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

[Jim Blandy <jimb@codesourcery.com>]

"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.