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int MLPutUnicodeString(MLINK link, unsigned short *s, long n) puts a string of n 16-bit Unicode characters to the MathLink connection specified by link.
All characters are assumed to be 16 bit.
8-bit characters can be sent by having the higher-order byte be null.
See The Mathematica Book: Section 2.12.5.
See also: MLPutString, MLPutByteString.