your solution writes past the end of the output buffer: you are printing a trailing '.', and then erase it. this is not robust: the worst-case output size is 16 i.e. strlen("100.100.100.100") + 1, so the caller of the function can expect that 16 bytes should be enough, but your solution needs more bytes than that. as a result of writing out-of-bounds, you corrupt the tests suite

They do.