How fo force subprocess to refresh stdout buffer?

Question

Platform: windows 8.1 IDE: vs2013 use c/c++

Process A read stdout of subprocess using pipe redirect.

but subprocess dont invoke fflush after printf, so processs A cant read anything from pipe before subprocess run to end.

ps: I have souce code of subprecess, but its very hard to modify them.

So whether Process A can force subprocess refresh stdout buffer to read something before subprocess run to end? (the same effective as fflush)