int MLPutReal(MLINK link, double x) puts the floating-point number x to the MathLink connection specified by link.
You can send arbitrary-precision real numbers to Mathematica by giving lists of digits, then converting them to numbers using FromDigits.
MLPutReal() is normally equivalent to MLPutDouble().
See Section 2.13.4 and Section 2.13.12.
See also: MLPutRealList, MLPutFloat, MLPutDouble.