Character Sequences
ANSI 3.8.2 The mapping of source file character sequences
Preprocessor statements use the same character set as source file statements with the exception that escape sequences are not supported.
Thus, to specify a path for an include file, use only one backslash:
#include "path1\path2\myfile"
Within source code, two backslashes are necessary:
fil = fopen( "path1\\path2\\myfile", "rt" );