Re: Random Number Seed

[Jerome BENOIT <jgmbenoit at wanadoo dot fr>]

Hello,



Olaf Lenz wrote:
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>
> Hello!
>
> A few months ago, you suggested the following code snippet for seeding
> the RNG from /dev/random:
>
> - -------------------------------------------------
> #include <stdio.h>
> #include <sys/time.h>
>
> unsigned long int random_seed()
> {
>
> ~ unsigned int seed;
> ~ struct timeval tv;
> ~ FILE *devrandom;
>
> ~ if ((devrandom = fopen("/dev/random","r")) == NULL) {
> ~   gettimeofday(&tv,0);
> ~   seed = tv.tv_sec + tv.tv_usec;
> ~   if(verbose == D_SEED) printf("Got seed %u from gettimeofday()\n",seed);
> ~ } else {
> ~   fread(&seed,sizeof(seed),1,devrandom);
> ~   if(verbose == D_SEED) printf("Got seed %u from /dev/random\n",seed);
> ~   fclose(devrandom);
> ~ }
>
> ~ return(seed);
> }
> - -------------------------------------------------
>
> I've used the code for quite a while now and only today I noticed a big
> problem with it. The code tests, if /dev/random can be opened, but it
> does NOT test if the fread has actually read any number.

On recent kernel (2.4.x,2.6.x),
you can easily know if there are available random numbers
by reading /proc/sys/kernel/random/entropy_avail
(see the Documentation distributed with the kernel source).
Second, you can read /dev/urandom 
Third, if you have an apropriate hard ware,
you can use the rng-tools to feed with true random number your /dev/random.


hth,
Jerome


> In my case, this resulted in the fact that the seed was not seeded at
> all and all processes used the same seed.... P-(
>
> So to all who have been using the code, I would recommend to check their
> results. For the future, I would recommend to use the following code:
>
> - -------------------------------------------------
> #include <stdio.h>
> #include <sys/time.h>
>
> unsigned long int random_seed()
> {
>
> ~ unsigned int seed;
> ~ struct timeval tv;
> ~ FILE *devrandom;
>
> ~ if ((devrandom = fopen("/dev/random","r")) == NULL) {
> ~   gettimeofday(&tv,0);
> ~   seed = tv.tv_sec + tv.tv_usec;
> ~   if(verbose == D_SEED) printf("Got seed %u from gettimeofday()\n",seed);
> ~ } else {
> ~   if (fread(&seed,sizeof(seed),1,devrandom) == 1) {
> ~     if(verbose == D_SEED) printf("Got seed %u from /dev/random\n",seed);
> ~     fclose(devrandom);
> ~   } else {
> ~     gettimeofday(&tv,0);
> ~     seed = tv.tv_sec + tv.tv_usec;
> ~     if(verbose == D_SEED) printf("Got seed %u from
> gettimeofday()\n",seed);
>
> ~   }
> ~ }
>
> ~ return(seed);
>
> }
> - -------------------------------------------------
>
> Cheers
>     Olaf
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Dr. Jerome BENOIT
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email: jgmbenoit@wanadoo.fr or benoit@cii.fc.ul.pt
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