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  time "Bill Cunningham" <nospam@nspam.invalid> - 2012-11-19 19:11 -0500
    Re: time Geoff <geoff@invalid.invalid> - 2012-11-19 16:26 -0800
      Re: time Keith Thompson <kst-u@mib.org> - 2012-11-19 16:51 -0800
        Re: time Geoff <geoff@invalid.invalid> - 2012-11-19 17:01 -0800
        Re: time "Bill Cunningham" <nospam@nspam.invalid> - 2012-11-19 20:07 -0500
          Re: time Keith Thompson <kst-u@mib.org> - 2012-11-19 23:54 -0800
            Re: time pete <pfiland@mindspring.com> - 2012-11-20 07:31 -0500
            Re: time "Bill Cunningham" <nospam@nspam.invalid> - 2012-11-20 17:09 -0500
    Re: time Keith Thompson <kst-u@mib.org> - 2012-11-19 16:30 -0800
      Re: time "Bill Cunningham" <nospam@nspam.invalid> - 2012-11-19 20:04 -0500
        Re: time Nick Keighley <nick_keighley_nospam@hotmail.com> - 2012-11-25 05:52 -0800
    Re: time Nobody <nobody@nowhere.com> - 2012-11-20 03:07 +0000
      Re: time Keith Thompson <kst-u@mib.org> - 2012-11-19 23:58 -0800
      Re: time Anand Hariharan <mailto.anand.hariharan@gmail.com> - 2012-11-21 08:30 -0800
        Re: time "osmium" <r124c4u102@comcast.net> - 2012-11-21 11:11 -0600
        Re: time James Kuyper <jameskuyper@verizon.net> - 2012-11-21 12:22 -0500
          Re: time gof@somewhere.invalid (Adam Wysocki) - 2012-11-21 19:09 +0000
            Re: time Keith Thompson <kst-u@mib.org> - 2012-11-21 12:01 -0800
              Re: time gof@somewhere.invalid (Adam Wysocki) - 2012-11-23 08:15 +0000
            Re: time "Bill Cunningham" <nospam@nspam.invalid> - 2012-11-21 15:16 -0500
              Re: time "osmium" <r124c4u102@comcast.net> - 2012-11-21 14:23 -0600
                Re: time Rosario1903 <Rosario@invalid.invalid> - 2012-11-22 10:25 +0100
              Re: time Keith Thompson <kst-u@mib.org> - 2012-11-21 12:30 -0800
                Re: time "Bill Cunningham" <nospam@nspam.invalid> - 2012-11-21 18:24 -0500
              Re: time Nick Keighley <nick_keighley_nospam@hotmail.com> - 2012-11-25 05:56 -0800
            Re: time Nobody <nobody@nowhere.com> - 2012-11-22 02:00 +0000
              Re: time Keith Thompson <kst-u@mib.org> - 2012-11-21 19:57 -0800
                Re: time gof@somewhere.invalid (Adam Wysocki) - 2012-11-23 08:31 +0000
              Re: time army1987 <army1987@ask-for-it.invalid> - 2012-11-24 13:58 +0000
            Re: time Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-11-22 11:36 +0000
              Re: time gof@somewhere.invalid (Adam Wysocki) - 2012-11-23 08:20 +0000
                Re: time Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-11-23 14:31 +0000
        Re: time Nick Keighley <nick_keighley_nospam@hotmail.com> - 2012-11-25 05:54 -0800
    Re: time Mark Storkamp <mstorkamp@yahoo.com> - 2012-11-20 08:34 -0600
      Re: time "Bill Cunningham" <nospam@nspam.invalid> - 2012-11-20 20:37 -0500
        Re: time Keith Thompson <kst-u@mib.org> - 2012-11-20 20:25 -0800
          Re: time "Bill Cunningham" <nospam@nspam.invalid> - 2012-11-21 15:11 -0500

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#28542

"Bill Cunningham" wrote:

> btw to  stay on-topic here that caret and those << indicators. I've never 
> seen that in C except maybe headers. Are they preprocessing tokens?

K&R has one of the best indexes I have ever seen in a technical book.  It 
starts on p 263.  But the whole point of writing a book is for people to 
*read* it.  Try it some time. 

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#28559

On Wed, 21 Nov 2012 14:23:26 -0600, "osmium" wrote:

>K&R has one of the best indexes I have ever seen in a technical book.  

yes

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#28543

"Bill Cunningham" <nospam@nspam.invalid> writes:
> Adam Wysocki wrote:
>> Maybe adding process ID to the seed calculation would do the job.
>>
>> Instead of:
>>
>> srand(time(NULL))
>>
>> Use:
>>
>> srand(time(NULL) ^ getpid())
>>
>> Or even:
>>
>> srand(time(NULL) ^ (getpid() << 16))
>
>     Is that really that random? As long as a process has fork() 'ed and the 
> process is running it's pid isn't going to change is it until the system 
> kills it.

