Why can't I open a file by its explicit path? The call

fopen("c:\newdir\file.dat", "r")

is failing.

The file you actually requested--with the characters \n and \f in its name--probably doesn't exist, and isn't what you thought you were trying to open.
In character constants and string literals, the backslash \ is an escape character, giving special meaning to the character following it. In order for literal backslashes in a pathname to be passed through to fopen (or any other function) correctly, they have to be doubled, so that the first backslash in each pair quotes the second one:
fopen("c:\newdir\file.dat", "r")
Alternatively, under MS-DOS, it turns out that forward slashes are also accepted as directory separators, so you could use
fopen("c:/newdir/file.dat", "r")
(Note, by the way, that header file names mentioned in preprocessor #include directives are not string literals, so you may not have to worry about backslashes there.)