Comment by _3u10
It’s just two pointers the current place to write and the current place to read, escapes are always more characters than they represent so there’s no danger of overwriting the read pointer. If you support compression this can become somewhat of and issue but you simply support a max block size which is usually defined by the compression algorithm anyway.
If you have a place to write, then it's not zero allocation. You did an allocation.
And usually if you want maximum performance, buffered read is the way to go, which means you need a write slab allocation.