Re: Inspecting memory address contents in GDB

[Jonny Grant <jg@jguk.org>]

On 09/04/2023 20:50, Andreas Schwab wrote:
> On Apr 09 2023, Jonny Grant wrote:
> 
>> Hi
>> Pasting some output lines from a core dump gdb session below.
>> I noticed after "x/8c" the line seems stuck in ASCII mode. So when I do "x/8b" it doesn't go back to output in hex, is that expected?
> 
>>From help x:
> 
>     Defaults for format and size letters are those previously used.
>     Default count is 1.  Default address is following last thing printed
>     with this command or "print".
> 
> 'c' is a format, 'b' is a size letter.  Each one will independently
> remain in effect until a different format and size, resp., is used.
> 

Many thanks Andreas, that makes sense.

I tried to make a dump, but got arguments wrong, gdb core dumped itself. (feels like the macro inhibited the prompt). It says to file as a gdb bug 

Regards, Jonny


$ gdb -c crash2_S11_1681076589.core
GNU gdb (Ubuntu 12.1-3ubuntu2) 12.1
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[New LWP 830133]

This GDB supports auto-downloading debuginfo from the following URLs:
https://debuginfod.ubuntu.com 
Enable debuginfod for this session? (y or [n]) n
Debuginfod has been disabled.
To make this setting permanent, add 'set debuginfod enabled off' to .gdbinit.
Core was generated by `./crash2'.
Program terminated with signal SIGSEGV, Segmentation fault.
#0  0x0000562e40e0b13d in ?? ()
(gdb) define mem_dump
Type commands for definition of "mem_dump".
End with a line saying just "end".
>dump binary memory dump.bin $arg0 $arg0+$arg1
>end
(gdb) mem_dump 100 0x0000562e40e0b13d
/build/gdb-hsz3w3/gdb-12.1/gdb/utils.c:712: internal-error: virtual memory exhausted: can't allocate 94756656951613 bytes.
A problem internal to GDB has been detected,
further debugging may prove unreliable.
----- Backtrace -----
0x559d96041fb6 ???
0x559d963a08b4 ???
0x559d963a0af0 ???
0x559d964efef4 ???
0x559d9639be91 ???
0x559d964f3c4a ???
0x559d9607b044 ???
0x559d96077714 ???
0x559d963611ef ???
0x559d96083bb2 ???
0x559d96083ff9 ???
0x559d96084365 ???
0x559d9636104d ???
0x559d96141c64 ???
0x559d96141ff0 ???
0x559d961426a2 ???
0x7f306352374c ???
0x559d961427fd ???
0x559d961429b3 ???
0x559d96140b4c ???
0x559d964f04a5 ???
0x559d964f0a66 ???
0x559d961fb66c ???
0x559d961fd3a4 ???
0x559d95fa49bd ???
0x7f306222350f __libc_start_call_main
	../sysdeps/nptl/libc_start_call_main.h:58
0x7f30622235c8 __libc_start_main_impl
	../csu/libc-start.c:381
0x559d95faa524 ???
0xffffffffffffffff ???
---------------------
/build/gdb-hsz3w3/gdb-12.1/gdb/utils.c:712: internal-error: virtual memory exhausted: can't allocate 94756656951613 bytes.
A problem internal to GDB has been detected,
further debugging may prove unreliable.
Quit this debugging session? (y or n) [answered Y; input not from terminal]

This is a bug, please report it.  For instructions, see:
<https://www.gnu.org/software/gdb/bugs/>.

/build/gdb-hsz3w3/gdb-12.1/gdb/utils.c:712: internal-error: virtual memory exhausted: can't allocate 94756656951613 bytes.
A problem internal to GDB has been detected,
further debugging may prove unreliable.
Create a core file of GDB? (y or n) [answered Y; input not from terminal]
Aborted (core dumped)