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backtrace/1436: manual fails to say what ?? means in backtrace


>Number:         1436
>Category:       backtrace
>Synopsis:       manual fails to say what ?? means in backtrace
>Confidential:   no
>Severity:       non-critical
>Priority:       low
>Responsible:    unassigned
>State:          open
>Class:          doc-bug
>Submitter-Id:   net
>Arrival-Date:   Thu Oct 30 19:08:00 UTC 2003
>Closed-Date:
>Last-Modified:
>Originator:     Jacob Burckhardt
>Release:        20031028
>Organization:
>Environment:
This GDB was configured as "i686-pc-linux-gnu"...Using host libthread_db library "/lib/libthread_db.so.1".
gdb and the test program were both compiled with gcc 3.3.2 and Linux sumatra.ca.metsci.com 2.4.18-17.7.xsmp #1 SMP Tue Oct 8 12:37:04 EDT 2002 i686 unknown
>Description:
Sometimes backtraces list ?? instead of the function name, filename
and line number.  But that behavior is omitted from the section of the
manual which describes call stacks:

http://sources.redhat.com/gdb/download/onlinedocs/gdb_7.html#SEC42

Please document what ?? means, including why it happens and how to
prevent it.
>How-To-Repeat:
Here is an example along with shell output showing the gcc and gdb
commands I used in which gdb prints ??:

The file t.c says:

#include <stdlib.h>
#include <assert.h>

void f() {
  malloc(1);
  f();
}

int main() {
  f();
  return 0;
}

~ sumatra 46 jb % gcc t.c -g
~ sumatra 47 jb % a.out 
Segmentation fault (core dumped)
~ sumatra 48 jb % /disk/tmp/bjacob/gdb+dejagnu-20031028_install/bin/gdb a.out core.18725 
GNU gdb 20031028
Copyright 2003 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 "i686-pc-linux-gnu"...Using host libthread_db library "/lib/libthread_db.so.1".

Core was generated by `a.out'.
Program terminated with signal 11, Segmentation fault.
Cannot access memory at address 0x400136c0
#0  0x4207a2f9 in ?? ()
(gdb) backtrace 9
#0  0x4207a2f9 in ?? ()
Cannot access memory at address 0xbf800000
(gdb) 
>Fix:

>Release-Note:
>Audit-Trail:
>Unformatted:


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