I don't know any portable way for a process to find out the number of files it has open.

But consider this case...I open 16 files and allocate fd 0 through fd 15. Then I close fd 0 through fd 14. This leaves only fd 15 open. If you could magically find out that only one file is open, how would that help you? You still don't know which fd to close.

The shell will allocate fd 0, 1, and 2 and pass these to you. Your program should keep track of which files it opens.

If you have lost track of want files are open, the only thing I can suggest to do a getrlimit() to obtain the highest possible fd that could ever be allocated. Then close all possible fd's. The close call will fail if the file is not open, so you just ignore that error.