Re: thread local storage (__thread) variables aren't working in gdb 6.5/6.6?

["John Bates" <johnnybates at gmail dot com>]

Interesting.. it does work with C, but not with C++. I tried -lpthread
in all cases with a C++ build (on fedora core and debian) with the
same problem.

jbates@psycho:~/test/gdbtls$ mv tls.c tls.cpp
jbates@psycho:~/test/gdbtls$ gcc -g tls.cpp -o tls -lpthread
jbates@psycho:~/test/gdbtls$ gdb tls
GNU gdb 6.4.90-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/libthread_db.so.1".

(gdb) start
Breakpoint 1 at 0x8048392: file tls.cpp, line 6.
Starting program: /home/jbates/test/gdbtls/tls
Failed to read a valid object file image from memory.
[Thread debugging using libthread_db enabled]
[New Thread -1209833792 (LWP 4199)]
[Switching to Thread -1209833792 (LWP 4199)]
main () at tls.cpp:6
6        i = 5;
(gdb) p i
Cannot access memory at address 0x0
(gdb)

On 6/26/07, Jim Blandy <jimb@codesourcery.com> wrote:
> "John Bates" <johnnybates@gmail.com> writes:
> > It would be helpful to update the GDB man/info/docs to state that
> > thread local storage is not supported. It would also help to print an
> > error when attempting to access TLS __thread variable that says
> > "thread local storage is not supported." Currently, GDB implies that
> > __thread variables are bad addresses--for example, "Cannot access
> > memory at address 0x0". I have verified the same result in GDB 6.4,
> > 6.5 and 6.6, with and without libpthread linked with the ELF. A simple
> > test case is in the email below if anyone is interested in checking
> > this.
>
> GDB does support thread-local storage --- but you need to link your
> program against -lpthread.
>
> $ cat tls.c
> __thread int i;
>
> int
> main (int argc, char **argv)
> {
>   i = 5;
>   return i;
> }
> $ gcc -g tls.c -o tls -lpthread
> $ gdb tls
> GNU gdb Red Hat Linux (6.5-15.fc6rh)
> 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 "i386-redhat-linux-gnu"...Using host libthread_db library "/lib/libthread_db.so.1".
>
> (gdb) start
> Breakpoint 1 at 0x8048382: file tls.c, line 6.
> Starting program: /home/jimb/play/tls
> [Thread debugging using libthread_db enabled]
> [New Thread -1208985920 (LWP 7664)]
> [Switching to Thread -1208985920 (LWP 7664)]
> main () at tls.c:6
> 6         i = 5;
> (gdb) p i
> $1 = 0
> (gdb) step
> 7         return i;
> (gdb) p i
> $2 = 5
> (gdb) info address i
> Symbol "i" is a thread-local variable at offset 0 in the thread-local storage for `/home/jimb/play/tls'.
> (gdb)
>
> The current GDB sources do print a slightly more helpful error message
> when they can't do this:
>
> $ gcc -g tls.c -o tls
> $ gdb tls
> GNU gdb Red Hat Linux (6.5-15.fc6rh)
> 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 "i386-redhat-linux-gnu"...Using host libthread_db library "/lib/libthread_db.so.1".
>
> (gdb) start
> Breakpoint 1 at 0x8048352: file tls.c, line 6.
> Starting program: /home/jimb/play/tls
> main () at tls.c:6
> 6         i = 5;
> (gdb) print i
> Cannot find thread-local variables on this target
> (gdb)
>
> This doesn't suggest the workaround of linking against -lpthread.