...
The return value of this function should always be verified as non-NULL before the application proceeds to access the file. The first argument of this function is the path of the file; if working directories are disabled, this must be the absolute file path, beginning with either a volume name or a ‘\’ (see µC/FS File and Directory Names and Paths). The second argument of this function is a string indicating the mode of the file; this must be one of the strings shown in the table below . Note that in all instances, the ‘b’ (binary) option has no affect on the behavior of file accesses.
...