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

[Peng Fan <van.freenix@gmail.com>]

On 08/27/2014 08:32 AM, Yao Qi wrote:
> Peng Fan <van.freenix@gmail.com> 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.
Yeah. the program hits the breakpoint at 0x87809580. then 'c'.
> 
> 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.
Yeah. Detailed. Got it. Thanks.
> 
>>
>> 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: $c#63...Ack
>>
>> why two Z0 here? I only set break at 0x87809580, but gdb send a break
>> set at 0x87800000 which is the beginning of the elf entry.
> 
> What is the symbol on 0x87800000? GDB inserts some internal breakpoints
> for some purpose, collecting interesting events, for example.
> 
Actually, there is a pair of "$z0,87800000,4" and "$Z0,87800000,4" of each 'c' except the first time to executing 'c'.
The first time 'c', there is only "$Z0,87800000,4" to set a breakpoint. 
0x87800000 is the entry symbol address of u-boot elf of my platform.
    1                                                                                 
    2 u-boot:     file format elf32-littlearm                                         
    3                                                                                 
    4                                                                                 
    5 Disassembly of section .text:                                                   
    6                                                                                 
    7 87800000 <__image_copy_start>:                                                  
    8 87800000: 11 00 00 ea 14 f0 9f e5 14 f0 9f e5 14 f0 9f e5
    9 87800010: 14 f0 9f e5 14 f0 9f e5 14 f0 9f e5 14 f0 9f e5 

Regards,
Peng.