Number of bytes receives not the same after each cycle

[AUDI_ENG]

OK, I'm asking once more : How do you know that for example 01 is the packet ID and not its content ?
Is the sender sends the data all the time or only at your request?

OK, I'm asking once more : How do you know that for example 01 is the packet ID and not its content ?:
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because I have 9 bytes : one for ID and 8 for bytes , and I have a point of depart : when I click on start button : I receive data



Is the sender sends the data all the time or only at your request? :
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I have my application Qt in the raspberry and i receive data from microcontroller

I have 2 buttons : one for start and one for for stop
when i click start, the port serial is openning, and data (packets of 9 bytes) is receiving from µC
I will display this packets on a table ..


You are right, the processing of data in the sender is infinitely, so when i click on the start button, i receive data but i don't know where is the begenning of the packet, !
its an other problem , thank for your tip, but you have any idea about resolving this ?