installs the yield function yf for the link referenced by link.
Some WSTP API calls will block until data is available to read or space is available for writing. If an application needs to perform other useful processing during that blocking time, the application can install a yield function for the link and WSTP will automatically make calls to the yield function while it blocks.
To back out of a blocked read or write, the yield function should return a nonzero value.
WSSetYieldFunction() will return 0 in the event of an error, and a nonzero value if the function succeeds.