Does Python have a function to mimic the sequence of C's rand()?

0
votes

I am looking for a Python function that will mimic the behavior of rand() (and srand()) in c with the following requirements:

  1. I can provide the same epoch time into the Python equivalent of srand() to seed the function
  2. The equivalent of rand()%256 should result in the same char value as in c if both were provided the same seed.

So far, I have considered both the random library and numpy's random library. Instead of providing a random number from 0 to 32767 as C does though both yield a floating point number from 0 to 1 on their random functions. When attempting random.randint(0,32767), I yielded different results than when in my C function.

TL;DR Is there an existing function/libary in Python that follows the same random sequence as C?

3
pythoncrandom
I doubt there is only one implementation of rand across all C environments. – stark
The output of C's rand is completely unspecified. There is no guaranteed sequence. If you want a guaranteed sequence, you shouldn't be using rand. – user2357112 supports Monica
(Also, the statistical quality of rand output is typically terrible.) – user2357112 supports Monica
To clarify, this is in the context of a cryptography problem where I know a range of possible times that a file was encrypted and know that the srand() seed was set with the epoch time during the period with a c algorithm. I am looking for a way to solve this problem in python with the knowledge that the cipher key was generated with c's rand(). rand() will produce the same sequence given a particular srand() seed – user3439632
... so, this is a job for FFI, to let you call the actual C implementations from Python, with the understanding that you need to call it in a system that's similar enough to the system you're trying to emulate (same libc, same architecture, maybe same compile time flags, etc). See, for example, docs.python.org/3/library/ctypes.html – Charles Duffy

3 Answers

0
votes

You can use random.seed(). The following example should print the same sequence every time it runs:

import random
random.seed(42)
for _ in range(10):
    print(random.randint(0, 32768))

Update: Just saw the last comment by the OP. No, this code won't give you the same sequence as the C code, because of reasons given in other comments. Two different C implementations won't agree either.

4
votes

You can't make a Python version of rand and srand functions "follo[w] the same random sequence" of C's rand and srand because the C standard doesn't specify exactly what that sequence is, even if the seed is given. Notably:

  • rand uses an unspecified random number algorithm, and that algorithm can differ between C implementations, including versions of the same standard library.
  • rand returns values no greater than RAND_MAX, and RAND_MAX can differ between C implementations.

In general, the best way to "sync" PRNGs between two programs in different languages is to implement the same PRNG in both languages. This may be viable assuming you're not using random numbers for information security purposes. See also this question: How to sync a PRNG between C#/Unity and Python?

1
votes

You can accomplish this with CDLL from https://docs.python.org/3/library/ctypes.html

from ctypes import CDLL

libc = CDLL("libc.so.6")
libc.srand(42)
print(libc.rand() % 32768)