Thread: A simple question about GDB debugging

I use gdb to debug code. Each time I use "next" command to execute code line by line, GDB shows the line number of the current executing line, but it doesn't show the content of the code at the line. So I have to find it in my source code with the line number. It's very boring.

How can I see the code content of a line this is being executing in GDB? Thanks!

are you using the "-g" switch when compiling? (assuming gcc)

yes, of course. Otherwise I even can't use GDB.
The problem is GDB does not show the code line content but the line number.
I remembered it showed content before I changed some configure. Now I just want to change it back. :-(

does the 'list' command work as expected?

Thanks, but what is "list" ?
I tried to google it but can't find any clue.

Originally Posted by pheres

does the 'list' command work as expected?

Code:

cyberfish@cyberfish-laptop:~$ gdb
GNU gdb 6.6-debian
Copyright (C) 2006 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 "i486-linux-gnu".
(gdb) help list
List specified function or line.
With no argument, lists ten more lines after or around previous listing.
"list -" lists the ten lines before a previous ten-line listing.
One argument specifies a line, and ten lines are listed around that line.
Two arguments with comma between specify starting and ending lines to list.
Lines can be specified in these ways:
  LINENUM, to list around that line in current file,
  FILE:LINENUM, to list around that line in that file,
  FUNCTION, to list around beginning of that function,
  FILE:FUNCTION, to distinguish among like-named static functions.
  *ADDRESS, to list around the line containing that address.
With two args if one is empty it stands for ten lines away from the other arg.
(gdb)

does it refer to my source code to fetch the code content?
If I, say, delete the source code and use GDB to debug the binary, is GDB still able to show the code content in any way? (I don't have my linux machine at hand today, so I can't verify it now)
Anybody knows?

I just tried it on my Linux machine. Yes, that is the case. gdb no longer shows line content after I delete the source file.

Code:

cyberfish@cyberfish-laptop:/tmp$ echo "int main() { int a[10]; a[10000] = 5; }" > a.cpp
cyberfish@cyberfish-laptop:/tmp$ g++ -g a.cpp
cyberfish@cyberfish-laptop:/tmp$ gdb ./a.out
GNU gdb 6.6-debian
Copyright (C) 2006 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 "i486-linux-gnu"...
Using host libthread_db library "/lib/tls/i686/cmov/libthread_db.so.1".
(gdb) r
Starting program: /tmp/a.out 

Program received signal SIGSEGV, Segmentation fault.
main () at a.cpp:1
1       int main() { int a[10]; a[10000] = 5; }
(gdb) q
The program is running.  Exit anyway? (y or n) y
cyberfish@cyberfish-laptop:/tmp$ rm a.cpp
cyberfish@cyberfish-laptop:/tmp$ gdb ./a.out
GNU gdb 6.6-debian
Copyright (C) 2006 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 "i486-linux-gnu"...
Using host libthread_db library "/lib/tls/i686/cmov/libthread_db.so.1".
(gdb) r
Starting program: /tmp/a.out 

Program received signal SIGSEGV, Segmentation fault.
main () at a.cpp:1
1       a.cpp: No such file or directory.
        in a.cpp
(gdb) q
The program is running.  Exit anyway? (y or n) y