Manipulating memory

Manipulating memory

[Eran Ifrah]

Hi,

I have read the GDB manual, and I could not find a way to manipulate a
memory, for example: set values at given address. I only found a way
to view it using '-data-read-memory' command,
is this true or did I miss something?

-- 
Eran Ifrah
eran.ifrah@gmail.com

Re: Manipulating memory

[Joel Brobecker]

> I have read the GDB manual, and I could not find a way to manipulate a
> memory, for example: set values at given address. I only found a way
> to view it using '-data-read-memory' command,
> is this true or did I miss something?

To view memory at a given address, I usually use the "x" command.
To write memory, I use "set {TYPE}ADDRESS := VALUE" (or something
like that, I'm completely jetlagged right now). For instance, to
set a byte at 0xdeadbeef to 0x48, I would do:

    (gdb) set {char} 0xdeadbeef = 0x48

(This assume that the current language is C)

-- 
Joel

Re: Manipulating memory

[Sheng-Liang Song]

 Here is an example: (if and if only the memory can be accessed by your 
CPU directly).

char mac[20]; (a memory on stack)

(gdb) p mac
$23 = "\0050:1b:21:01:b2:71\000\000"
(gdb) p /x mac
$24 = {0x5, 0x30, 0x3a, 0x31, 0x62, 0x3a, 0x32, 0x31, 0x3a, 0x30, 0x31, 
0x3a,
  0x62, 0x32, 0x3a, 0x37, 0x31, 0x0, 0x0, 0x0}
(gdb) p &mac
$25 = (char (*)[20]) 0x7fbfffe940
(gdb) p *((char*) 0x7fbfffe940)
$26 = 5 '\005'
(gdb) set  *((char*) 0x7fbfffe940)=208
(gdb) p *((char*) 0x7fbfffe940)
$27 = -48 ''
(gdb) set  *((char*) 0x7fbfffe940)=11
(gdb) p *((char*) 0x7fbfffe940)
$28 = 11 '\v'


Note: For some CPUs,  some memories can only be only accessed with 
word-aligned (addr%4==0) addresses.
If you try to cast a random memory address to a (char*), it may not work 
(or crash!).

Sheng-Liang Song

Joel Brobecker wrote:
>> I have read the GDB manual, and I could not find a way to manipulate a
>> memory, for example: set values at given address. I only found a way
>> to view it using '-data-read-memory' command,
>> is this true or did I miss something?
>>     
>
> To view memory at a given address, I usually use the "x" command.
> To write memory, I use "set {TYPE}ADDRESS := VALUE" (or something
> like that, I'm completely jetlagged right now). For instance, to
> set a byte at 0xdeadbeef to 0x48, I would do:
>
>     (gdb) set {char} 0xdeadbeef = 0x48
>
> (This assume that the current language is C)
>
>   

Re: Manipulating memory

[André Pönitz]

On Tuesday 05 August 2008 21:03:38 Eran Ifrah wrote:
> Hi,
> 
> I have read the GDB manual, and I could not find a way to manipulate a
> memory, for example: set values at given address. I only found a way
> to view it using '-data-read-memory' command,
> is this true or did I miss something?

I'd be glad if someone had a real solution for that task, too.

So far I used something along the lines of

   call sprintf(address, "%d %s ..", token, ... )

to write something to target memory, but I am looking for an alternative
too, as this gets mis-parsed in some cases.

Maybe  

   restore filename [binary] bias start end

as described in section 8.16 of the manual would help, but that seems to
require the contents to be put in a file first. Not really convenient either...

Regards,
André

Re: Manipulating memory

[Nick Roberts]

 > > I have read the GDB manual, and I could not find a way to manipulate a
 > > memory, for example: set values at given address. I only found a way
 > > to view it using '-data-read-memory' command,
 > > is this true or did I miss something?
 > 
 > I'd be glad if someone had a real solution for that task, too.
 > 
 > So far I used something along the lines of
 > 
 >    call sprintf(address, "%d %s ..", token, ... )
 > 
 > to write something to target memory, but I am looking for an alternative
 > too, as this gets mis-parsed in some cases.
 > 
 > Maybe  
 > 
 >    restore filename [binary] bias start end
 > 
 > as described in section 8.16 of the manual would help, but that seems to
 > require the contents to be put in a file first. Not really convenient either.


GDB has an undocumented MI command called -data-write-memory.  I've not used it
though, and don't know if it works or does what you want.

-- 
Nick                                           http://www.inet.net.nz/~nickrob

Re: Manipulating memory

[Eran Ifrah]

On Thu, Aug 7, 2008 at 10:15 AM, Nick Roberts <nickrob@snap.net.nz> wrote:
>  > > GDB has an undocumented MI command called -data-write-memory.  I've not
>  > > used it though, and don't know if it works or does what you want.
>  > >
>  >
>  > Thanks ! this is exactly what I was looking for: the sibling function
>  > of -data-read-memory.
>  >
>  > Any ideas why this command is undocumented?
>
> Not really.  If it works for you, perhaps you could documentation.
I was not referring to the actual documentation, but maybe there is a
reason for that? unsupported?

But, I will document it once I will explore all its options.

>
> --
> Nick                                           http://www.inet.net.nz/~nickrob
>

-- 
Eran Ifrah
eran.ifrah@gmail.com

Re: Manipulating memory

[André Pönitz]

On Thursday 07 August 2008 09:42:23 Eran Ifrah wrote:
> On Thu, Aug 7, 2008 at 10:15 AM, Nick Roberts <nickrob@snap.net.nz> wrote:
> >  > > GDB has an undocumented MI command called -data-write-memory.  I've not
> >  > > used it though, and don't know if it works or does what you want.
> >  > >
> >  >
> >  > Thanks ! this is exactly what I was looking for: the sibling function
> >  > of -data-read-memory.
> >  >
> >  > Any ideas why this command is undocumented?
> >
> > Not really.  If it works for you, perhaps you could documentation.
> I was not referring to the actual documentation, but maybe there is a
> reason for that? unsupported?
> 
> But, I will document it once I will explore all its options.

As side note: And maybe documentation for non-implemented 
features could be removed.

For the original problem: looks like

    set {char[100]} buffer_in_debuggee = { 's','o','m','e',' ','d','a','t','a',0 }

works. 

André

Re: Manipulating memory

[Eli Zaretskii]

> Date: Thu, 7 Aug 2008 10:42:23 +0300
> From: "Eran Ifrah" <eran.ifrah@gmail.com>
> 
> But, I will document it once I will explore all its options.

Thanks!