std::random_device::entropy (3) - Linux Manuals

std::random_device::entropy: std::random_device::entropy

NAME

std::random_device::entropy - std::random_device::entropy

Synopsis

double entropy() const noexcept; (since C++11)

Obtains an estimate of the random number device entropy, which is a floating-point value between 0 and log
2(max()+1) (which is equal to std::numeric_limits<unsigned int>::digits). If the device has n states whose individual probabilities are P
0,...,P
n-1, the device entropy S is defined as
S = -Σn-1
i=0P
ilog(P
i)
A deterministic random number generator (e.g. a pseudo-random engine) has entropy zero.

Return value

The value of the device entropy, or zero if not applicable.

Notes

This function is not fully implemented in some standard libraries. For example, LLVM_libc++ always returns zero even though the device is non-deterministic. In comparison, Microsoft Visual C++ implementation always returns 32, and boost.random returns 10.
The entropy of the Linux kernel device /dev/urandom may be obtained using ioctl_RNDGETENTCNT - that's what std::random_device::entropy() in GNU_libstdc++ uses as of version 8.1

Example

Example output on one of the implementations
// Run this code

  #include <iostream>
  #include <random>

  int main()
  {
      std::random_device rd;
      std::cout << rd.entropy() << '\n';
  }

Possible output:

  32