How to modify memory contents using GDB

Question

I know that we can use several commands to access and read memory  for example  print  p  x     But how can I change the contents of memory at any specific location  while debugging in GDB

User · Answer

As Nikolai has said you can use the gdb 'set' command to change the value of a variable.

You can also use the 'set' command to change memory locations. eg. Expanding on Nikolai's example:

(gdb) l
6       {
7           int i;
8           struct file *f, *ftmp;
9
(gdb) set variable i = 10
(gdb) p i
$1 = 10

(gdb) p &i
$2 = (int *) 0xbfbb0000
(gdb) set *((int *) 0xbfbb0000) = 20
(gdb) p i
$3 = 20

This should work for any valid pointer, and can be cast to any appropriate data type.

User · Answer

Expanding on the answers provided here   You can just do set idx   1 to set a variable  but that syntax is not recommended because the variable name may clash with a set sub-command  As an example set w 1 would not be valid   This means that you should prefer the syntax  set variable idx    1 or set var idx   1   Last but not least  you can just use your trusty old print command  since it evaluates an expression  The only difference being that he also prints the result of the expression     gdb  p idx   1  1   1   You can read more about gdb here

User · Answer

The easiest is setting a program variable  see GDB  assignment     gdb  l 6         7           int i  8           struct file  f   ftmp  9  gdb  set variable i   10  gdb  p i  1   10   Or you can just update arbitrary  writable  location by address    gdb  set  int 0x83040   4   There s more  Read the manual

[c] How to modify memory contents using GDB?

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