Problem in GDB debugging

Question

I use GDB to debug a C program, but I find GDB execute some codes twice.

For example,

 ....
    stream_t *s = stream_CommonNew( VLC_OBJECT(p_access) );
    stream_sys_t *p_sys;
    if( !s )
    return NULL;
    s->p_input = p_access->p_input;
    s->psz_path = strdup( p_access->psz_path );
  ....

GDB Debugging,

292     stream_t *s = stream_CommonNew( VLC_OBJECT(p_access) );
Missing separate debuginfos, use: debuginfo-install dbus-libs-1.2.16-9.fc12.i686 libcap-ng-0.6.2-3.fc12.i686
(gdb) next
295     if( !s )
(gdb) 
292     stream_t *s = stream_CommonNew( VLC_OBJECT(p_access) );
(gdb) 
295     if( !s )
(gdb) 
298     s->p_input = p_access->p_input;
(gdb) 
299     s->psz_path = strdup( p_access->psz_path );
(gdb) 
298     s->p_input = p_access->p_input;
(gdb) 
299     s->psz_path = strdup( p_access->psz_path );

I am confused. Could you explain why?

Thanks

Do you have compiler optimisations enabled?

Oliver Charlesworth
– Oliver Charlesworth

2011-04-29 08:20:01 +00:00
Commented Apr 29, 2011 at 8:20 — Oliver Charlesworth
– Oliver Charlesworth, Commented Apr 29, 2011 at 8:20
@Oli Charlesworth: yes, I think it is enabled.

JavaMobile
– JavaMobile

2011-05-03 04:01:36 +00:00
Commented May 3, 2011 at 4:01 — JavaMobile
– JavaMobile, Commented May 3, 2011 at 4:01

mark4o · Accepted Answer · 2011-04-29 08:31:16Z

4

It is not actually executing the same code twice. Compiler optimizations can cause machine instructions to be reordered, such that some instructions that were generated for the second source line are placed before the last instruction for the first source line. Gdb's "next" command stops when the source line corresponding to the instruction changes, even though it may actually just be executing the rest of a source line that was not finished yet.

answered Apr 29, 2011 at 8:31

mark4o

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2 Comments

JavaMobile Over a year ago

I see. If it is, how could I know or trace the program`s running?

mark4o Over a year ago

If it bothers you, you can compile without optimization as Pih suggested. If you are interested in the actual optimizations you can always disassemble the code and step by machine instructions instead of by source line (nexti, stepi).

Pih · Accepted Answer · 2011-04-29 08:30:37Z

1

Try to compile without any optimization (-O0) and run again. Another idea, it is to put a watch on s->p_input and see if this structure field is modified twice.

answered Apr 29, 2011 at 8:30

Pih

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2 Comments

Pih Over a year ago

Worked? What was the problem?

JavaMobile Over a year ago

I think the reason is compiler optimization. So I just turn it off by using 'g++ -o0'

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