How GDB knows a breakpoint is hit?

How GDB knows a breakpoint is hit?

[kceiwH]

Hi,

I wonder how GDB knows it should stop the inferior when a breakpoint  
is hit. I try to read the code but only to find how GDB creates a  
breakpoint. I thought GDB might change some instructions of inferior  
so that when the inferior executes the modified instructions, GDB  
should know. But what the function "create_breakpoint" does is to add  
a breakpoint to the list "breakpoint_chain". It is not what I thought.  
So I am confused. Does GDB monitor each instruction of the inferior  
and check whether there is a breakpoint and stop the inferior before  
the instruction is executed? If so, how GDB monitor the inferior? And  
it will be too slow. Could anybody tell how GDB handle it and plus the  
codes. Thanks in advance.

Regards,

Mao

Re: How GDB knows a breakpoint is hit?

[Satendra...]

whenever you put a breakpoint at some address using GDB then GDB
replaces instruction at
that address with a trap, illegal divide, or some other instruction
that will cause an exception.
So when it's encountered, GDB will take the exception and stop the program.

Before you browse the code, go through
http://sourceware.org/gdb/current/onlinedocs/gdbint_toc.html
once.

-Satendra

On 19/08/2009, kceiwH <kceiwh@gmail.com> wrote:
> Hi,
>
> I wonder how GDB knows it should stop the inferior when a breakpoint
> is hit. I try to read the code but only to find how GDB creates a
> breakpoint. I thought GDB might change some instructions of inferior
> so that when the inferior executes the modified instructions, GDB
> should know. But what the function "create_breakpoint" does is to add
> a breakpoint to the list "breakpoint_chain". It is not what I thought.
> So I am confused. Does GDB monitor each instruction of the inferior
> and check whether there is a breakpoint and stop the inferior before
> the instruction is executed? If so, how GDB monitor the inferior? And
> it will be too slow. Could anybody tell how GDB handle it and plus the
> codes. Thanks in advance.
>
> Regards,
>
> Mao
>

Re: How GDB knows a breakpoint is hit?

[Andreas Schwab]

kceiwH <kceiwh@gmail.com> writes:

> I wonder how GDB knows it should stop the inferior when a breakpoint is
> hit. I try to read the code but only to find how GDB creates a
> breakpoint. I thought GDB might change some instructions of inferior  so
> that when the inferior executes the modified instructions, GDB  should
> know. But what the function "create_breakpoint" does is to add  a
> breakpoint to the list "breakpoint_chain".

Breakpoints are only inserted immediately before the inferior is
resumed, see insert_breakpoints.

Andreas.

-- 
Andreas Schwab, schwab@linux-m68k.org
GPG Key fingerprint = 58CA 54C7 6D53 942B 1756  01D3 44D5 214B 8276 4ED5
"And now for something completely different."

Re: How GDB knows a breakpoint is hit?

[kceiwH]

Thanks very much.

Mao

On Aug 19, 2009, at 5:09 PM, Andreas Schwab wrote:

> kceiwH <kceiwh@gmail.com> writes:
>
>> I wonder how GDB knows it should stop the inferior when a  
>> breakpoint is
>> hit. I try to read the code but only to find how GDB creates a
>> breakpoint. I thought GDB might change some instructions of  
>> inferior  so
>> that when the inferior executes the modified instructions, GDB   
>> should
>> know. But what the function "create_breakpoint" does is to add  a
>> breakpoint to the list "breakpoint_chain".
>
> Breakpoints are only inserted immediately before the inferior is
> resumed, see insert_breakpoints.
>
> Andreas.
>
> -- 
> Andreas Schwab, schwab@linux-m68k.org
> GPG Key fingerprint = 58CA 54C7 6D53 942B 1756  01D3 44D5 214B 8276  
> 4ED5
> "And now for something completely different."

Re: How GDB knows a breakpoint is hit?

[Hui Zhu]

I think you can read the code around function bpstat_what.

Hui

On Wed, Aug 19, 2009 at 23:28, kceiwH <kceiwh@gmail.com> wrote:
>
> Hi,
>
> I wonder how GDB knows it should stop the inferior when a breakpoint is hit. I try to read the code but only to find how GDB creates a breakpoint. I thought GDB might change some instructions of inferior so that when the inferior executes the modified instructions, GDB should know. But what the function "create_breakpoint" does is to add a breakpoint to the list "breakpoint_chain". It is not what I thought. So I am confused. Does GDB monitor each instruction of the inferior and check whether there is a breakpoint and stop the inferior before the instruction is executed? If so, how GDB monitor the inferior? And it will be too slow. Could anybody tell how GDB handle it and plus the codes. Thanks in advance.
>
> Regards,
>
> Mao