gdb shows source line multiple times before executing !!

gdb shows source line multiple times before executing !!

[mohanlal jangir]

While single stepping with gdb, quite frequently I see sorce lines appearing
multiple times i.e. gdb shows some line that will be executed on command
"next", but that is actully not. After next "next" command, I see same line
appearing. This happens two or three times and then finally that line is
executed.

gdb --version command gives following output:
GNU gdb Red Hat Linux 7.x (5.0rh-15) (MI_OUT)
Copyright 2001 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain
conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB.  Type "show warranty" for details.
This GDB was configured as "i386-redhat-linux".


Regards
Mohanlal

Re: gdb shows source line multiple times before executing !!

[Nikola Kolev]

the executable is probably optimized (gcc/g++ -O2 ...)

mohanlal jangir wrote:

>While single stepping with gdb, quite frequently I see sorce lines appearing
>multiple times i.e. gdb shows some line that will be executed on command
>"next", but that is actully not. After next "next" command, I see same line
>appearing. This happens two or three times and then finally that line is
>executed.
>
>gdb --version command gives following output:
>GNU gdb Red Hat Linux 7.x (5.0rh-15) (MI_OUT)
>Copyright 2001 Free Software Foundation, Inc.
>GDB is free software, covered by the GNU General Public License, and you are
>welcome to change it and/or distribute copies of it under certain
>conditions.
>Type "show copying" to see the conditions.
>There is absolutely no warranty for GDB.  Type "show warranty" for details.
>This GDB was configured as "i386-redhat-linux".
>
>
>Regards
>Mohanlal
>
>  
>

Re: gdb shows source line multiple times before executing !!

[mohanlal jangir]

Actully yes. I will try to figure out if this is only reason, but can you
explain why this happens if executable is optimized.

> the executable is probably optimized (gcc/g++ -O2 ...)
>
> mohanlal jangir wrote:
>
> >While single stepping with gdb, quite frequently I see sorce lines
appearing
> >multiple times i.e. gdb shows some line that will be executed on command
> >"next", but that is actully not. After next "next" command, I see same
line
> >appearing. This happens two or three times and then finally that line is
> >executed.

Re: gdb shows source line multiple times before executing !!

[Daniel Jacobowitz]

Because GCC splits the code for the line into multiple parts.  Part of
the line has executed when you step over it, but it may not be
immediately obvious how.  For instance, given:
  a = b + c;
the first time you step over it may load b and c from memory; the
second time may add them; and only the third will store the result into
a.

On Thu, Jul 24, 2003 at 08:08:24PM +0530, mohanlal jangir wrote:
> Actully yes. I will try to figure out if this is only reason, but can you
> explain why this happens if executable is optimized.
> 
> > the executable is probably optimized (gcc/g++ -O2 ...)
> >
> > mohanlal jangir wrote:
> >
> > >While single stepping with gdb, quite frequently I see sorce lines
> appearing
> > >multiple times i.e. gdb shows some line that will be executed on command
> > >"next", but that is actully not. After next "next" command, I see same
> line
> > >appearing. This happens two or three times and then finally that line is
> > >executed.
> 
> 

-- 
Daniel Jacobowitz
MontaVista Software                         Debian GNU/Linux Developer

Re: gdb shows source line multiple times before executing !!

[mohanlal jangir]

> Because GCC splits the code for the line into multiple parts.  Part of
> the line has executed when you step over it, but it may not be
> immediately obvious how.  For instance, given:
>   a = b + c;
> the first time you step over it may load b and c from memory; the
> second time may add them; and only the third will store the result into
> a.
hmm... but sometimes it behaves even worse. For example there are two lines
a = b + c;
d = e + f;
then first gdb shows line a = b + c; then d = e + f and then again a = b +
c. And while showing d = e + f first time if I try to print value of a, it
will be some uninitialised (that indicates that a = b + c has not been
executed). Then on next "next" command I will see line a = b +c again and
then valute of a will be correct one.

> > mohanlal jangir wrote:
> >
> >While single stepping with gdb, quite frequently I see sorce lines
> appearing
> >multiple times i.e. gdb shows some line that will be executed on command
> >"next", but that is actully not. After next "next" command, I see same
> line
> >appearing. This happens two or three times and then finally that line is
> >executed.

Re: gdb shows source line multiple times before executing !!

[Nikola Kolev]

why don't you take the optimization flags (-ON) out, live the debug flag 
(-g) in, recompile, fire up the debugger, do your stuff, instead of 
wasting time, energy, bandwidth, etc. to explain something that is 
obvious to why it's happening...

