I want to change the program to C so that some of the files that it creates are hidden in Windows. Which Windows or (even better) POSIX API will set the hidden file attribute?
You can do this by calling SetFileAttributes and setting the FILE_ATTRIBUTE_HIDDEN flag. See http://msdn.microsoft.com/en-us/library/aa365535%28VS.85%29.aspx
This is not POSIX. To create a “hidden” file on a regular POSIX system such as Linux, simply run the file name with a dot (.).
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Windows SetFileAttributes.
CreateFile FILE_ATTRIBUTE_HIDDEN
GetFileAttributesEx, GetFileAttributes SetFileAttributes API Win32.