Re: GDB Remote debug: Why several "$Z0,address,length" msg when only set one breakpoint?

Peng Fan <[email protected]> writes:

It is a typical step-over-breakpoint process in GDB,

> Sending packet: $m87809580,4#7a...Ack
> Packet received: f0452de9
> Sending packet: $m87809580,4#7a...Ack
> Packet received: f0452de9

Looks like the program hits the breakpoint on 0x87809580, and 'c'
command is being executed.  Before GDB resume the inferior, it will
execute the instruction in a single-step fashion on the address
where the breakpoint is set, because it isn't executed yet.

GDB read the instruction on address 0x87809580, decode it and know what
is the address of next instruction,

> Sending packet: $Z0,87809584,4#c7...Ack
> Packet received: OK

and insert the breakpoint on next instruction for software single step,

> Sending packet: $Hc0#db...Ack
> Packet received: 
> Sending packet: $c#63...Ack
> Packet received: T050f:84958087;0d:a41355bf;
> Sending packet: $z0,87809584,4#e7...Ack
> Packet received: OK
> Sending packet: $qTStatus#49...Ack

resume the program, and it hits the breakpoint on 0x87809584.  Then,
instruction on 0x8780950 is executed.

> Packet received: 
> Sending packet: $Z0,87800000,4#ad...Ack
> Packet received: OK
> Sending packet: $Z0,87809580,4#c3...Ack
> Packet received: OK

Restore the breakpoint on 0x87809580.

> Sending packet: $Hc0#db...Ack
> Packet received: 
> Sending packet: $c#63...Ack

Resume the inferior and keep waiting.

>
> Actually the breakpoint is at 0x87809580, but why first set breakpoint
> at 0x87809584 then remove it then set breakpoint at 0x87809580?
>

My comments above answer your question.

> Also the first time set a breakpoint then continue.
> (gdb) c
> Continuing.
> Sending packet: $qTStatus#49...Ack
> Packet received: 
> Sending packet: $Z0,87800000,4#ad...Ack
> Packet received: OK
> Packet Z0 (software-breakpoint) is supported
> Sending packet: $Z0,87809580,4#c3...Ack
> Packet received: OK
> Sending packet: $vCont?#49...Ack
> Packet received: 
> Packet vCont (verbose-resume) is NOT supported
> Sending packet: $Hc0#db...Ack
> Packet received: 
> Sending packet: $c#63...Ack
>
> why two Z0 here? I only set break at 0x87809580, but gdb send a break
> set at 0x87800000 which is the beginning of the elf entry.

What is the symbol on 0x87800000? GDB inserts some internal breakpoints
for some purpose, collecting interesting events, for example.

-- 
Yao (齐尧)