Automatically insertion of 00 in EIP in Stack Overflow 64bit

Automatically insertion of 00 in EIP in Stack Overflow 64bit

I am trying to overflow a stack in C sample code to execute uncalled function in code.

The problem is when i overflow stack by entering a(24 times), $EIP is 0x555555550061. I want it to be 0x555555555561. Why these two zeros are inserted automatically?

I retrieved this values from gdb. For Instance, if that uncalled function address is 0x555555555561, I can not call that function because of these zeros.

I tried using:

./(name of executable) $(cat input.txt) #input.txt has input string 24 times a with no '\n'

but same problem.

I am running 5.5.0-kali2-amd64.