You've probably got something like the following code:

   char *filename = "c:\foo\bar\mumble.dat";
   FILE *fptr;
   /*.*/
   fptr = fopen(filename, "r");

 The problem is that \f is a form feed, \b is a backspace, and \m is m.
 Whenever you want a backslash in a string constant in C, you must use
 two backslashes:

 char *filename = "c:\\foo\\bar\\mumble.dat";

 This is a feature of every C compiler, because Dennis Ritchie designed C
 this way. It's a problem only on MS-DOS systems, because only DOS (and
 Atari ST/TT running TOS) uses the backslash in directory paths. But even
 in DOS this backslash convention applies _only_ to string constants in
 your source code. For file and keyboard input at run time, \ is just a
 normal character, so users running your program would type in file specs
 the same way as in DOS commands, with single \ characters.

 Another possibility is to code all paths in source programs with /
 rather than \ characters:

 char *filename = "c:/foo/bar/mumble.dat";

 Ralf Brown writes, "All versions of the DOS kernel accept either forward
 or backslashes as directory separators. I tend to use this form more
 frequently than backslashes since it is easier to type and read." This
 applies to DOS function calls (and therefore to calls to the file
 library of every programming language), but not to DOS commands.