Programming

There is a reason - most systems double datatype has 15 digits of precision.
see limits.h DBL_DIG

Try long double instead. I also thought you needed 16 digits, not a numeric datatype like double.

For example, suppose you get 0000000123456789 from /dev/urandom. If that becomes a numeric datatype it no longer has 16 digits. It has only nine digits.

From man pages, random seem to be more suitable for key generation than urandom character stream

I should add that a floating point datatype is the wrong kind of variable to hold integer data. Even a very high precision floating point value does not have complete precision. a 64-bit 'long long' type seems better suited. Or better yet, include stdint.h and use int64_t. To convert from a string, see atoll.

/dev/random has the problem that sometimes it may block. When there's not enough entropy available the read() call will wait until there is. crypto-grade randomness may not be needed unless this really is for crypto.

Random is a loaded word. If you want somewhat random numbers, use the system call 'rand'.

If you need something more rigorous, you need to look into encryption software. Just feed any robust encryption engine a text string and you will get a random sequence of bits back that you can slice and dice as you like. Feed it the same string concateneated to itself, you will still get a random stream of non-repeating bits. Change the key and you get a different random stream of bits.

Google 'aes encryption source code' [AES stands for advanced encryption standard] There are other packages, but that is the standard.

You can do cool graphics stuff with true random numbers. Quasi-random numbers quickly reveal visual artifacts.