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# Re: What is the reason for multiplying 10 in __srandom_r?

```It is highly desirable for a pseudo-random number generation library routine
to have good statistical properties. Supposedly minor changes to
an algorithm can play havoc with the algorithm's statistical properties.

```
These days, if an efficient integer multiplication is not available in the HW,
```the compiler can easily generate a "x+x+8*x" sequence which is nearly
as efficient as "8*x".  Certainly it is a tiny cost compared to
the 10*x calls to " (void) __random_r (buf, &discard);"

```
In addition, for Monte-Carlo simulation, part of the value of pseudo-random number generators is the ability to reproduce results for a given seed in order to allow
```comparisons with different input conditions, etc. Any proposal to change
```
a standard library routine that provides a pseudo-random number will rightfully
```be met with great resistance.

Pseudo-random number generation is an area where there has been a lot of
deep mathematical study for a long time.  See
https://en.wikipedia.org/wiki/Pseudorandom_number_generator
```
for an introduction to the field and follow up with the links to that article
```if you find a need for further study.

- patrick

On 5/30/2018 10:03 PM, Xinzhen Chen wrote:
```
```Hello, glibc developers :-)

I was trying to understand what functions like rand() do under the hood. I
compiled glibc 2.27 from source and link the new compiled libc.so.6 to my
test C program in the following:

#include <stdlib.h>
#include <stdio.h>

int main(int argc, char const* argv[])
{
int randt = {0};
initstate(2, (char*)randt, 128);
setstate((char*)randt);
printf("%d\n", rand());
return 0;
}

I used gdb to step into initstate() function and got confused when I
stepped into the following line of  __srandom_r function:

kc = buf->rand_deg;
for (i = 1; i < kc; ++i)
{
/* This does:
state[i] = (16807 * state[i - 1]) % 2147483647;
but avoids overflowing 31 bits.  */
}

buf->fptr = &state[buf->rand_sep];
buf->rptr = &state;
kc *= 10;
while (--kc >= 0)
{
}

I also read the comment of  __srandom_r function which says "Lastly, it
cycles the state information a given number of times to get rid of any
initial dependencies introduced by the L.C.R.N.G.".

My questions are:
1. Why the " given number of times" is "kc * 10" instead of "kc * 8" or "kc
* 16" which is more efficient in multiplication?
2. Are there papers or books which dive into glibc random implementation
```