Qt application with a C "sub" application - compilation

[jpn]

If I define it in the main .pro file it searches for the sub-projects .pro which doesn't exist.

Why don't you create one with "qmake -project"? qmake is a flexible makefile generator not restricted to Qt projects only.

[high_flyer]

In addition to what jpn posted, if you are working on linux and KDE, I warmly recommend to use KDevelop, it will take you two minutes to add this subproject.
I don't know for sure, but probably QDevelop, Edyuk and Monkeystudio will manage it easy too.
(This is the same as what jpn said, it only will be done with a nice GUI, which will generate the pro file for your subproject)