std::random_device::entropy (3) - Linux Man Pages
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
n-1, the device entropy S is defined as
S = -Σn-1
A deterministic random number generator (e.g. a pseudo-random engine) has entropy zero.
The value of the device entropy, or zero if not applicable.
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 output on one of the implementations
// Run this code