Why does gdb get wrong "optind" variable value?

Question

I am writing a simple program which using getopt function:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

int main(int argc, char **argv)
{

        char *fname;
        int c;

        printf("Before getopt: optind is %d, address is %p \n", optind, &optind);

        while ((c = getopt(argc, argv, "f:")) != -1)
                switch (c) {
                case 'f':
                        fname = optarg;
                        break;
                }

        printf("After getopt: optind is %d, address is %p \n", optind, &optind);
        return 0;
}

Executing it, outputs:

# ./test -f 1
Before getopt: optind is 1, address is 0x601040
After getopt: optind is 3, address is 0x601040

But when using gdb to debug it, some weird things happen:

13              printf("Before getopt: optind is %d, address is %p \n", optind, &optind);
(gdb) n
Before getopt: optind is 1, address is 0x601040
15              while ((c = getopt(argc, argv, "f:")) != -1)
(gdb) p &optind
$1 = (int *) 0x7ffff7dd42a0 <optind>
(gdb) n
16                      switch (c) {
(gdb) n
18                              fname = optarg;
(gdb)
19                              break;
(gdb) p &optind
$2 = (int *) 0x7ffff7dd42a0 <optind>
(gdb) p optind
$3 = 1

I can see using p optind command, it outputs 1 (should be 3), and this variable address is 0x7ffff7dd42a0, not 0x601040.

Using readelf command:

# readelf -a test | grep optind
000000601040  000600000005 R_X86_64_COPY     0000000000601040 optind + 0
 6: 0000000000601040     4 OBJECT  GLOBAL DEFAULT   25 optind@GLIBC_2.2.5 (2)
54: 0000000000601040     4 OBJECT  GLOBAL DEFAULT   25 optind@@GLIBC_2.2.5

It also display one optind, and its address should be 0x601040. So when using gdb, why does it get optind from 0x7ffff7dd42a0? What is it?

Update:
Using the newest gdb 7.11, and find this issue has been fixed.

Answer 1

This is an obscure gdb bug, triggered by another obscure feature known as a copy relocation.