console stdout captured only at prog end

[Amrouch]

I am trying to make a gui application that hides a console application that uses menu. In order to do that, I want to capture stdout and replace it by a pipe and replace stin by a pipe. I tried the following sollution :

Using CreateProcess with

StartupInfo.dwFlags = STARTF_USESHOWWINDOW | STARTF_USESTDHANDLES;
StartupInfo.hStdOutput = StdOutWrite;

The problem with this solution is that I get all of the output only at the ending of the console application (I can't get the output byte by byte as it is sent to the screen).

An other thing to say is that the console app, I didn't write it bye my self (I can't change it)

Does someone know why ?