multiple results from a function

I see that I can use PQgetResult to get results from an async query, and this allows for multiple results, presumably
whenthe query is like "SELECT 123, 456; SELECT 'ABC', 'DEF';". Is there a way for a C function, using SPI, to send
resultsets without waiting for completion and returning a potentially huge set of results right at the end of the
function?

Failing that, is there a way for a C function to send to STDOUT in the same way that a COPY (...) TO STDOUT would?

Thanks

James