If the data on the link corresponds to a real number, MLGetInteger64() will round it to a 64-bit integer.
If the data on the link corresponds to an integer too large to store in a 64-bit C type, or the 32-bit structure on your computer system, then MLGetInteger64() will fail, and return 0.
MLGetInteger64() returns a nonzero value if the function succeeds.
Use MLError() to retrieve the error code if MLGetInteger64() fails.
MLGetInteger64() is declared in the MathLink header file mathlink.h.