If the data on the link corresponds to a C int, MLGetReal64() will coerce it to a double before storing it in x.
If the data on the link corresponds to a number outside the range that can be stored in a C double on your computer system, then MLGetReal64() will fail, and return 0.
Arbitrary-precision real numbers are transmitted between MathLink programs as the number sequence generated by RealDigits, typically as an integer array or as a C string of decimal characters. This sequence can be read using MLGetInteger32List() or MLGetString().
MLGetReal64() returns 0 in the event of an error, and a nonzero value if the function succeeds.
Use MLError() to retrieve the error code if MLGetReal64() fails.
MLGetReal64() is declared in the MathLink header file mathlink.h.