Fopen() failed to open a file

Question

I'm working in win 7,64-bit,c++. I have a multi-threaded program. My program open and close files repeatedly, I checked and I'm closing all of the files that I open. but after some time the program runs and I try to use fopen I get the error "too many open files".

Is there any way to print the open files? Is there any way to clean all the handles?(without closing the module)

thanks for the help!