Wolfram Research, Inc.
int MLEvaluateString(MLINK link, char *string) sends a string to Mathematica for evaluation, and discards any packets sent in response.
The code for MLEvaluateString() is not included in the MathLink library, but is generated automatically by mcc or mprep in processing MathLink template files.
MLEvaluateString("Print[\"string\"]") will cause string to be printed in a Mathematica session at the other end of the link.
See The Mathematica Book: Section 2.12.3 and Section 2.12.9.