You only call srand() once during the execution of the program.

>          btw to  stay on-topic here that caret and those << indicators. I've 
> never seen that in C except maybe headers. Are they preprocessing tokens?

^ is the exclusive-or operator.  << is the left-shift operator.
You'll find both in the index of the C standard and of K&R, which
you could have checked before wasting everyone's time by asking
about it here.

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
    Will write code for food.
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"

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#28548

Keith Thompson wrote:
> "Bill Cunningham" <nospam@nspam.invalid> writes:
>> Adam Wysocki wrote:
>>> Maybe adding process ID to the seed calculation would do the job.
>>>
>>> Instead of:
>>>
>>> srand(time(NULL))
>>>
>>> Use:
>>>
>>> srand(time(NULL) ^ getpid())
>>>
>>> Or even:
>>>
>>> srand(time(NULL) ^ (getpid() << 16))
>>
>>     Is that really that random? As long as a process has fork() 'ed
>> and the process is running it's pid isn't going to change is it
>> until the system kills it.
>
> You only call srand() once during the execution of the program.
>
>>          btw to  stay on-topic here that caret and those <<
>> indicators. I've never seen that in C except maybe headers. Are they
>> preprocessing tokens?
>
> ^ is the exclusive-or operator.  << is the left-shift operator.
> You'll find both in the index of the C standard and of K&R, which
> you could have checked before wasting everyone's time by asking
> about it here.

    Oh yes that's right. I was thinking shells and posix in this discussion 
about arc4random. The bit-wise operators ok I know what they are. Fell 
asleep for a minute.

Bill

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#28644

On Nov 21, 8:16 pm, "Bill Cunningham" <nos...@nspam.invalid> wrote:
> Adam Wysocki wrote:
> > Maybe adding process ID to the seed calculation would do the job.
>
> > Instead of:
>
> > srand(time(NULL))
>
> > Use:
>
> > srand(time(NULL) ^ getpid())
>
> > Or even:
>
> > srand(time(NULL) ^ (getpid() << 16))
>
>     Is that really that random? As long as a process has fork() 'ed and the
> process is running it's pid isn't going to change is it until the system
> kills it.btw to  stay on-topic here that caret and those << indicators. I've
> never seen that in C except maybe headers. Are they preprocessing tokens?

good grief. RTFM!

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#28554

On Wed, 21 Nov 2012 19:09:18 +0000, Adam Wysocki wrote:

>> If srand(time(NULL)) were used in that program, it would generate the
>> same exact sequence of random numbers for all requests received during
>> the same second.
> 
> Maybe adding process ID to the seed calculation would do the job.

It would, and that's common. But there's no portable way to read the PID
(if such a concept even exists on a given platform).

The main issue is that time() appears to be about the only portable source
of non-determinism available, and it's typically rather weak.

On Wed, 21 Nov 2012 12:01:46 -0800, Keith Thompson wrote:

> If it's worth going to that much trouble, it's probably worth using a
> better random number generator than rand() (one provided by the
> implementation but not specified by the C standard).

But that doesn't do anything about the lack of non-determinism (entropy).
A better PRNG may give you better random numbers, but without something
other than time(), they'll be exactly the same random numbers as any
other invocation which gets the same value from time().

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#28556

Nobody <nobody@nowhere.com> writes:
> On Wed, 21 Nov 2012 19:09:18 +0000, Adam Wysocki wrote:
[...]
> On Wed, 21 Nov 2012 12:01:46 -0800, Keith Thompson wrote:
>> If it's worth going to that much trouble, it's probably worth using a
>> better random number generator than rand() (one provided by the
>> implementation but not specified by the C standard).
>
> But that doesn't do anything about the lack of non-determinism (entropy).
> A better PRNG may give you better random numbers, but without something
> other than time(), they'll be exactly the same random numbers as any
> other invocation which gets the same value from time().

Some PRNGs are self-seeding.  For example, if you can read
data from /dev/random, you'll get numbers that are pretty much
cryptographically secure (and /dev/urandom is nearly as good and
much faster).

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
    Will write code for food.
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"

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#28586

Keith Thompson <kst-u@mib.org> wrote:

> Some PRNGs are self-seeding.  For example, if you can read
> data from /dev/random, you'll get numbers that are pretty much
> cryptographically secure (and /dev/urandom is nearly as good and
> much faster).

Remember that /dev/random can quickly run out of numbers when entropy 
pool gets empty.

AW

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#28604

On Thu, 22 Nov 2012 02:00:19 +0000, Nobody wrote:

> On Wed, 21 Nov 2012 19:09:18 +0000, Adam Wysocki wrote:
> 
>>> If srand(time(NULL)) were used in that program, it would generate the
>>> same exact sequence of random numbers for all requests received during
>>> the same second.

[...]

