Thank you for the reply,

Circular buffer would be cool, but as I mentioned my problem is with the fragmentation.
Imagine this situation (the numbers are the packages sizes of bytes in the cyclic buffer g-get pointer, p-put pointer, f-free place):

Now the buffer is full:
5 9 4 5
^
g
^
p

there are 2 timeouts of packages and I clear the first and the third package with size 4 and 5 bytes. So I get 9 bytes free. The next telegram that arrives has 9 bytes, I have 9 bytes free but I am still not able to save it in the buffer.
F 9 F 5
^ _ ^
g _ p


I didn't really underestand your second suggestion according start, end characters.

Each package contains autonomous information for a specific sender. The package contains all the info the receiver needs. I can't have a start or end package, because it's possible that I receive only 1 package all the other are lost.

But I also can't start processing the package until I am not sure that all the repeated package were received (upon timeout), because I would think I received 2x the same package (during processing).

greetings
Attila