Re: Handling aliases in debug information

[Elena Zannoni <ezannoni at redhat dot com>]

Andrew Haley writes:
 > In this program, a and b are aliases for the same variable:
 > 
 > int
 > main (int argc, char *argv[])
 > {
 >   union
 >   {
 >     int a;
 >     int b;
 >   };
 >   a = argc;
 >   return b;
 > }
 > 
 > But debugging is hard:
 > 
 > cuddles:~ $ g++ -v
 > Reading specs from /usr/local/lib/gcc/i686-pc-linux-gnu/3.4/specs
 > Configured with: '/local/aph/gcc/gcc/configure' '--enable-threads=posix '--enable-languages=c,c++,java : (reconfigured) 
 > Thread model: posix
 > gcc version 3.4 20030915 (experimental)
 > cuddles:~ $ g++ union.cc -g
 > cuddles:~ $ gdb ./a.out
 > GNU gdb 5.2.1
 > Copyright 2002 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 "i686-pc-linux-gnu"...
 > (gdb) b main
 > Breakpoint 1 at 0x8048434: file union.cc, line 10.
 > (gdb) r
 > Starting program: /cuddles/aph/home/a.out 
 > 
 > Breakpoint 1, main (argc=1, argv=0xbfffdbc4) at union.cc:10
 > 10        a = argc;
 > (gdb) n
 > 11        return b;
 > (gdb) p a
 > No symbol "a" in current context.
 > (gdb) info locals
 > No locals.
 > 
 > Is this because
 > 
 > a.  g++ is not generating good debug info for anonymous unions?
 > b.  gdb is not interpreting g++ debug info correctly?
 > c.  Some other reason?

Hmm, I do not see this:

[ezannoni@topanga ezannoni]$ gcc -g aph.cc -o aph
[ezannoni@topanga ezannoni]$ gdb ./aph
GNU gdb Red Hat Linux (5.3.90-0.20030710.39rh)
Copyright 2003 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-gnu"...Using host libthread_db library "/lib/tls/libthread_db.so.1".

(gdb) b main
Breakpoint 1 at 0x8048324: file aph.cc, line 9.
(gdb) r
Starting program: /home/ezannoni/aph 

Breakpoint 1, main (argc=1, argv=0xbfffc5b4) at aph.cc:9
9	  a = argc;
(gdb) p a
$1 = 134513528
(gdb) p b
$2 = 134513528
(gdb) info locals
a = 134513528
b = 134513528
(gdb) n
10	  return b;
(gdb) info locals
a = 1
b = 1
(gdb) 

[ezannoni@topanga ezannoni]$ gcc -v
Reading specs from /usr/lib/gcc-lib/i386-redhat-linux/3.2.3/specs
Configured with: ../configure --prefix=/usr --mandir=/usr/share/man --infodir=/usr/share/info --enable-shared --enable-threads=posix --disable-checking --with-system-zlib --enable-__cxa_atexit --host=i386-redhat-linux
Thread model: posix
gcc version 3.2.3 20030502 (Red Hat Linux 3.2.3-20)

But I see you are using an ancient gdb. So maybe try a newer one.

elena