I assume that you want to preserve the older entries and just overwrite existing ones. The relevent question in this case is whether each line or log block is of fixed size.

For example, if you structure your log messages to be, say, 384 bytes or less (any size will do), then open your log in r/w mode. If the file is of the correct size, position the pointer at the beginning of the file and begin writing your standard blocks. If the file is not yet of the correct size, position the file pointer at the end of the file and keep adding.

The only thing you need is a MAXSZ, CURRSZ and BLOCKSZ variable to track location.

I'm sure there are other ways but this was the first one that came to mind.