Red Hat ENTERPRISE LINUX 3 - SECURITY GUIDE, Manual

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136 Chapter 16. Altering Execution
$1 = 1
(gdb) set g=4
(gdb) p g
$2 = 1
(gdb) r
The program being debugged has been started already.
Start it from the beginning? (y or n) y
Starting program: /home/smith/cc_progs/a.out
"/home/smith/cc_progs/a.out": can’t open to read symbols:
Invalid bfd target.
(gdb) show g
The current BFD target is "=4".
The program variable
g
did not change, and you silently set the
gnutarget
to an invalid value. In
order to set the variable
g
, use
(gdb) set var g=4
gdb allows more implicit conversions in assignments than C; you can freely store an integer value into
a pointer variable or vice versa, and you can convert any structure to any other structure that is the
same length or shorter.
To store values into arbitrary places in memory, use the
{...}
construct to generate a value of spec-
ified type at a specified address (refer to Section 10.1 Expressions ). For example,
{int}0x83040
refers to memory location
0x83040
as an integer (which implies a certain size and representation in
memory), and
set {int}0x83040 = 4
stores the value 4 into that memory location.
16.2. Continuing at a different address
Ordinarily, when you continue your program, you do so at the place where it stopped, with the
continue
command. You can instead continue at an address of your own choosing, with the fol-
lowing commands:
jump
linespec
Resume execution at line
linespec
. Execution stops again immediately if there is a break-
point there. Refer to Section 9.1 Printing source lines , for a description of the different forms of
linespec
. It is common practice to use the
tbreak
command in conjunction with
jump
. Refer
to Section 7.1.1 Setting breakpoints .
The
jump
command does not change the current stack frame, or the stack pointer, or the contents
of any memory location or any register other than the program counter. If line
linespec
is in a
different function from the one currently executing, the results may be bizarre if the two functions
expect different patterns of arguments or of local variables. For this reason, the
jump
command
requests confirmation if the specified line is not in the function currently executing. However,
even bizarre results are predictable if you are well acquainted with the machine-language code
of your program.