In the very rare cases that you really need to serialize complicated data structures, something like s-expressions or (gasp) json, or even a memory dump is perf

In the very rare cases that you really need to serialize complicated data structures, something like s-expressions or (gasp) json, or even a memory dump is perfectly appropriate. However, most of the times your data structures should be lists of numbers or lists of strings. For those, you do not really need a "format", you can simply print and scan them from a text file.