What is the main error of this code?

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VC++

In my vc++ code,

I want to change a filename xxx.000001.log to xxx.000002.log, xxx.000003.log.... using CString.

struct idx_name{
    char type[4];
    char digit[6];
    char ext[4];
}

idx_name idx_logname;

	memset(&idx_logname, 0, sizeof(idx_logname));

        // get the char fname[100]; = "xxx.000001.log"
	memcpy(&idx_logname, fname, sizeof(idx_logname));

	CString s = CString(idx_logname.digit).GetBufferSetLength(6);
	int idx = _ttoi(s);
	idx++;

        ::ZeroMemory(&idx_logname.digit, sizeof(idx_logname.digit));
	sprintf_s(idx_logname.digit, sizeof(idx_logname.digit), "%06d", idx);
	::ZeroMemory(filename, sizeof(filename));
	memcpy(filename, &idx_logname, sizeof(idx_logname));

        // get Current_folder as current_directory
	CString folder = CString(Current_folder);
	s = folder + CString(filename);

This has no compile error.
But in the run time, process has running error at the line of "sprintf_s" like as

"Expression : ("Buffer too small",0);

As a novice of VC++, please let me know the error and who knows the simple ways to change filenames?

What I have tried:

One day wasted for this problem.