When would tmpfile(); not work?

A program I have written will be ran on Windows as a normal user. An installer I have created will install it into the C:\Program Files directory. I will need to use tmpfile(); for many things. Is this effective or is there a better method? Would the system deny the program the ability to use tmpfile(); because the program is being ran from that directory? I have used tmpfile(); in a variety of situations and it has never returned NULL for me, and I need it to still be reliable in this situation. What would make it return NULL, and why? Do I have nothing to worry about? Thanks! :D

This is essentially a Windows question.
tmpfile (CRT)

Thank you for the link. I take it I should use something other than tmpfile();, in this case? What if I use tmpnam(); as well to put it in the Application Data directory? Will there be any chances of it returning null after that? I need a 100% success rate. Maybe I should write my own functions for this instead of using tmpfile(); and fgetc(); and the rest of it?

First result of Google search of "windows temp file c++". Note that the example, which states that it is C++, is in fact C code.

After looking at all of this, I have decided to just store the file in an array and replace all of my fgetc and similar code with functions made to treat the array like a file. Thank you all for contributing, though. :)