[Bug exp/28428] New: Global variable is always null

[Bug exp/28428] New: Global variable is always null

[peter.weber at flapflap dot eu]

https://sourceware.org/bugzilla/show_bug.cgi?id=28428

            Bug ID: 28428
           Summary: Global variable is always null
           Product: gdb
           Version: 11.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: exp
          Assignee: unassigned at sourceware dot org
          Reporter: peter.weber at flapflap dot eu
  Target Milestone: ---

Created attachment 13709
  --> https://sourceware.org/bugzilla/attachment.cgi?id=13709&action=edit
optarg

Hello!
I'm using GNU getopt[1] and during debugging the global variable 'char *
optarg' always prints 0x00 i.e. nullptr. While the variable actually contains a
valid pointer address as a value. Upon assignment, the correct value is
therefore used and the program executes well in defined behavior. My
expectation is, that GDB shows the same value as is used by the actual program
in execution. I'm using Archlinux with GCC 11.1 and GDB 11.1. The attachment
contains a sample code.

Steps to reproduce:
--------------------------------------------------------------------------------
$ gcc -o getopt getopt.c -g # debugging ON, optimizations OFF
$ gdb getopt
(gdb) b 26 # set breakpoint
(gdb) r -cfoo
(gdb) p optarg
$1 = 0x0 # no "foo"?
(gdb) n # executes cvalue = optarg
(gdb) p cvalue
$2 = 0x7fffffffea99 "foo" # here it is, please don't expect to get the same
memory-address
--------------------------------------------------------------------------------

When I stop the execution on line 26 and print 'optarg' I receive 0x00. When I
step further to line 28 and print 'cvalue' it contains the passed command line
parameter value. The 'optarg' still shows 0x00 as a value. I figured out that a
syntax which is used to refer to hidden variables or similar[3] can be used as
a workaround: p (char*)'optarg@GLIBC_2.2.5'

I figured that out during reading other C++-related questions and playing with
tab completion in GDB. For further info, you can read the corresponding
question at Stackoverflow[2]. 

Do I make something wrong? Is this a bug?
Thank you
Peter


[1] https://www.gnu.org/software/libc/manual/html_node/Using-Getopt.html
[2] https://stackoverflow.com/q/69409047/1054324
[3] https://sourceware.org/gdb/current/onlinedocs/gdb/Variables.html

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[Bug exp/28428] Global variable is always null

[peter.weber at flapflap dot eu]

https://sourceware.org/bugzilla/show_bug.cgi?id=28428

Peter <peter.weber at flapflap dot eu> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |DUPLICATE
             Status|UNCONFIRMED                 |RESOLVED

--- Comment #1 from Peter <peter.weber at flapflap dot eu> ---
https://sourceware.org/bugzilla/show_bug.cgi?id=8588

GDB seems to fail to handle the Global-Offset-Table of Position-Indepdent-Code
here.

*** This bug has been marked as a duplicate of bug 8588 ***

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