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The conversion "%lf" is not valid for printf() (it should be used in scanf() with pointers to doubles).
To print a value of type double, use "%f"; you cannot print a value of type float because the...

Try compiling both your source (containing the main() function) and the source you found on the internet

$ gcc rt.c prng.c -Wall -o rt -lm

Oh! I also included the option to warn about stuff gcc...