gdb optimized out values when using __thread - c

Question

All my static __thread values shown as <optimized out> when, in debugging, I want to watch the value of a variable; even with -o0 and/or volatile.
Static Variables without __thread are shown correctly.
Is there anyway to show the values of the variable even though I'm working with threads?
Using Win10 (CreateThread), eclipse CDT, c11, mingw64-w64 and gdb 7.11.1

Answer 1

A workaround may be: add some printers of thread local variables in your code, and let gdb call them. (Or if you are familiar with x86 assembly, write some hackish plugin to modify the executable memory to read out fs:offset/gs:offset (thread local variable value) and recover the memory & register)
To be more specific, add a function to the C code that does nothing but return the interesting __thread variable, and when you break the program using gdb, you can always make gdb call that function for you (assume that the function is not optimized) without corrupting the stack frame of the original program. it should be as easy as:
(gdb) p rand()
$1 = 1804289383
(gdb) p rand()
$2 = 846930886
(gdb) p rand()
$3 = 1681692777
although rand is not a good example since it has side effect. TLS variable read does not have side effect.
Example: (under Ubuntu 16.04, but things shouldn't have much difference since the functionality is very basic)
tls.cpp:
#include <stdio.h>
__thread long thread_cand;
long what_is_thread_cand()
{
return thread_cand;
}
int main()
{
while ( !feof ( stdin ) )
{
scanf ( "%ld", &thread_cand );
printf ( "%p : %ld\n", &thread_cand, thread_cand );
}
return 0;
}
terminal:
$ g++ -O2 -g3 tls.cpp -o tls
tls.cpp: In function ‘int main()’:
tls.cpp:14:38: warning: ignoring return value of ‘int scanf(const char*, ...)’, declared with attribute warn_unused_result [-Wunused-result]
scanf ( "%ld", &thread_cand );
^
$ gdb tls --nh
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Reading symbols from tls...done.
(gdb) r
Starting program: /home/ubuntu/tls
123
0x7ffff7fcb6f8 : 123
432
0x7ffff7fcb6f8 : 432
^C
Program received signal SIGINT, Interrupt.
0x00007ffff7b04230 in __read_nocancel ()
at ../sysdeps/unix/syscall-template.S:84
84 ../sysdeps/unix/syscall-template.S: No such file or directory.
(gdb) p thread_cand
Cannot find thread-local storage for process 6472, executable file /home/ubuntu/tls:
Cannot find thread-local variables on this target
(gdb) p what_is_thread_cand()
$1 = 432
(gdb)