--Nik

mohanlal jangir wrote:

>>Because GCC splits the code for the line into multiple parts.  Part of
>>the line has executed when you step over it, but it may not be
>>immediately obvious how.  For instance, given:
>>  a = b + c;
>>the first time you step over it may load b and c from memory; the
>>second time may add them; and only the third will store the result into
>>a.
>>    
>>
>hmm... but sometimes it behaves even worse. For example there are two lines
>a = b + c;
>d = e + f;
>then first gdb shows line a = b + c; 
>
that's when it loded b and c from memory, and possibly stored their sum 
in a temporary register

>then d = e + f and then again a = b +
>c. And while showing d = e + f first time if I try to print value of a, it
>will be some uninitialised (that indicates that a = b + c has not been
>executed). 
>
It is uninitialized, because it has not assigned the the sum to a yet, 
but skipped to load e and f because it determined it will be faster that 
way (here's where optimization comes to play)...

>Then on next "next" command I will see line a = b +c again and
>then valute of a will be correct one.
>  
>
Well, that's when it assigned a with what you're looking for...

Re: gdb shows source line multiple times before executing !!

[Daniel Jacobowitz]

On Thu, Jul 24, 2003 at 08:21:02PM +0530, mohanlal jangir wrote:
> 
> > Because GCC splits the code for the line into multiple parts.  Part of
> > the line has executed when you step over it, but it may not be
> > immediately obvious how.  For instance, given:
> >   a = b + c;
> > the first time you step over it may load b and c from memory; the
> > second time may add them; and only the third will store the result into
> > a.
> hmm... but sometimes it behaves even worse. For example there are two lines
> a = b + c;
> d = e + f;
> then first gdb shows line a = b + c; then d = e + f and then again a = b +
> c. And while showing d = e + f first time if I try to print value of a, it
> will be some uninitialised (that indicates that a = b + c has not been
> executed). Then on next "next" command I will see line a = b +c again and
> then valute of a will be correct one.

Then there's some other cleanup to be done that the compiler hasn't
finished with yet.  I recommend either not worrying about it, or
looking at the assembly to understand what's going on.

> 
> > > mohanlal jangir wrote:
> > >
> > >While single stepping with gdb, quite frequently I see sorce lines
> > appearing
> > >multiple times i.e. gdb shows some line that will be executed on command
> > >"next", but that is actully not. After next "next" command, I see same
> > line
> > >appearing. This happens two or three times and then finally that line is
> > >executed.
> 
> 

-- 
Daniel Jacobowitz
MontaVista Software                         Debian GNU/Linux Developer

Re: gdb shows source line multiple times before executing !!

[Doug Evans]

Daniel Jacobowitz writes:
 > I recommend either not worrying about it, or
 > looking at the assembly to understand what's going on.

It might also help to step through the assembly
instead of stepping line-by-line.

(gdb) display/i $pc
(gdb) stepi
[and keep repeating stepi, which will happen if at this point
you just keep hitting <return>]

Re: gdb shows source line multiple times before executing !!

[Elena Zannoni]

mohanlal jangir writes:
 > 
 > > Because GCC splits the code for the line into multiple parts.  Part of
 > > the line has executed when you step over it, but it may not be
 > > immediately obvious how.  For instance, given:
 > >   a = b + c;
 > > the first time you step over it may load b and c from memory; the
 > > second time may add them; and only the third will store the result into
 > > a.
 > hmm... but sometimes it behaves even worse. For example there are two lines
 > a = b + c;
 > d = e + f;
 > then first gdb shows line a = b + c; then d = e + f and then again a = b +
 > c. And while showing d = e + f first time if I try to print value of a, it
 > will be some uninitialised (that indicates that a = b + c has not been
 > executed). Then on next "next" command I will see line a = b +c again and
 > then valute of a will be correct one.

Yes, only part of the assembly instructions that correspond to the
line a = b+c; has been executed at that time, and the result hasn't
been transfered into the location of a yet.  Try to use stepi.  Also
compare with the execution on the same program compiled with -O0 or just
with -g.

elena

 > 
 > > > mohanlal jangir wrote:
 > > >
 > > >While single stepping with gdb, quite frequently I see sorce lines
 > > appearing
 > > >multiple times i.e. gdb shows some line that will be executed on command
 > > >"next", but that is actully not. After next "next" command, I see same
 > > line
 > > >appearing. This happens two or three times and then finally that line is
 > > >executed.