The f_mkdir creates a directory.
FRESULT f_mkdir ( const char* DirName // Pointer to the directory name );
The f_mkdir creates an empty directory. This function is not supported in read only configuration.
    res = f_mkdir("/sub1");
    if (res) die(res);
    res = f_mkdir("/sub1/sub2");
    if (res) die(res);
    res = f_mkdir("/sub1/sub2/sub3");
    if (res) die(res);