Your "**" is a string constant. Writing into is is normally not recommended and can even cause a crash. Handling strings is a little more complex than numeric variables, so I suggest a C online course as listed in the manual for learning how to use strings.

In your case a simple solution to avoid a crash would be setting s1 to a long empty string before writing into it, like this:

s1 = "____________________";
sprintf(s1,..);