C's stdlib rand() function in Java

Question

I've generated a series of random numbers from a known seed in C using srand() and rand() from stdlib. I now need to generate the same series of numbers using the same seed from C in Java.

Java's Random class documentation states it uses a "linear congruential formula". The documentation I've found on rand() says it uses a "linear congruential" generator, although I'm not sure if this is for one specific implementation.

Does anyone know if both generators will produce the same numbers if given the same seed or if a port of srand() and rand() exists for Java?

Answer 1

The C standard does not dictate the implementations of srand() and rand(). As such, different environments (OS, C libraries, architecture, etc.) will more than likely produce sequences of numbers that are different for the same seed value.

Also, the implementation Java's Random class is not bound to any particular algorithm. Here again, different JVMs may very well produce different sequences for the same seed value. In addition, the implementation will more than likely not be tied to the standard C functions. Which means, the Java produced sequence will be different than a C sequence using the same seed.

If you truly need to generate random sequence in Java to match exactly that of the standard C functions, the best you could hope to do is replicate the sequence for a particular environment. This would require creating a JNI library to proxy calls to srand() and rand() or creating some other external process that makes the calls and is invoked from Java. Either way, that's a lot of complexity and more program maintenance.

If in fact all you need are random sequences that appear to be uniformly distributed, regardless of the exact values, then use Random as is. It is more than sufficient for most RNG needs.

Answer 2

As said in other answer, the C standard doesn't even specify that rand() will return the same sequence across different C platforms (libraries), and likewise nothing in Java guarantees that it matches any given C (or other Java) implementation. You could use JNI to call the specific C implementation on that platform, but this would only guarantee the same sequence to be produced when both the C and Java programs are run on the same platform using the same C library.

If you wish to ensure the same sequence in all situations, you need to implement the same random number generator in both languages. A simple example can be found in POSIX.1-2001, and is quoted on many man 3 rand pages:

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 seed) {
    next = seed;
}

It is trivially portable to Java. The quality of randomness produced this generator is not very high, but it's not really guaranteed to be any better with rand() either, so you would need to implement something fancier if better randomness is required.

Answer 3

In both C and Java, the same seed will generate the same random values. While the underlying mechanism might be different, this property is maintained in every programming language I know about.

C and Java will probably not generate the same set of random numbers given a seed. The only property that is maintained is that given a seed, a programming language will generate the same random numbers.