C language - rand() datatype (max value)

John Kauffman

In one source I see Propeller C rand() generates from 0 to 2,147,483,647, implying data type of int.
But I generated 100k rand() values and got a range from 0 to 32767 implying a data type of short int, just the positives.
A general C reference said it depends on the implementation.

Can anyone confirm the data type, or why my test for max is 32767 instead of 2,147,483,647?

test code:
#include "simpletools.h"
int main(void)
{
srand(CNT);
int myRandNew=1; int countTrial=1;
int maxSoFar=1; int minSoFar=1;

for(countTrial;countTrial<=100000;countTrial++){
myRandNew=rand();
if(myRandNew>maxSoFar) maxSoFar=myRandNew;
if(myRandNew<minSoFar) minSoFar=myRandNew;
}//for
print("count of trials %d\nminSoFar=%d\tmaxSoFar=%d\n",countTrial,minSoFar,maxSoFar);
}

Dave Hein

The source for the rand function is

#define A 1103515245
#define C 12345
#define S 16  /* amount to shift before retrieving result */

int
rand(void)
{
  _TLS->rand_seed = _TLS->rand_seed * A + C;
  return (_TLS->rand_seed >> S) & RAND_MAX;
}

RAND_MAX is defined to be 0x7fff. So rand returns an int that is between 0 and 32767.

Heater.

Interesting, according to my rand man page:

"The rand() function returns a pseudo-random integer in the range 0 to RAND_MAX inclusive"

However it does not say what RAND_MAX might be.

It goes on to:

"POSIX.1-2001 gives the following example of an implementation of rand() and srand(), possibly useful when one needs the same sequence on two different machines.

           static unsigned long next = 1;

           /* RAND_MAX assumed to be 32767 */
           int myrand(void) {
               next = next * 1103515245 + 12345;
               return((unsigned)(next/65536) % 32768);
           }

           void mysrand(unsigned int seed) {
               next = seed;
           }

All in all I would never use rand(). Firstly because it is generally a really bad pseudo random number generator and secondly because it is not thread safe (for that you should use rand_r()).

Like many C standard functions it is to be avoided.

Can you printf the value of RAND_MAX ? That will tell you something.

yeti

(yeti@aurora:8)~/wrk/propeller/propgcc/propgcc/lib/include$ grep -i rand stdlib.h 
#define RAND_MAX    0x7fff
  int rand(void);
  void srand(unsigned int seed);

... and reading lib/stdlib/rand.c ...

int
rand(void)
{
  _TLS->rand_seed = _TLS->rand_seed * A + C;
  return (_TLS->rand_seed >> S) & RAND_MAX;
}

...looks like propgcc4's lib produces 15-bit random integers...
(propgcc4-1.9.whatsowver-version here...)

John Kauffman

Thanks for discussion. Looks like combined methods of analyses point to 32767.

Heater. wrote: »

Can you printf the value of RAND_MAX ? That will tell you something.

I tried below but rand_max not recognized (tried UC and LC). Do I have to do something more to "pull it out from the library"?

#include "simpletools.h"
int main(void){
srand(CNT); 
int myRandNew=1;
myRandNew=rand();
//print("\nrand_max = %d",RAND_MAX());
//print("\nrand_max = %d",rand_max());
print("\nrand_max = %d",rand_Max());
}

Dave Hein

You have to include stdlib.h.

John Abshier

This may be of interest to random persons http://web.stanford.edu/class/ee380/Abstracts/150218.html The linked PCG website has C code.

John Abshier

Heater.

John,

Wow thanks for the heads up on PGC. Melissa O'Neill's lecture on random number generators is a real treat for us random nerds.

I had not realized there were such recent developments in PRNGs and statistical tests for randomness. (TestU01)

Somewhere here I posted a Spin object for the jkiss32 PRNG, a version of George Marsaglia's KISS algorithm, which dates back to 1999 http://www.math.niu.edu/~rusin/known-math/99/RNG, looks like we now need a Spin / PASM PGC!