Save and restart random chain (drand48) from checkpoint in C - c

Question

I'm trying to write a program that gives the same result either if is executed entirely or if is stopped and restarted from some checkpoint. To do that I need to be able to repeat exactly the same random number sequence in any scenario. So, here a piece of code where I tried to do that, but of course, I'm not successful. Could you help me to fix this code?
int main(){
int i;
long int seed;
// Initial seed
srand48(3);
// Print 5 random numbers
for(i=0;i<5;i++) printf("%d %f\n",i,drand48());
// CHECKPOINT: HOW TO PROPERLY SET seed?
seed=mrand48(); // <--- FIXME
// 5 numbers more
for(i=5;i<10;i++) printf("%d %f\n",i,drand48());
// Restart from the CHECKPOINT.
srand48(seed);
// Last 5 numbers again
for(i=5;i<10;i++) printf("%d %f\n",i,drand48());
}

Answer 1

If you need to be able to resume the random number sequence, you can't let the drand48() package hide the seed values from you, so you need to use different functions from the package. Specifically, you should be calling:
double erand48(unsigned short xsubi[3]);
instead of:
double drand48(void);
and you'll keep an array of 3 unsigned short values around, and at each checkpoint, you'll record their values as part of the state. If you need to resume where things left off, you'll restore the values from the saved state into your array, and then go on your merry way.
This is also how you write library code that neither interferes with other code using the random number generators nor is interfered with by other code using the random number generators.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main(void)
{
unsigned short seed[3] = { 0, 0, 3 };
// Print 5 random numbers
for (int i = 0; i < 5; i++)
printf("%d %f\n", i, erand48(seed));
// CHECKPOINT
unsigned short saved[3];
memmove(saved, seed, sizeof(seed));
// 5 numbers more
for (int i = 5; i < 10; i++)
printf("%d %f\n", i, erand48(seed));
// Restart from the CHECKPOINT.
memmove(seed, saved, sizeof(seed));
// Last 5 numbers again
for (int i = 5; i < 10; i++)
printf("%d %f\n", i, erand48(seed));
return 0;
}
Example run:
0 0.700302
1 0.122979
2 0.346792
3 0.290702
4 0.617395
5 0.059760
6 0.783933
7 0.352009
8 0.734377
9 0.124767
5 0.059760
6 0.783933
7 0.352009
8 0.734377
9 0.124767
Clearly, how you set the seed array initially is entirely up to you. You can easily allow the user to specify the seed value, and report the seed you're using so that they can do so. You might use some elements from the PID or the time of day and the sub-seconds component as a default seed, for example. Or you could access a random number device such as /dev/urandom and obtain 6 bytes of random value from that to use as the seed.
How can I allow the user to specify the seed value using only a long int? In this approach, it seems that the user need to define 3 numbers but I would like to ask only 1 number (like a safe prime) in the input file.
You can take a single number and split it up in any way you choose. I have a program that takes option -s to print the random seed, -S to set the seed from a long, and that sometimes splits the long into 3 unsigned short values when using a random Gaussian distribution generator. I mostly work on 64-bit systems, so I simply split the long into three 16-bit components; the code also compiles safely under 32-bit systems but leaves the third number in the seed as 0. Like this:
case 'q':
qflag = true;
break;
case 'r':
check_range(optarg, &min, &max);
perturber = ptb_uniform;
break;
case 's':
sflag = true;
break;
case 't':
delim = optarg;
break;
case 'S':
seed = strtol(optarg, 0, 0);
break;
case 'V':
err_version("PERTURB", &"#(#)$Revision: 1.6 $ ($Date: 2015/08/06 05:05:21 $)"[4]);
/*NOTREACHED*/
default:
err_usage(usestr);
/*NOTREACHED*/
}
}
if (sflag)
printf("Seed: %ld\n", seed);
if (gflag)
{
unsigned short g_seed[3] = { 0, 0, 0 };
g_seed[0] = (unsigned short)(seed & 0xFFFF);
g_seed[2] = (unsigned short)((seed >> 16) & 0xFFFF);
if (sizeof(seed) > 4)
{
/* Avoid 32-bit right shift on 32-bit platform */
g_seed[1] = (unsigned short)(((seed >> 31) >> 1) & 0xFFFF);
}
gaussian_init(&g_control, g_seed);
}
else
srand48(seed);
filter_anon(argc, argv, optind, perturb);
return 0;
}
For my purposes, it is OK (not ideal, but OK) to have the even more restricted seeding values for 32-bit. Yes, I could use unsigned long long and strtoull() etc instead, to get 64-bit numbers even on a 32-bit platform (though I'd have to convert that to a long to satisfy srand48() anyway. An alternative that I considered is to accept an argument -S xxxx:yyyy:zzzz with the three seed components set separately. I'd then have to modify the seed printing code as well as the parsing code. I use a separate program randseed to read numbers from /dev/urandom and format the result so it can be passed to programs which need a random seed:
$ randseed -b 8
0xF45820D2895B88CE
$