LinkRead[link] reads one expression from the specified MathLink connection.
LinkRead[link, h] wraps h around the expression read before evaluating it.
link must be an active LinkObject, as returned by functions like LinkLaunch or Links.
LinkRead will wait until it has read a complete expression before returning.
You can test whether an expression is ready to be read from a particular link using LinkReadyQ.
You can use LinkRead[link, Hold] to get an expression from a link without evaluating it.
See The Mathematica Book: Section 2.12.6.
See also: LinkReadyQ, LinkWrite, Read.