There's no standard for this - Linux folks came up & implemented it first and other platforms have taken that as compatibility target. So "non-portable implementation details" is all there is, and it works fine for CSPRNG.
There are many standards and cross-platform interfaces defined outside of POSIX. Some explicit and top-down, some organically evolved.
The spec for /dev/*random is here, as was published in the mid-90s: http://git.kernel.org/cgit/linux/kernel/git/stable/linux-sta...