> The main issue is that time() appears to be about the only portable
> source of non-determinism available, and it's typically rather weak.

Well, there's tmpnam()...



-- 
[  T  H  I  S     S  P  A  C  E     I  S     F  O  R     R  E  N  T  ]
Troppo poca cultura ci rende ignoranti, troppa ci rende folli.
  -- fathermckenzie di it.cultura.linguistica.italiano
<http://xkcd.com/397/>

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#28563

gof@somewhere.invalid (Adam Wysocki) writes:

> James Kuyper <jameskuyper@verizon.net> wrote:
>
>> If srand(time(NULL)) were used in that program, it would generate 
>> the same exact sequence of random numbers for all requests received 
>> during the same second.
>
> Maybe adding process ID to the seed calculation would do the job.
>
> Instead of:
>
> srand(time(NULL))
>
> Use:
>
> srand(time(NULL) ^ getpid())
>
> Or even:
>
> srand(time(NULL) ^ (getpid() << 16))

There a small thing you can do that might help on some (possibly
theoretical) systems:

  srand(time(NULL) * 1000ULL);

It's largely harmless on most systems since all you normally want is
something that varies using the least significant bits of the time and
1000 times that is as good anything else, but on a system with a
floating point time_t you get more of the variation.

(The use of ULL is just to make it unlikely that the multiplication will
overflow in the case of a signed integer time_t.)

-- 
Ben.

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#28585

Ben Bacarisse <ben.usenet@bsb.me.uk> wrote:

> but on a system with a floating point time_t you get more of the 
> variation.

POSIX says that "time_t and clock_t shall be integer or real-floating 
types", so it is possible, but can you give an example of such system? 
I've never seen one.

AW

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#28592

gof@somewhere.invalid (Adam Wysocki) writes:

> Ben Bacarisse <ben.usenet@bsb.me.uk> wrote:
>
>> but on a system with a floating point time_t you get more of the 
>> variation.
>
> POSIX says that "time_t and clock_t shall be integer or real-floating 
> types", so it is possible, but can you give an example of such system?

No, hence my "possibly theoretical" remark.  Maybe it should have been
"probably theoretical".  I see that there was a proposal to The Open
Group to amend the its standard (AKA POSIX) to rule out all floating
type for time_t.  I don't know what happened to it -- I found the
document tracking system rather confusing.

> I've never seen one.

Nor I.

-- 
Ben.

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#28643

On Nov 21, 4:30 pm, Anand Hariharan <mailto.anand.hariha...@gmail.com>
wrote:
> On Nov 19, 9:07 pm, Nobody <nob...@nowhere.com> wrote:
>
>
>
> > The simple solution is srand(time(NULL)). However, time() only has a one
> > second resolution, so consecutive invocations of the program may produce
> > identical results.
>
> Given an application for which, the standard rand family of functions
> is sufficient, why would one want to make consecutive invocations to
> srand?

I ran into trouble once when I started several identical executables
at once. Thet all got the same "random" seed.

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#28471

In article <k8ehrs$ddf$1@dont-email.me>,
 "Bill Cunningham" <nospam@nspam.invalid> wrote:

>     I am using the time and date in my code using asctime () time(), and 
> localtime(). I know in POSIX they are obsolete but I am concerned with 
> c89-99. strftime() is suppoed to be the function used now. I get the time 
> and date just fine with the code I've written but I want to use the time to 
> seed a random number generator. I don't know if I want seconds from epoch or 
> what. Any input?
> 
> Bill

You could consider using arc4random() instead (if your system supports 
it) and then not worry about having to seeding it.

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#28518

Mark Storkamp wrote:
> In article <k8ehrs$ddf$1@dont-email.me>,

> You could consider using arc4random() instead (if your system supports
> it) and then not worry about having to seeding it.

    It evidently doesn't. I looked at everything defined and declared in my 
stdlib.h and it wasn't there. It didn't come up on my manpage either. I use 
fedora 17.

Bill

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#28521

"Bill Cunningham" <nospam@nspam.invalid> writes:
> Mark Storkamp wrote:
>> In article <k8ehrs$ddf$1@dont-email.me>,
>
>> You could consider using arc4random() instead (if your system supports
>> it) and then not worry about having to seeding it.
>
>     It evidently doesn't. I looked at everything defined and declared in my 
> stdlib.h and it wasn't there. It didn't come up on my manpage either. I use 
> fedora 17.

arc4random() (which I'd never heard of before Mark mentioned it)
is a BSD library function.  On my system (Ubuntu), it's part of the
"libbsd-dev" package.

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
    Will write code for food.
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"

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#28539

Keith Thompson wrote:

> arc4random() (which I'd never heard of before Mark mentioned it)
> is a BSD library function.  On my system (Ubuntu), it's part of the
> "libbsd-dev" package.

    OK I got it. There's several functions there I haven't sorted out yet. 
But arc4random() seems to work fine.

Bill

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