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Date: Tue, 05 Aug 2008 19:37:00 -0000
From: Sheng-Liang Song <ssl@baymicrosystems.com>
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To: Joel Brobecker <brobecker@adacore.com>
CC: Eran Ifrah <eran.ifrah@gmail.com>, gdb@sourceware.org
Subject: Re: Manipulating memory
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 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)
>
>