 64-bit behaviour occur.
I'm wondering if the following wording will disuade people from playing
this stupid deterministic game, or arguing that these specific functions
serve such a purpose. I suppose the two lessons are:
- if you need determinism, write your own.
- do not rely upon an external function which will make your seed
produce a different result approximately every 8 years.
People keep talking about the value of determinism, and in particular for
these specific function names, so I'm hoping to put the argument to rest.
I stopped short by avoiding the phrase "undefined behaviour".
Index: stdlib/rand.3
===================================================================
RCS file: /cvs/src/lib/libc/stdlib/rand.3,v
retrieving revision 1.19
diff -u -p -u -r1.19 rand.3
--- stdlib/rand.3 9 Dec 2014 21:55:39 -0000 1.19
+++ stdlib/rand.3 12 Feb 2021 05:36:52 -0000
@@ -78,6 +78,9 @@ can be substituted for
then subsequent
.Fn rand
calls will return results using the deterministic algorithm.
+The deterministic sequence algorithm changed a number of times since
+original development, is underspecified, and should not be relied up to
+remain consistant between platforms and over time.
.Pp
The
.Fn rand
Index: stdlib/random.3
===================================================================
RCS file: /cvs/src/lib/libc/stdlib/random.3,v
retrieving revision 1.28
diff -u -p -u -r1.28 random.3
--- stdlib/random.3 9 Dec 2014 21:55:39 -0000 1.28
+++ stdlib/random.3 12 Feb 2021 05:36:52 -0000
@@ -96,6 +96,9 @@ a default table of size 31 long integers
numbers in the range from 0 to (2**31)\-1.
The period of this random number generator is very large, approximately
16*((2**31)\-1), but the results are a deterministic sequence from the seed.
+The deterministic sequence algorithm changed a number of times since
+original development, is underspecified, and should not be relied up to
+remain consistant between platforms and over time.
.Pp
The
.Fn initstate
